MACHINE GUN SEARS AND CONVERSION PARTS
The definition of "machinegun" in the NFA (26 USC sec.
5845(b) includes parts to convert a gun into a machine gun. Note
that conversion parts are not included in the definition of
"firearm" under the Gun Control Act, one of the few things I know
of that is a firearm under the NFA, but not the GCA. Thus the
purchaser of a conversion part from an FFL need not do a 4473
form, unlike other NFA weapons. Of course the host gun, if
purchased from an FFL, will require the 4473. This reading of
the law is based on numerous statements from ATF, and the
definition of "firearm" under the GCA, which requires it be able
to expel a shot. However, at least one very slow judge has
decided that somehow the definition of "firearm" in the GCA
"incorporates" the definition of "machine gun" under the GCA
(even though the law doesn't say that) and that a machine gun
conversion part is a "firearm" under the GCA as well as the NFA.
I think the judge is clearly wrong, even ATF reads the law better
than that, but the point is to be careful. The case is U.S. v.
Hunter, 843 F.Supp 235 (E.D. Mich. 1994), and see also the same
judge's second opinion in the same case, at 863 F.Supp. 462 (E.D.
Mich. 1994). These parts are called registered sears, as well as
other parts or sets of parts to convert a gun into a machine gun.
In every case, the part(s) are installed into a
semi-automatic gun, and without any alteration to the semi-auto
gun's receiver, the new part(s) will allow the gun to fire as a
machine gun.
As a general rule a sear conversion is less
desirable than an original gun, or a registered receiver
conversion. This is because if the registered part breaks or
wears out it cannot be replaced, only repaired, if possible.
BATF considers replacing it with a new part to be the new
manufacture of a machine gun, and a civilian could not own it, as
it would have been made after the 1986 ban. This wear/breakage
thing is also true of the receiver on a gun where that is the
registered part, but in general the receiver is less subject to
wear or breakage than a small part, like a sear. Being larger,
a receiver may also be easier to repair. The sear conversion
will most likely not be just like the factory machine gun
version; it will be working in the semi-auto version of the gun.
A registered receiver conversion can (and should, but isn't
always) be mechanically identical to the original full auto
version of the gun, and factory spare parts may be used. Some
sear conversions require altered parts, in addition to the
registered sear.
http://www.subguns.com/laws/iif1.htm
You can't repair a broken or fractured sear.
Robert