unless they are bayonetted with a "post-postban" firearm.
BTW:
Short barreled rifles ("rifle" meaning any firarm with a shoulder stock, other than a shotgun) with barrels less than 16" require the BATFE NFA tax stamp. To get the stamp, follw these instructions:
http://www.atf.gov/firearms/faq/faq2.htm#m15(M15) What are the required transfer procedures for an individual who is not qualified as a manufacturer, importer, or dealer of NFA firearms?
ATF Form 4 (5320.4) must be completed, in duplicate. The transferor must first complete the face of the form. The transferee must complete the transferee's certification on the reverse of the form and have the "Law Enforcement Certification" completed by the chief law enforcement officer.
The transferee is to affix, on each copy of the form, a 2-inch by 2-inch photograph of the transferee taken within the past year (proofs, group photographs or photocopies are unacceptable). The transferee's address must be a street address, not a post office box. If there is no street address, specific directions to the residence must be included.
If state or local law requires a prior permit or license to purchase, possess, or receive NFA firearms, a copy of the transferee's permit or license must accompany the application. A check or money order for $200 ($ 5 for transfer of "any other weapon") shall be made payable to ATF by the transferor. All signatures on both copies must be in ink. Submit fingerprints on FBI Form FD-258, in duplicate. Fingerprints must be taken by a person qualified to do so, and must be clear and classifiable. If wear or damage to the fingertips do not allow clear prints, and if the prints are taken by a law enforcement official, a statement on his or her official letterhead giving the reason why good prints are unobtainable should accompany the fingerprints.
Forward completed information and appropriate tax payment to the Bureau of ATF, P. O. Box 73201, Chicago, IL 60673. Transfer of the NFA firearm may be made only upon approval of the ATF Form 4 by the NFA Branch. If the application is approved, the original of the form with the cancelled stamp affixed showing approval will be returned to the applicant. Otherwise, the tax will be refunded.
Upon approval of the ATF Form 4, the transferor should transfer the firearm as soon as possible, since the firearm is now registered to the transferee. <26 U. S. C. 5812, 27 CFR 179.83-179.86> Any rifle must also be at least 26" long (including the 16" barrel) to meet these NFA requirements (even with the stock folded up).
*end BATFE quote*
If that firearm picture is of a semi-auto firearm, then it would be illegal without the NFA tax stamp and accompanying registration with BATFE. If it is a full-auto firearm manufactured after 1986, then it illegal for a any US citizen to personally own (even law enforcement).