¶1
Madsen, J.Matthew Williams challenges his conviction for possession of an unlawful firearm pursuant to RCW 9.41.190. Williams claims that instruction 11, the “to convict” instruction, was a misstatement of the law because it failed to inform the jury that in order to convict him, it must find he knew the characteristics of the firearm that made it unlawful under RCW 9.41.190. We hold the State must prove that a defendant knows, or should know, the characteristics of the firearm that make it unlawful.
¶2 Nevertheless, we conclude that the defect in instruction 11 was harmless. There is no dispute that Williams’ knowledge of the law prohibiting him from possessing the shotgun at issue is irrelevant to the question of guilt. He is *906presumed to know the law. Because the barrel of the shotgun that Williams possessed was shortened to only 13 inches, 5 inches shorter than the law permits, the evidence of his guilt is overwhelming. Although Williams arguably presented a sympathetic explanation for possessing the gun, such considerations were for the prosecuting attorney in deciding initially whether to charge Mr. Williams with possession of an unlawful firearm. We affirm.
FACTS
¶3 In April 2003, Mr. Williams was helping his grandmother move out of her house and into another residence. While he was cleaning out his grandmother’s garage, he came across his deceased grandfather’s shotgun. Mr. Williams took the shotgun and placed it in the bathroom that was inside the back bedroom — the bedroom that had been his grandmother’s — because there was a lock on that door and the garage did not have a lock. He then locked the door to the bedroom to prevent others from stumbling upon the gun and hurting themselves.
¶4 The following week, Mr. Williams was leaving his grandmother’s house to run some errands when Deputy Sheriff Mark Malloque approached him and inquired about a certain juvenile suspect for whom Malloque was looking. Mr. Williams said that the juvenile was not at his grandmother’s house. At Deputy Malloque’s request, Mr. Williams allowed him to search the house for the juvenile. He unlocked the bedroom door to allow Deputy Malloque to look for the juvenile. Inside the bathroom, Malloque saw the shotgun sitting on top of the toilet tank and noticed that the barrel was shorter than allowed by law. When asked about the weapon, Williams initially denied knowing anything about the gun. Upon further inquiry, he said the gun came from the garage. Deputy Malloque arrested Mr. Williams. Williams said he did not understand why he was being arrested until Deputy Malloque informed him that the gun was too short. The barrel on the shotgun measured 13 1/8 *907inches, with an overall length of 24 3/8 inches. Report of Proceedings (jury trial, July 29, 2003) (RP) at 105. The State charged Mr. Williams with one count of possession of an unlawful firearm pursuant to RCW 9.41.190(1).
¶5 At trial, the court advised the jury in instruction 11 that to convict the defendant it must find beyond a reasonable doubt “(1) [t]hat on or about April 23, 2003 [Williams] did knowingly have in his possession or under his control a short-barreled shotgun and (2) [t]hat the acts occurred in the State of Washington.” Clerk’s Papers (CP) at 56. The State proposed that the jury be instructed that “[i]t is not a defense to a criminal charge that the Defendant believe his or her conduct was lawful. Ignorance of the law is no excuse for criminal conduct.” RP at 132. The State argued that the purpose of the instruction was to preclude defense counsel from arguing that the defendant did not know this particular weapon was illegal. The trial court declined to give the proposed instruction.
¶6 In closing argument defense counsel argued that under instruction 11, Williams had to know that the shotgun was a short-barreled shotgun but did not have to know it was illegal. In contrast, the State argued that Williams did not have to know the weapon was a short-barreled shotgun in order for the jury to convict him.
¶7 The jury found Mr. Williams guilty, and the court sentenced him to 45 days in jail (with possibility of jail alternatives). Williams appealed his conviction, arguing that the State needed to prove that he knew the facts that made the firearm illegal. The Court of Appeals disagreed, holding that the State need only prove that a defendant knowingly possessed the unlawful firearm. State v. Williams, 125 Wn. App. 335, 340-41, ¶15, 103 P.3d 1289 (2005).
