Opinion by Judge GRABER; Dissent by Judge REINHARDT.

Defendant Kelly David Ankeny, Sr., was indicted on four counts of being a felon in possession of a firearm and one count of possession of an unregistered sawed-off shotgun. The district court denied his motion to suppress and, reserving the right to appeal that decision, Defendant pleaded guilty. The district court sentenced him to 262 months’ imprisonment pursuant to the Armed Career Criminal Act of 1984 (“ACCA”) and the Career Offender provision of the United States Sentencing Guidelines (“U.S.S.G.”).

On appeal, Defendant argues that: (1) the evidence seized during the search of his residence should have been suppressed; (2) the district court should have dismissed all but one count of felon in possession; (3) the government should have been required to allege Defendant’s prior convictions in the indictment and prove them to a jury beyond a reasonable doubt; (4) his prior convictions do not qualify as predicate felonies under ACCA; and (5) the district court erred in applying the Career Offender guideline.

We hold that the motion to suppress was properly denied, but that the convictions were multiplicitous and that material errors were made at sentencing. Thus, we affirm the convictions but vacate the sentence and remand for resentencing.

FACTUAL AND PROCEDURAL HISTORY

On October 21, 2003, Michele Rayley reported to the Portland police that Defendant, with whom she has an 18-year-old son, had choked and kicked her. The altercation with Defendant took place when Rayley went to the house where their son was living, located at 936 N.E. 94th Avenue in Portland, and found that Defendant was living there. She confronted Defendant about her belief that he was supplying drugs to their son, at which point Defendant became angry and attacked her. He then ran to another floor of the house and returned waving a semi-automatic handgun. Rayley told police that she believed Defendant was using methamphet-amines and that he might flee or shoot at police.

The case was referred to Officer Rhodes of the Domestic Violence Reduction Unit. In ongoing conversations, Rayley reported to Rhodes that several other people, including an infant and a prison associate of Defendant, also were living in the house. Rayley told Rhodes that, on October 31, 2003, she and Defendant had another argument during which he displayed a handgun.

Officer Rhodes conducted a background check on Defendant and found that he had several outstanding arrest warrants and an extensive criminal history, including convictions for possession and delivery or manufacture of controlled substances, attempting to elude a police officer, escape, felon in possession of a firearm, and robbery. He also had been charged with, but not convicted of, assault on a police officer and aggravated assault.

*748 The police considered various options for how to proceed, including arresting Defendant during a traffic stop, and ultimately decided that it was necessary to arrest Defendant at the house. The police believed that a street arrest would pose a risk to public safety because Defendant had a lengthy record of violence and hostility toward the police. Further, the police believed that an arrest outside the house would be risky because there was evidence of drug and firearm activity inside the house, in addition to the presence of a prison associate of Defendant.

A warrant was authorized on November 18, 2003, and executed on November 20, 2003, at around 5:30 a.m. The house was dark, and there was no noise or movement from within. The Special Emergency Reaction Team (“SERT”) led the operation. Thirteen officers were assigned to enter the home and, in total, 44 officers participated in the execution of the warrant.

Officer Stradley yelled “police, search warrant” while pounding on the door and, about one second later, officers used a battering ram to break open the door. Officer Wilcox entered and directed a light-mounted weapon into the house. Defendant had been sleeping on a recliner near the front door; he stood up as the officers broke down the door. Officer Wilcox instructed Defendant to show his hands and get down. Officer Forsyth then threw a flash-bang device into the center of the room. Officer Forsyth testified at the suppression hearing that he heard Officer Wilcox tell Defendant to show his hands; he did not recall hearing him tell Defendant to get down. The flash-bang device had a fuse delay of one to one-and-a-half seconds. Officer Forsyth stated that Defendant went down to the floor during that delay, and the device exploded near his upper body. Because of his proximity to the flash-bang device when it exploded, Defendant suffered first and second-degree burns to his face and chest and second-degree burns to his upper arms.

Meanwhile, officers stationed outside the house shot out the second-story windows with rubber bullets. Officers securing the second level of the house threw a second flash-bang device into an open area. A man and a woman were lying in bed in that area, and the explosion caused the bed to catch fire. After attempting to extinguish the fire, officers threw the mattress and box spring out of a window.

