I stumbled across the following text on another website, regarding AR15 fire control parts. It talks about the danger of doubling with the AR15 bolt carrier. It has me concerned about the "shrouded" AR15 bolt carriers.
The AR-15 hammer, unlike the M16 hammer, has a notch on the front face, which is supposed to catch the ring or collar on the back of the AR-15 firing pin, if the disconnector is disabled. The collar on the AR-15 firing pin is larger than that on the M-16 firing pin, for this purpose. The hammer notch will only catch the ring on the AR-15 firing pin if an AR-15 style bolt carrier is also used, as they have a ramp milled along the underside of the carrier for this purpose; the M16 carrier, in addition to the sear trip surface, also does not have this ramp milled.
So the AR-15 hammer, bolt carrier and firing pin are all required to prevent the hammer follow down malfunction.
This hammer follow down malfunction can also be induced by physically removing the disconnector from the firearm, if either an M-16 bolt carrier or M-16 hammer are also used, and ATF has done just that, when the rifle lacked an M-16 disconnector, trigger or selector.
Removing the disconnector entirely is the same mechanically as putting the M-16 selector in the full automatic position, when the gun also is assembled in combination with an M-16 disconnector and trigger. This hammer follow down malfunction is much more reliable if handloaded ammunition is used, made with softer pistol primers, rather than rifle primers.
ATF has used .223 ammunition loaded with softer than normal primers to make an AR-15 type rifle with no auto sear fire more than one shot, in combination with inducing the hammer follow down malfunction.
This hammer follow down phenomenon is really a malfunction, as it is possible for the rifle to fire the cartridge before the action is locked, with potentially disastrous consequences for both the rifle and the person firing it.
"It is not the function of the government to keep the citizen from falling into error; it is the function of the citizen to keep the government from falling into error."--- U.S. Supreme Court Justice Robert H. Jackson