ANALYSIS
¶8 Mr. Williams argues that an essential element of possession of an unlawful firearm is knowledge that the firearm in question has the characteristics of an unlawful *908firearm. He claims that the “to convict” instruction given in this case erroneously omitted this knowledge requirement. Accordingly, he contends, his conviction must be reversed and a new trial ordered under proper instructions. The State argues that it need only prove that a defendant knowingly possessed the firearm.1
¶9 RCW 9.41.190(1) provides that “[i]t is unlawful for any person to manufacture, own, buy, sell, loan, furnish, transport, or have in possession or under control, any machine gun, short-barreled shotgun, or short-barreled rifle.” Because the statute contains no express knowledge element, we must decide whether the legislature intended to hold those who possess these weapons strictly liable.
¶10 Statutory interpretation is a question of law that this court reviews de novo. Am. Cont’l Ins. Co. v. Steen, 151 Wn.2d 512, 518, 91 P.3d 864 (2004). The primary goal of statutory interpretation is to ascertain and give effect to the legislature’s intent and purpose. Dep’t of Ecology v. Campbell & Gwinn, LLC, 146 Wn.2d 1, 9, 43 P.3d 4 (2002). This is done by considering the statute as a whole, giving effect to all that the legislature has said, and using related statutes to help identify the legislative intent embodied in the provision in question. Id. at 9-10.
¶11 While this court has not decided whether knowledge is an element of RCW 9.41.190, we have determined that knowledge is an element of RCW 9.41.040(2)(a), second degree unlawful possession of a firearm. State v. Anderson, 141 Wn.2d 357, 5 P.3d 1247 (2000). RCW 9.41.040 prohibits convicted felons from possessing firearms. In Anderson, this court relied on the test from Staples v. United States, 511 U.S. 600, 114 S. Ct. 1793, 128 L. Ed. 2d 608 (1994), adopted by this court in State v. Bash, 130 Wn.2d 594, 925 P.2d 978 (1996), in finding that the legisla*909ture did not intend second degree unlawful possession of a firearm to be a strict liability crime. In Staples, the defendant argued that the government was required to prove that he knew the features of his firearm that brought the weapon within the requirements of the National Firearms Act, 26 U.S.C. §§ 5801-5872, which criminalized possession of an unregistered machine gun. Because the act did not contain a knowledge element, the Court considered several factors to decide whether Congress intended to create strict liability. These factors include: (1) the background rules of the common law and its conventional mens rea requirement, (2) whether the crime can be characterized as a public welfare offense, (3) the extent to which a strict liability reading of the statute would encompass innocent conduct, (4) the harshness of the penalty, (5) the seriousness of the harm to the public, (6) the ease or difficulty of the defendant ascertaining the true facts, (7) relieving the prosecution of time-consuming and difficult proof of fault, and (8) the number of prosecutions expected. Staples, 511 U.S. at 615-19. The considerations identified in Staples must be applied “in light of the principle that offenses with no mental element are generally disfavored.” Anderson, 141 Wn.2d at 363.
¶12 In Anderson, application of the Staples’ factors led to the conclusion that the legislature did not intend to eliminate mens rea from RCW 9.41.040. Id. Accordingly, we held that the State must prove knowing possession to sustain a conviction.
¶13 The gun possession offenses created in RCW 9.41-.040 and .190 implicate various levels of knowledge. State v. Warfield, 119 Wn. App. 871, 878, 80 P.3d 625 (2003). Under RCW 9.41.190, at issue here, there are three possible points at which knowledge might be relevant: knowledge of the possession itself; knowledge that possessing a short-barreled shotgun, short-barreled rifle, or machine gun is illegal; and knowledge of the weapon’s characteristics that makes possession of the weapon unlawful.