Extensive damage was done to the house during the entry. The police shot out approximately ten windows, kicked in many doors, burned carpet, and made holes in the walls and ceilings with the rubber bullets. 1

Thereafter, the police recovered a 9mm semiautomatic handgun from the crack between the arm and the bottom cushion of the chair in which Defendant was sitting when the police entered the house. They also recovered a semiautomatic handgun on an adjacent chair. The police found a 12-gauge sawed-off shotgun and a .22-caliber long rifle in a closet in an upstairs bedroom and another .22-caliber rifle in the basement of the house. The police seized approximately $3,000, ammunition, and suspected drugs and drug paraphernalia.

Defendant was indicted on four counts of being a felon in possession of a firearm and one count of possession of an unregistered sawed-off shotgun. Soon thereafter, the government filed notice of its intent to seek a sentence enhancement under the *749 ACCA and identified three predicate felonies. Defendant moved to suppress the evidence seized during the search. After an evidentiary hearing, briefing, and argument, the district court denied the motion. Defendant entered a conditional guilty plea, reserving the right to appeal the denial of his motion to suppress.

The Presentence Investigation Report (“PSR”) specified a base offense level of 37, which included application of the Career Offender guideline. After a three-level downward adjustment for acceptance of responsibility, Defendant’s total offense level was 34 with a criminal history category of VI, resulting in an advisory Sentencing Guidelines range of 262 to 327 months.

Defendant filed a motion to dismiss as multiplicitous all but one count of his being a felon in possession of a firearm. He also filed objections to the application of the ACCA and the Career Offender guideline. He argued both that his prior convictions were not predicate felonies under ACCA and that prior convictions must be alleged in the indictment and proved to a jury in order to comport with the Sixth Amendment. The district court rejected all of Defendant’s arguments. The court considered various sentencing factors and acknowledged that the Guidelines were advisory, but declined to go below the Guideline range because of the seriousness of the offense and Defendant’s extensive criminal record. The court sentenced Defendant to 262 months’ imprisonment for each count of being a felon in possession of a firearm and 120 months’ imprisonment for the count of possession of an unregistered sawed-off shotgun, with the sentences to run concurrently. Defendant timely appealed.

DISCUSSION

A. Motion to Suppress

We first address Defendant’s contention that the evidence found in the house should have been suppressed' because the police failed to knock and announce their presence, a failure not justified by exigency, and because the extent of force used by the police rendered the search unreasonable.

In a published opinion, the district court found that the amount of time between the knock and the entry was “so brief in time to be virtually the equivalent to a no-knock entry,” but held that there were exigent circumstances justifying the police action. United States v. Ankeny, 358 F.Supp.2d 998, 1000-01 (D.Or.2005). The district court declined to decide whether the overall manner of execution of the warrant was unreasonable because the “inevitable discovery” doctrine applied: 2 the police had a warrant, and “[a]ny excess force used does not change” the fact that they would have discovered the evidence in the house. Id. Thus, the court held that, “for general objections to the manner of executing a search, suppression requires a causal link between those complained of behaviors and the seizure of the evidence,” and there was no such causal link here. Id. at 1002 .

On de novo review, United States v. Crawford, 372 F.3d 1048, 1053 (9th Cir.2004) (en banc), we agree with the district *750 court: even if the knock-and-announce violation and the other aspects of the search amounted to Fourth Amendment violations, suppression is not warranted.

1. Knock and Announce

Turning first to the alleged knock- and-announce violation, see Wilson v. Arkansas, 514 U.S. 927, 930 , 115 S.Ct. 1914 , 131 L.Ed.2d 976 (1995) (“[The] common-law knock and announce principle forms a part of the reasonableness inquiry under the Fourth Amendment.”), we hold that suppression is foreclosed by the Supreme Court’s decision in Hudson v. Michigan, — U.S. -, 126 S.Ct. 2159 , 165 L.Ed.2d 56 (2006).

