Will any upper work on a full auto M16 lower?

Colt, Yes. Other brands no guarantee but they should work.

As long as you have a M16 bolt carrier pretty much any standard AR upper will work.

With out a m16 carrier you have not way to "trip" the auto sear. Your upper receiver must also have a relief cut for the Auto Sear. There are a few old A1s that do not have it.

The upper has nothing to do with the auto sear. Any upper with an M16 bolt carrier will work. Colt won't work any better than any other brand.

Slight correction fellas. Not ALL will work, on the earlier Colt models, at least up thru the A/2 versions no relief cut was made on the underside of the upper to accommodate the auto sear.

I own a converted A/2, purchased in the immediate time frame before Hughes went into effect. Had a dealer do the machine work and put it on his books. When I finally got to take possession of the lower I found it unreliable. Essentially it was firing by virtue of hammer follow thru........did the machine cut on the upper and problem solved.

Bottom line....if your upper is cut for the AS you are good to go. Without that cut the upper will partially or fully depress that sear and keep it from engaging the hammer extension.....a loose upper might let it work sometimes but usually you'll get the follow thru I mention.

Most any will work for at least one round - whether or not it will run in full auto? It depends... Some of your run-of-the-mill AR15's will run too fast and trap a shell between the ejection port and bolt. As mentioned above, an M16 carrier would be needed as well.

That's an ejection or extraction problem, it has nothing to do with the speed of the gun cycling.

Either the extractor spring is to weak to hold the extractor in the groove of the casing during extraction and slipping off and leaving the case behind or your ejector is gummed up and or the ejector spring is bad which is causing the FTE.

Cyclic rate is controlled by the gas port size and buffer weight. Even over gassed guns eject, most do it violent and pitch the brass WAY over there. A casing jammed in the port has nothing to do with the ROF but rater the failure of the above parts doing their job.

In the event that the gun is short stroking you may get a jammed case in the port but short stroking guns are not "running to fast" , they are actually not running fast enough.


If your M16 lower has all the parts it was intended to have you have a FA BCG with a upper that has the relief cut you all you need from a mechanical stand point.

Sorry, but that's not entirely correct. You can absolutely have a gun that's running too fast trap shells as they come off the shell deflector and bounce straight forward, never fully leaving the ejection port. I test guns for a living and see it all the time when rate of fire gets above 1000/1100 rounds per minute on an AR-style gun.

What are running that is giving you a 1000+ PRM m16 platform?

Test guns. Some of those being high round count guns with gas port erosion, other are... well... other stuff. You're correct about FTE's usually being extractor/ejector related, but that's a completely different failure from what I meant. It's very easy to see the difference on high speed video, but I'm not at liberty to post it.