*910¶14 With regard to knowledge of possession, in State v. O’Neal, 126 Wn. App. 395, 414-15, 109 P.3d 429 (2005), the court dismissed the defendant’s conviction for possession of a machine gun without prejudice because the charging document failed to include knowledge of the possession as an element of the crime. Similarly, in Warfield, 119 Wn. App. 871, the court held that knowing possession is an element of the crime of possession of an unlawful firearm under RCW 9.41.190. The decisions in O’Neal and Warfield were based, in part, on this court’s decision in Anderson. We agree that our reasoning in Anderson compels the result reached by the Court of Appeals in those cases.
¶15 Here, the trial court instructed the jury in instruction 11 that to convict Williams of the crime of unlawfully possessing a short-barreled shotgun, it must find that “[Williams] did knowingly have in his possession or under his control a short-barreled shotgun.” CP at 56. The trial court’s instruction was proper and in accord with O’Neal and Warfield.2
¶16 The only remaining question here is whether the legislature intended to make persons strictly liable for possessing certain types of weapons, regardless of whether the person knows the characteristics of the weapon that make its possession illegal. As in Anderson, we are guided by the factors adopted from Staples. The first factor does not further our analysis because there is no common law predecessor to RCW 9.41.190. However, the second factor, whether the crime is a public welfare offense, does shed some light on the inquiry. As noted, the eight-factor test has previously been applied by this court in construing RCW *9119.41.040. See Anderson, 141 Wn.2d 357. Notably, the legislature amended RCW 9.41.040 at the same time as RCW 9.41.190. Laws op 1994, 1st Spec. Sess., ch. 7, §§ 402, 420. Both amendments were part of a comprehensive effort by the legislature to reduce violence, especially among youths. Id. § 101. The legislature stated that the purpose of the amendments and new enactments was to
(1) [p]revent acts of violence by encouraging change in social norms and individual behaviors that have been shown to increase the risk of violence; (2) reduce the rate of at-risk children and youth . . .; (3) increase the severity and certainty of punishment for youth and adults who commit violent acts; (4) reduce the severity of harm to individuals when violence occurs; (5) empower communities to focus their concerns and allow them to control the funds dedicated to empirically supported preventive efforts in their region; and (6) reduce the fiscal and social impact of violence on our society.
Id. Because RCW 9.41.040 and RCW 9.41.190 were each amended with these purposes, our analysis of this legislative history in Anderson to determine whether RCW 9.41.040 was intended as a public welfare offense is persuasive. The court noted that the legislature was trying to address the problem of increasing violence in society. Anderson, 141 Wn.2d at 362. While the court found that the legislative intent was inconclusive, it did not believe that the legislature intended a strict public welfare offense. Id. at 362, 364. Likewise, the Warfield court found that “although the [legislative] findings certainly reference the public’s health and safety, the link is not so strong that it conclusively establishes RCW 9.41.190 as a public welfare offense.” Warfield, 119 Wn. App. at 879. Thus, case law supports the finding that the legislative history does not positively establish RCW 9.41.190 as a public welfare statute.
¶17 Case law is more conclusive on the third factor, i.e., whether a strict liability reading would encompass seemingly entirely innocent conduct. We begin the analysis with *912Staples because the Washington cases dealing with the issue cite Staples with approval. See Anderson, 141 Wn.2d 357; Warfield, 119 Wn. App. 871; State v. Semakula, 88 Wn. App. 719, 946 P.2d 795 (1997). In Staples, the petitioner was convicted of unlawful possession of an unregistered machine gun pursuant to § 5861(d) of the National Firearms Act. Staples, 511 U.S. at 614-15. Section 5861(d) of the act was silent regarding a knowledge requirement for violation. Staples, 511 U.S. at 605. In determining whether Congress intended a knowledge requirement, the Court expressed concern about criminalizing a broad range of apparently innocent conduct. Id. at 615. Under a strict liability reading of the statute, criminal sanctions would potentially be imposed on “a class of persons whose mental state — ignorance of the characteristics of weapons in their possession— makes their actions entirely innocent.” Id. at 614-15. Using a factual hypothetical, the Court pointed out that in the government’s view, any person who has purchased what he believes to be a semiautomatic rifle or handgun, or who simply has inherited a gun from a relative and left it untouched in an attic or basement, can be subject to imprisonment, despite absolute ignorance of the gun’s firing capabilities, if the gun turns out to be automatic. Id. at 615. The Court determined that Congress did not intend to imprison a person who genuinely believed what he possessed was a semiautomatic weapon but turned out to have been worn down into, or to have been secretly modified to, an automatic weapon. Id. Thus, the Court held that the government should have been required to prove that petitioner knew of the features of his firearm that brought it within the scope of the act. Id. at 624-25.