In Hudson , police entered a suspect’s home, with a warrant, after announcing their presence and waiting three to five seconds. The state conceded that there was a knock-and-announce violation. The Court held that violation of the knock-and-announce rule did not merit suppression of evidence found in the search, because the purposes of the knock-and-announce rule — to protect bodily safety, property, and privacy — are not vindicated by excluding evidence obtained after the rule has been violated. Id. at 2165 . The Court emphasized that the knock-and-announce rule does not protect “one’s interest in preventing the government from seeing or taking evidence described in a warrant” and that the social costs of exclusion for knock-and-announce violations outweigh the benefits of deterrence. Id. at 2165-66 . Thus, we need not resolve whether the knock-and-announce rule was violated and, if so, whether the violation was justified by exigent circumstances. Under Hudson , the evidence should not be suppressed in any event.

Defendant asserted at oral argument that this case is not governed by Hudson , because the police could have obtained a no-knock warrant but failed to do so. 3 According to Defendant, that failure rendered the entry “essentially warrant-less.” We disagree. There is no requirement that the police obtain a no-knock warrant simply because one is available. See Richards v. Wisconsin, 520 U.S. 385 , 396 n. 7, 117 S.Ct. 1416 , 137 L.Ed.2d 615 (1997) (commenting that the fact that a no-knock entry has not been authorized in advance “should not be interpreted to remove the officers’ authority to exercise independent judgment concerning the wisdom of a no-knock entry at the time the warrant is being executed”). Further, we decline to limit Hudson so narrowly to its facts. The Supreme Court made it clear that, because the knock-and-announce rule protects interests that “have nothing to do with the seizure of ... evidence, the exclusionary rule is inapplicable” to knock-and-announce violations. Hudson, 126 S.Ct. at 2165 .

2. Manner of Entry

Defendant contends that, beyond the alleged knock-and-announce violation, the police’s manner of entry violated the *751 Fourth Amendment. He urges us to hold that the overall violence and destructiveness of the officers’ actions were unreasonable and, thus, that suppression is warranted.

It is true that “the manner in which a warrant is executed is subject to later judicial review as to its reasonableness.” Dalia v. United States, 441 U.S. 238, 258 , 99 S.Ct. 1682 , 60 L.Ed.2d 177 (1979). Unnecessary destruction of property or use of excessive force can render a search unreasonable. Boyd v. Benton County, 374 F.3d 773, 780 (9th Cir.2004); Liston v. County of Riverside, 120 F.3d 965, 979 (9th Cir.1997). Deciding whether officers’ actions were reasonable requires us to balance “the nature and quality of the intrusion on the individual’s Fourth Amendment interests against the countervailing governmental interests at stake.” Graham v. Connor, 490 U.S. 386, 396 , 109 S.Ct. 1865 , 104 L.Ed.2d 443 (1989) (internal quotation marks omitted).

Whether this entry and search were conducted reasonably is a close question. The police had legitimate concerns about their safety in entering and searching the house. Defendant had a substantial criminal record, which included violent crimes; there was reliable evidence that he was armed and aggressive; there were several other people in the house, including a former prison inmate; and certain physical characteristics of the house made it difficult to secure. 4 Officers testified at the suppression hearing that the element of surprise was very important due to those factors and that they used the battering ram, rubber bullets, and flash-bang devices in order to surprise and distract the occupants of the house. Thus, the destruction of property and use of force arguably were necessary to carry out the search safely and effectively. The fact that a gun was found stuffed into the cushions of the chair in which Defendant was sitting when the police entered suggests as much: if officers had entered more gently, perhaps Defendant would have had a chance to draw his weapon and injure or kill an officer or be injured or killed himself.

Further, the search did not exceed the scope of the warrant, which weighs in favor of a conclusion of reasonableness. See United States v. Penn, 647 F.2d 876 , 882 n. 7 (9th Cir.1980) (en banc) (“A warranted search is unreasonable if it exceeds in scope or intensity the terms of the warrant.”). The warrant authorized officers to search the house for guns, ammunition, and associated documents and paraphernalia, and they did just that. See United States v. Becker, 929 F.2d 442, 446 (9th Cir.1991) (holding that, where warrant authorized search of the defendant’s premises, it was reasonable for officers to use a jackhammer to break up a concrete slab in the backyard in order to search for evidence underneath).

