Will any AR-15 upper function on a full-auto M-16 lower receiver?

[Viking]

Assume you have an M-16, and want something shorter. You look around, and find a nice 10.5 inch upper receiver for sale. You buy it and put it on your M-16 receiver. Will the gun still function as an assault rifle (ie safe-semi-full auto)? Or are there some parts in the original upper receiver required for full auto not present in the 10.5 inch upper?

[charby]

I believe you need a full auto bolt carrier to function as a machine gun.

[Boomhauer]

So long as it has a full auto bolt carrier- which are readily available and no problem to acquire.

[Jamisjockey]

Which FTR the M16 BCG has more mass than the AR15 BCG and functions better in an AR.

[MikeB]

Depends on what you mean by M-16 lower receiver sort of. The early AR15 SP1 models had a different larger takedown pin for attaching the upper than most modern variants. My "M-16" is a AR15 SP1 conversion to FA and uses the different pin. There are conversion pins you can buy, they come with some uppers. So long story short, yes any upper will work, but depending on the actual lower and upper you might need a conversion pin like this:

http://www.bravocompanyusa.com/DPMS-Offset-Pin-Set-AR15-p/offset%20pin.htm

Also some companies still make large pin uppers, I think Daniel Defense does.

[Fly320s]

I believe you need a full auto bolt carrier to function as a machine gun.

As I understand it, a full auto bolt carrier is not needed to function correctly, but the extra mass of that bolt carrier is a good idea to have.

Look at Boomhauer's photo. The only difference is in the tail of the bolt carrier. That doesn't affect function of the fire control group.

[bedlamite]

As I understand it, a full auto bolt carrier is not needed to function correctly, but the extra mass of that bolt carrier is a good idea to have.

You do need the full auto bolt carrier to trip the auto sear. The semi bolt is cut too far back to trip it.

[MikeB]

You do need the full auto bolt carrier to trip the auto sear. The semi bolt is cut too far back to trip it.

This is correct, but if someone already has a "M-16", then they should already have a bolt carrier that can be swapped to another upper unless it is perhaps an oddball caliber.

[AJ Dual]

Which FTR the M16 BCG has more mass than the AR15 BCG and functions better in an AR.

Yeah, no AR-15 "needs" an AR-15 bolt carrier. It was one of the PC-esque inventions of Colt to try and forestall bans and whatnot during the 90's. Like Colt "Large Pin" lowers, which used larger pins to hold in the trigger/sear/disconnector than the M16 standard.