¶18 Similar to the Staples Court, the Anderson court was most compelled by “the fact that entirely innocent conduct may fall within the net cast by [RCW 9.41.040].” Anderson, 141 Wn.2d at 366 (holding that knowledge is an element of RCW 9.41.040). The court found that the legislature did not intend to sweep entirely innocent conduct within the statute. Id. at 362. To support this proposition the court cited *913Staples’ concern with criminalizing perfectly innocent conduct. Id. at 364. The court surmised that “if the State is not required to prove knowing possession of a firearm [,] there is a distinct possibility that entirely innocent behavior would fall within the sweep of this statute.” Id.
¶19 The Warfield court also expressed concern about criminalizing innocent conduct when it held that a defendant must knowingly possess a firearm to be convicted under RCW 9.41.190. Warfield, 119 Wn. App. at 882-83. The court cited Staples and found that imposing strict liability on a person who lacks knowledge or reason to know that he or she possessed a prohibited item would be “overly and unfairly rigorous.” Id. at 879. Moreover, the court noted that “[w]ithout a knowing possession element, the offense would implicate a significant amount of innocent conduct, which is a result that the legislature likely did not intend.” Id. at 881.
¶20 In the present case, we are similarly concerned that possessing a firearm can be innocent conduct. Citizens have a constitutional right to bear arms under both the federal and state constitutions. U.S. Const, amend. II; Wash. Const. art. I, § 24. A person may lawfully own a shotgun so long as the barrel length is more than 18 inches in length and has an overall length of more than 26 inches. See RCW 9.41.010(6). RCW 9.41.190 precludes possession of a short-barreled shotgun. Moreover, the statute also criminalizes possession of a short-barreled rifle and a machine gun. The factor concerned with innocent conduct is particularly important in the case of a machine gun, which can be altered in ways not easily observable.3 If strict liability is imposed, a person could innocently come into the possession of a shotgun, rifle, or weapon meeting the definition of a machine gun but then be subject to imprisonment, despite *914ignorance of the gun’s characteristics, if the barrel turns out to be shorter than allowed by law or the weapon has been altered, making it a machine gun. The legislature likely did not intend to imprison persons for such seemingly innocent conduct.
¶21 The fourth factor, the harshness of the penalty, has also been considered in recent Washington cases. In Anderson we found that the harshness of the penalty, a five year maximum term, weighed in favor of requiring a knowledge element. Anderson, 141 Wn.2d at 365. Warfield followed our decision in Anderson, concluding that the harshness of the penalty, also a five year maximum term, weighed against strict liability. Warfield, 119 Wn. App. at 882. The maximum imprisonment term for the present statute is likewise five years. RCW 9.41.190(4); RCW 9A.20.021(1)(c). Therefore, the harshness of the penalty in this case also weighs against strict liability.