On the other hand, the extent of the property damage, and particularly the use of two flash-bang devices, one of which seriously injured Defendant, weigh in favor of a conclusion of unreasonableness. See id. (noting that even where the search is within the scope of the warrant, it can be unreasonable because of its “intolerable intensity”). The record is unclear with respect to whether and why it was necessary to shoot out so many windows and break down so many doors. Cf. San Jose Charter of the Hell’s Angels Motorcycle Club v. City of San Jose, 402 F.3d 962 , 974 *752 (9th Cir.2005) (holding that it was unreasonable for officers to cut a mailbox off its post, jackhammer the sidewalk, and break a refrigerator); Mena v. City of Simi Valley, 226 F.3d 1031, 1041 (9th Cir.2000) (holding that it was unreasonable for officers to break down doors that they already knew were open). And in Boyd, 374 F.3d at 779 , we held that, “given the inherently dangerous nature of the flash-bang device, it cannot be a reasonable use of force under the Fourth Amendment to throw it ‘blind’ into a room occupied by innocent bystanders absent a strong governmental interest, careful consideration of alternatives and appropriate measures to reduce the risk of injury.” It is not clear that the officers took all appropriate and available measures to reduce the risk of injury here. For instance, Officer Forsyth testified at the suppression hearing that he was trained to deploy the flash-bang device away from the outer walls of rooms and away from furniture and curtains that could catch on fire, so he aimed for the center of the room. Although his concern for fire safety was valid, Forsyth threw the flash-bang close to Defendant.

Ultimately, we need not determine whether the entry was unreasonable because we agree with the district court that suppression is not appropriate in any event. The alleged Fourth Amendment violation and the discovery of the evidence lack the causal nexus that is required to invoke the exclusionary rule.

The principle that the exclusionary rule applies only when discovery of evidence results from a Fourth Amendment violation is well-established. See, e.g., Hudson, 126 S.Ct. at 2164 (“[B]ut-for causality is ... a necessary ... condition for suppression.”); Segura v. United States, 468 U.S. 796, 804 , 104 S.Ct. 3380 , 82 L.Ed.2d 599 (1984) (noting that the exclusionary rule reaches “evidence obtained as a direct result of an illegal search or seizure,” or “found to be derivative of an illegality”); United States v. Pulliam, 405 F.3d 782, 791 (9th Cir.2005) (denying suppression because “the indispensable causal connection” between the unlawful act and discovery of the evidence was absent).

United States v. Ramirez, 523 U.S. 65 , 118 S.Ct. 992 , 140 L.Ed.2d 191 (1998), is instructive. There, the police obtained a no-knock warrant to search a home. Approximately 45 officers gathered, announced by loudspeaker that they had a search warrant, broke one window in the garage, and pointed a gun through the opening. The Supreme Court noted that “[e]xcessive or unnecessary destruction of property in the course of a search may violate the Fourth Amendment, even though the entry itself is lawful and the fruits of the search are not subject to suppression.” Id. at 71, 118 S.Ct. 992 (emphasis added). Although the Court concluded that the police conduct in that case did not violate the Fourth Amendment, the Court noted that, had the search been unreasonable, it then would have had to determine “whether ... there was [a] sufficient causal relationship between the breaking of the window and the discovery of the guns to warrant suppression of the evidence.” Id. at 72 n. 3, 118 S.Ct. 992 (emphasis added).

Here, the discovery of the guns was not causally related to the manner of executing the search. The police had a warrant, the validity of which is not questioned, and the guns, money, and other contraband were not hidden. Even without the use of a flash-bang device, rubber bullets, or any of the other methods that Defendant challenges, “the police would have executed the warrant they had obtained, and would have discovered the [evidence] inside the house.” Hudson, 126 S.Ct. at 2164 ; cf. United States v. Hector, 474 F.3d 1150, *753 1155 (9th Cir.2007) (holding that suppression was inappropriate where “[t]he causal connection between the failure to serve [a] warrant and the evidence seized is highly-attenuated, indeed non-existent”). Accordingly, we affirm the district court’s denial of Defendant’s motion to suppress the evidence. 5

B. Multiplicitous Convictions

Defendant asserts that three of the four counts of being a felon in possession of a firearm should have been dismissed prior to sentencing because they were mul-tiplicitous. We review de novo whether a sentence is multiplicitous, unless the defendant fails to raise the issue before the district court, in which case review is for plain error. United States v. Smith, 424 F.3d 992, 999-1000 (9th Cir.2005), cert. denied, 547 U.S. 1008 , 126 S.Ct. 1477 , 164 L.Ed.2d 257 (2006). Under either standard, we agree with Defendant.

