The M16 bolt was intended solely for the M16, thus the name.............It does work in later designed semi auto's, however , it was not intended for that use.
That is the way it has been explained to me.
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The same could be said for almost every other major component of an AR-15, or AKM, or any number of other semi-auto rifles based on earlier military designs. The barrel, the pistol grip, the sights, the sling loops, etc.... Heck, a lot of imported AK kits started their lives as full-auto military guns and they were imported under very strict rules that still allowed most of their parts, including the bolts to be imported and used in semi-auto guns.
The part that matters here is that it is not intended solely for use in a MG. People get hung up on the name, "Full Auto carrier" . So what if that's the name. a very large number of manufacturers build and sell those carriers in rifles that are semi-auto only. That means they are not intended to be used solely in a MG.
I get that some manufacturers worry about this stuff and it's just as easy for them to sell semi-auto carriers, so why not. It's still a dumb argument and the name "full-auto carrier" does not make it a carrier that is intended solely for use in FA weapons.