¶22 The fifth factor requires us to consider the seriousness of the harm to the public. A person who possesses a weapon prohibited under RCW 9.41.190 but who does not know the characteristics that make it unlawful may still pose a danger. However, that firearms may be dangerous must be considered along with the fact that it is lawful to own many of them. In Staples, the Court rejected the arguments that “all guns . . . are dangerous devices that put gun owners on notice that they must determine . . . whether their weapons come within the scope of the [National Firearms] Act.” Staples, 511 U.S. at 608. Instead, the Court focused on the fact that guns are lawful possessions. Id. at 610. There is a “long tradition of widespread lawful gun ownership by private individuals in this country.” Id.
¶23 While Washington’s legislature expressed its intent to protect the public from increasing violence in society, Laws of 1994, 1st Spec. Sess., ch. 7, § 101, in Anderson we noted that a person’s unwitting possession of a firearm poses no significant danger to the public. Anderson, 141 *915Wn.2d at 365. Moreover, punishing unwitting possessors does not further the goal of deterrence. Id.
¶24 The sixth factor, the ease or difficulty of defendant ascertaining the true facts, is not particularly useful in our analysis. Although the length of the barrel and/or stock of a short-barreled rifle or shotgun may easily be seen, the statute also criminalizes possession of a machine gun, and a semiautomatic firearm that has been altered into an automatic weapon may not be obvious, particularly to one who is unfamiliar with firearms. Because the statute’s prohibition extends to both types of firearms, the same mental element, if any, must apply to both types.
¶25 The seventh factor requires us to consider whether requiring proof of knowledge would place too heavy a burden on the prosecution. In Staples, the Court noted that “knowledge can be inferred from circumstantial evidence, including any external indications signaling the nature of the weapon.” Staples, 511 U.S. at 615 n.ll. Moreover, the Court stated that “we are confident that when the defendant knows of the characteristics of his weapon that bring it within the scope of the [National Firearms] Act, the Government will not face great difficulty in proving that knowledge.” Id. Likewise, here, proving that a defendant knew the characteristics of his firearm would not place too heavy a burden on the State.
¶26 The final factor considers the number of prosecutions to be expected. Generally, the fewer expected prosecutions, the more likely intent is required. Anderson, 141 Wn.2d at 365 (citing 1 Wayne R. LaFave & Austin W. Scott, Substantive Criminal Law § 3.8, at 344 (1986)). In 2004, 16 persons were convicted of violating RCW 9.41.190.4 The relatively few convictions pursuant to RCW 9.41.190 support the finding that knowledge is an element under the statute.
¶27 Considering all of the above factors leads to the conclusion that the legislature intended that the State *916prove that a person knew, or should have known, the characteristics that make a firearm illegal to be convicted under RCW 9.41.190. Thus, we hold that a defendant must know the characteristics of the firearm that make the firearm illegal.
¶28 We are not unmindful that the difference between knowing the facts that make a person’s conduct illegal and ignorance of the law is a fine distinction. As noted earlier, the State is not required to prove a gun owner knows that possession of a shotgun with a barrel less than 18 inches or a stock and barrel of less than 26 inches is unlawful. The State’s burden is to prove that the defendant knowingly possessed an unlawful weapon and that he knew, or should have known, the characteristics that make the weapon unlawful. These facts will be readily apparent in the case of a short-barreled rifle or short-barreled shotgun since, in most cases, the owner of such a weapon can easily determine its length. As stated above, an individual is held to know that possession of such weapons is unlawful and is also held to know the legal definition of a short-barreled shotgun, a short-barreled rifle, or a machine gun. RCW 9.41.010(4), (6), (7).5
¶29 Next we must determine if instruction 11 properly informed the jury of the State’s burden. Instruction 11 can be read two ways: it can be read to require only knowing possession of the weapon, or it can be read, as defense counsel did, to require both knowledge of possession and knowledge that the weapon is a short-barreled shotgun. Because this instruction has the potential of creating con*917fusion, we conclude that it is deficient. However, as this court has held, “ ‘not every omission or misstatement in a jury instruction relieves the State of its burden’ ” so as to require reversal. State v. Berube, 150 Wn.2d 498, 505, 79 P.3d 1144 (2003) (quoting State v. Brown, 147 Wn.2d 330, 339, 58 P.3d 889 (2002)); Neder v. United States, 527 U.S. 1, 9, 119 S. Ct. 1827, 144 L. Ed. 2d 35 (1999). Thus, we must determine whether the error was harmless.