In United States v. Szalkiewicz, 944 F.2d 658 , 653-54 (9th Cir.1991) (per curiam), we held that, regardless of the number of firearms involved, there is only one offense of being a felon in possession unless there is a showing that the firearms were “stored or acquired at different times and places.” Further, the separateness of acquisition or possession must be found by a jury (or, presumably, admitted by the defendant). Id.; see also United States v. Keen, 104 F.3d 1111 , 1118 n. 11 (9th Cir.1997) (same).

The indictment in this case makes no reference to separate acquisition or possession of the four firearms. Defendant was charged with possessing each of the firearms on November 20, 2003. His written plea and plea colloquy made no reference to separate acquisition or possession. The government concedes that it presented no evidence of separate acquisition or possession. Instead, the government argues that, because Defendant moved to dismiss the allegedly multiplici-tous counts only after his guilty plea was accepted, he waived any claim of error with respect to the indictment, and any objection to his sentence may be reviewed only for plain error.

The government’s procedural argument is unpersuasive. The defendant in Szalk-iewicz similarly did not object to the multiple counts of felon in possession in the indictment&emdash;and, indeed, any objection to the indictment presumably would have been overruled because the government still would have had the opportunity to present proof of separate acquisition and possession. Defendant lodged his objec- tion at the appropriate time, after the gov- ernment had missed that opportunity. The plain error standard applies when the defendant failed to raise the issue of multi- plicitous sentences in the district court, which was not the case here. See Smith, 424 F.3d at 1000 . [10]Further, even

Further the plain error standard did apply, see United States v. Olano, 507 U.S. 725, 732 , 113 S.Ct. 1770 , 123 L.Ed.2d 508 (1993) (explaining that plain error is error that is plain and that affects substantial rights), Defendant’s conviction of multiple counts of being a felon in possession of a firearm, rather than a single count, harmed his substantial rights, see Guam v. Torre, 68 F.3d 1177, *754 1180 (9th Cir.1995) (noting that “multiple convictions, apart from concurrent sentences, carry ‘adverse collateral consequences that may not be ignored’ ” (quoting Ball v. United States, 470 U.S. 856, 864-65 , 105 S.Ct. 1668 , 84 L.Ed.2d 740 (1985))).

Accordingly, we remand with instructions for the district court to dismiss all but one count of being a felon in possession of a firearm.

C. Predicate Felonies for the ACCA

As an initial matter, Defendant’s argument that prior convictions must be alleged in the indictment and either admitted or proved beyond a reasonable doubt to a jury is foreclosed by Almendarez-Torres v. United States, 523 U.S. 224, 243-47 , 118 S.Ct. 1219 , 140 L.Ed.2d 350 (1998). Defendant’s argument that we should invoke the doctrine of constitutional doubt to avoid Almendarez-Torres is foreclosed by United States v. Grisel, 488 F.3d 844, 846-47 (9th Cir.2007) (en banc).

Defendant also argues that several of his prior convictions do not qualify as predicate felonies under the ACCA. We review that claim de novo, United States v. Marks, 379 F.3d 1114, 1116 (9th Cir.2004), and hold that the prior convictions do qualify as predicate felonies under the ACCA.

To qualify for a sentence enhancement under the ACCA, a defendant must have three or more prior convictions for violent felonies or serious drug offenses. 18 U.S.C. § 924 (e)(1). The government alleged three prior convictions: a 2002 conviction for delivery of a Schedule II controlled substance under Oregon law, a 1998 conviction for delivery of a Schedule II controlled substance under Oregon law, and a 1977 conviction for Robbery II under Oregon law.

