However for HK guns it is pretty much all there is, especially if you want an MP-5 type gun. And if you want a version of the Colt 9mm AR machine gun, the auto sear route is more plentiful than the few registered receiver conversions, and the even fewer factory Colt guns, as the model was introduced (1985) right around the same time as the 1986 ban. And in general the sear or other registered part is cheaper to buy than the same gun as a registered receiver, both because you aren’t getting a gun also, and because it is less desirable. However you may find that due to the escalating value of the semi-auto host guns, the conversion part already installed in a host gun may cost as much as a registered receiver conversion of the same gun, like an IMI semi-auto UZI with a registered bolt installed versus a registered receiver UZI conversion. It pays to shop around.
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In all likelihood 18 USC sec. 922(o), the ban on civilian possession of machine guns registered after the law took effect, or never registered, precludes an Amnesty (as provided for under existing law) for machine guns. You could register it, and comply with the NFA, but you would still be in violation of sec. 922(o), because the gun would have been registered after the law took effect. The penalties are the same under either law. One could register all other categories of NFA guns at an Amnesty. Congress could also pass a law providing for an Amnesty, and override 922(o) in that manner.