¶30 Under the Neder test for harmless error, it must appear beyond a reasonable doubt that the error did not contribute to the ultimate verdict. Id. We believe that instruction 11, while potentially ambiguous, did not mislead the jury as to the State’s burden in this case. First, Williams himself argued that instruction 11, coupled with instruction 9, required the State to prove that he knowingly possessed a particular weapon, a short-barreled shotgun. Specifically, counsel told the jury,
[t]he State would submit to you that he only has to be aware of the fact that it’s something. I would submit to you that he has — that knowledge that the fact, in this Instruction Number 9, is the fact of it being a short-barrelled shotgun, not that it’s illegal, but the fact this it was a short-barrelled shotgun.
RP at 143.
¶31 Secondly, Williams’ weapon was considerably modified. The barrel was shortened to only 13 1/8 inches and the overall length of the shotgun was only 24 3/8 inches. We think the jury was more than justified in finding that he knew or should have known that the barrel of his shotgun was less than 18 inches (5 inches shorter than the law permits) and thus met the legal definition of a short-barreled shotgun. Accordingly, we find that any error in this case was harmless.
|32 The Court of Appeals is affirmed.
Owens and Fairhurst, JJ., concur.
¶33
The State also argues that even if the “to convict” instruction is erroneous, Mr. Williams invited the error because he did not object. Because the State raises this argument for the first time in its supplemental brief, we decline to address it. State v. Collins, 121 Wn.2d 168, 847 P.2d 919 (1993); Douglas v. Freeman, 117 Wn.2d 242, 814 P.2d 1160 (1991). Because we decline to reach this issue, we do not reach Mr. Williams’ motion to strike that portion of the State’s brief addressing it.
The parties agree that the State is not required to prove Williams knew that possessing a short-barreled shotgun is unlawful. In order to prevent Williams from arguing that he did not know that he was violating the law by possessing a short-barreled shotgun, the State proposed an instruction advising the jury that ignorance of the law is not an excuse. Although the trial court declined to give the State’s proposed instruction, the court told defense counsel that he could not argue that ignorance of the law excused Williams’ conduct. Counsel responded that he only intended to argue that Williams did not know the weapon was a short-barreled shotgun, that is, Williams did not know the characteristics of the weapon that made it illegal.
Amachine gun is defined as “any firearm known as a machine gun, mechanical rifle, submachine gun, or any other mechanism or instrument not requiring that the trigger be pressed for each shot and having a reservoir clip, disc, drum, belt, or other separable mechanical device for storing, carrying, or supplying ammunition which can be loaded into the firearm, mechanism, or instrument, and fired therefrom at the rate of five or more shots per second.” RCW 9.41.010(7).
Wash. Sentencing Guidelines Comm’n, Statistical Summary of Adult Felony Sentencing: Fiscal Year 2004, at 9, available at http://www.sgc.wa.gov/Informa tional/Publications.htm (last visited Nov. 28, 2005).
The concurrence apparently wants it both ways. On the one hand, the concurrence says that RCW 9.41.190 requires the State to prove only that the defendant knew he possessed the firearm at issue, concurrence at 918, but then, inconsistently, appears to agree the State must prove knowledge of the characteristics rendering possession of the firearm unlawful if those characteristics are not “readily apparent to the naked eye.” Id. at 918-19. RCW 9.41.190 either requires the State to prove that the defendant had knowledge of the weapon’s illegal characteristics or it does not. The better approach in cases such as the one presented here, where the illegal characteristic is visible, is to give the additional instruction that ignorance of the law is no excuse for possession of an unlawful firearm under RCW 9.41.190.