Defendant contends that his two prior drug offenses do not qualify as predicate felonies under the ACCA, because the maximum punishment for those offenses under the Oregon Sentencing Guidelines is less than 10 years. The ACCA provides that, in order to count as a “serious drug offense,” the maximum term of imprisonment “prescribed by law” must be ten years or more. 18 U.S.C. § 924 (e)(2)(A). Delivery of a Schedule II controlled substance is a Class B felony under Oregon law, Or.Rev.Stat. § 475.840(l)(b), and the maximum sentence allowed by statute is 10 years, Or.Rev.Stat. § 161.605(2). Nonetheless, Defendant argues that the maximum sentence under the Oregon Sentencing Guidelines is the maximum term “prescribed by law.” Defendant’s argument is foreclosed by United States v. Parry, 479 F.3d 722, 724-25 (9th Cir.2007). The district court properly found that Defendant’s drug convictions were predicate felonies under the ACCA.

Defendant also contends that his 1977 robbery conviction does not qualify as a violent felony under the ACCA because he was 17 years old at the time of that offense; the ACCA defines the term “violent felony” to include “any act of juvenile delinquency involving the use or carrying of a firearm, knife, or destructive device,” 18 U.S.C. § 924 (e)(2)(B); and the indictment and judgment do not prove that he used or carried a firearm.

It is irrelevant whether Defendant used or carried a firearm, because the documents associated with the 1977 robbery show that Defendant was prosecuted as an adult, rather than as a juvenile. Had Defendant not been prosecuted as an adult, the information would not have alleged a violation of Oregon Revised Statute section 164.405, and there would not be a judgment of conviction for Robbery II — instead, there would be only an adjudication *755 of juvenile delinquency. See Or.Rev.Stat. ch. 419C (juvenile code provisions concerning delinquency); cf. State v. Lawler, 144 Or.App. 456 , 927 P.2d 99, 103 (1996) (“[A]n adult criminal prosecution is not initiated by filing a delinquency petition.”). Section 924(e)(2)(B) concerns only an act “of juvenile delinquency,” a term of art that does not encompass the criminal conviction of a juvenile who was prosecuted in adult court. See 18 U.S.C. § 5032 (discussing the transfer of a juvenile for prosecution in adult court in lieu of prosecution for juvenile delinquency).

The only remaining question is whether Robbery II under Oregon law is a violent felony for purposes of the ACCA. The ACCA defines a “violent felony” as “any crime punishable by imprisonment for a term exceeding one year ... that has as an element the use, attempted use, or threatened use of physical force against the person of another.” 18 U.S.C. § 924 (e)(2)(B)(i). Oregon’s Robbery II statute provides:

Or.Rev.Stat. § 164.405. In turn, Oregon Revised Statute section 164.395 provides that a person commits robbery

Under the categorical approach described in Taylor v. United States, 495 U.S. 575, 602 , 110 S.Ct. 2143 , 109 L.Ed.2d 607 (1990), we look only to the fact of conviction and.the statutory definition of the offense to determine whether it was a violent felony. The Oregon statutory definition of second-degree robbery contains the required element of use, attempted use, or threatened use of physical violence. See United States v. Melton, 344 F.3d 1021, 1026 (9th Cir.2003) (holding that where the state’s definition of robbery has on its face the element of use or threatened use of physical force, a robbery conviction is properly used as a predicate under ACCA). We therefore hold that a conviction for second-degree robbery under Oregon law constitutes a violent felony for purposes of ACCA.

Thus, the district court properly found that Defendant’s prior convictions were predicate felonies under ACCA.

D. Career Offender Guideline

U.S.S.G. § 4B1.1 (2003) 6 establishes that a defendant is a career offender if the defendant was at least 18 years old at the time of the instant offense, the instant offense is a felony and either a crime of violence or a controlled substance offense, and the defendant has at least two prior felony convictions for crimes of violence or controlled substance offenses. Defendant argues that the Career Offend *756 er Guideline should not have been applied to him, because the instant offense was not a crime of violence. We review de novo the district court’s interpretation and application of the Sentencing Guidelines. United States v. Nielsen, 371 F.3d 574, 582 (9th Cir.2004).

Defendant is correct with respect to the counts of being a felon in possession of a firearm in violation of 18 U.S.C. § 922 (g)(1). “‘Crime of violence’ does not include the offense of unlawful possession of a firearm by a felon.... ” U.S.S.G. § 4B1.2 cmt. n. 1; see also Stinson v. United States, 508 U.S. 36, 45-47 , 113 S.Ct. 1913 , 123 L.Ed.2d 598 (1993) (holding that Sentencing Guidelines commentary must be given controlling weight unless it violates the Constitution or a federal statute, or is plainly inconsistent with the Guidelines itself). However, Defendant is incorrect with respect to the count of possessing an unregistered sawed-off shotgun in violation of 26 U.S.C. § 5861 (d). “The possession of an unregistered firearm of the kind defined in [26 U.S.C.] § 5845 ... involves the substantial risk of violence necessary to label the possession a crime of violence,” United States v. Dunn, 946 F.2d 615, 621 (9th Cir.1991), and Defendant’s sawed-off shotgun is of the type described in § 5845(a). 7

The district court therefore should have applied the Career Offender Guideline only to Count 4, possession of an unregistered sawed-off shotgun. Instead, the district court erred by imposing a Career Offender sentence on all counts. If a district court incorrectly applies the Sentencing Guidelines and its error is not harmless, then we vacate the sentence and remand for resentencing. United States v. Cantrell, 433 F.3d 1269, 1279 (9th Cir.2006); see also United States v. Menyweather, 447 F.3d 625, 634 (9th Cir.2006) (holding that any error was harmless because, “under the unusual circumstances present in this third-time appeal, we recognize that the district court could — and would — impose the same sentence again”). Here, we cannot say with certainty that Defendant would not have received a lower sentence if the district court had calculated his Guidelines range properly. Accordingly, we remand for resentencing.

Convictions AFFIRMED, except for the multiplicitous felon-in-possession convictions, which are REVERSED; sentence VACATED; REMANDED for resentenc-ing.

. The owner of the house estimated that the damage cost him $14,000 to repair. He filed a claim against the City of Portland for the loss, received $10,000, and decided, after consulting a lawyer, not to pursue a civil suit for the remainder.

. In this circuit, the "inevitable discovery” doctrine, first recognized in Nix v. Williams, 467 U.S. 431 , 104 S.Ct. 2501 , 81 L.Ed.2d 377 (1984), provides that if, “by following routine procedures, the police would inevitably have uncovered the evidence,” then the evidence will not be suppressed despite a constitutional violation. United States v. Ramirez-Sandoval, 872 F.2d 1392, 1399 (9th Cir.1989). That concept is closely related to the "independent source” doctrine, which provides that evidence discovered by independent legal means should not be suppressed even though there was an illegal search as well. Id. at 1396 .

. Defendant also asserted at oral argument that this case is different from Hudson because the federal knock-and-announce statute, 18 U.S.C. § 3109 , applies. That issue was waived. Defendant did not argue, either before the district court or in his opening brief to this court, that § 3109 applies. See United States v. Kama, 394 F.3d 1236, 1238 (9th Cir.2005) (holding that issues not raised in a party’s opening brief are waived); Smith v. Marsh, 194 F.3d 1045, 1052 (9th Cir.1999) (noting general rule that we will not consider arguments for the first time on appeal). Accordingly, we need not decide whether the statute independently might require suppression. Moreover, we note that the search was not conducted by federal officers. See United States v. Combs, 394 F.3d 739 , 742 n. 1 (9th Cir.2005) (stating that § 3109 does not govern the conduct of state officers).

. The residence had two stories and a basement, with a concrete barrier at the rear separating the house from the 1-205 freeway.

. The dissent's assertion that, "under the majority's approach even the most outrageous methods employed by invading officers would be irrelevant so long as the officers had obtained a warrant,” dissent at 762, is overstated. Separate from the question of exclusion, " 42 U.S.C. § 1983 and the Bivens doctrine have made tort damages an effective remedy for constitutional violations by feder- al or state law enforcement officers.” United States v. Langford, 314 F.3d 892, 895 (7th Cir.2002). (7th Cir.2002).

. Defendant was sentenced under the 2003 Sentencing Guidelines.

. The 2004 commentary to U.S.S.G. § 4B1.2 explicitly provides that being a felon in possession of the type of firearm described in 26 U.S.C. § 5845 (a) is a crime of violence. If this change from the 2003 Guidelines is “clarifying’' rather than “substantive/' it applies retroactively. United States v. Morgan, 376 F.3d 1002, 1010 (9th Cir.2004). We need not decide whether the amendment applies retroactively, though, because under Dunn , Count 4 properly was treated as a crime of violence in 2003.