Will an AR-15 upper receiver work well on an M4 lower?

[TheGewehrGuy]

At my local gunshop, the dealer told me that he gave his military friend an AR-15 pistol, piston driven upper to test out for him at the nearby military base.

And so today I got into an argument with about 5 guys who said that it would not fit, and/ or it would not function. I am no expert but I do believe almost all parts on an M4/ M16 platform are interchangeable with the AR-15 platform, are they not?

If I am wrong, please tell me which parts MUST be changed in order to function flawlessly.

I don't honestly see how they wouldn't, that would be dumb to build a civilian variant of a rifle with completely different and non-interchangeable specifications.

 

[MrCleanOK]

The M4 and M16 are specific configurations of AR-15 rifles. The upper receivers are all interchangeable, neglecting that different length gas systems require different weight buffers in the lower receiver, that commercial SEMI bolt carrier groups won't work properly with AUTO trigger groups, and other fancy boutique crap.

Are you saying that this piston upper with a presumably short barrel (it's for a pistol) is going to be mated up with a military M4 lower receiver to be run through its paces? If that is the case, it will most likely fit onto the lower, but whether or not it functions correctly is based on how it's gassed, and whether or not the BCG has the correct "auto" geometry.

 

[mljdeckard]

Range control doesn't appreciate privately owned weapons on the range.

But yes, it should work.

 

[killchain]

A semi-only bolt carrier group is going to have problems in a true M4 safe/semi/burst lower.

But if you're talking about a pistol grip/buffer tube AR15 lower receiver, it should "work..." the buffer tube and spring and the gas system have to be configured properly or it will short stroke, beat it self to death, etc.

Also, if it's a short AR15 pistol barrel on a normal AR15 lower, you are skirting the law.

 

[TheGewehrGuy]

Thanks

You've confirmed my general thought about that.

Of course it wont always be as easy as popping a new upper on without regard to bolt carrier or gas tubes and what not.

I had 4 guys on Yahoo Answers tell me that it would not work, and one guy said that a military M4 carbine has a different barrel specification that is completely illegal to the general public. I don't know what that guy was smoking.

I did post the question in the "military" category, so I probably wasn't getting very many answers from knowledgeable gun enthusiasts.

 

[F40]

Yahoo answers? Really? Any "ar" upper will fit on an M4/m16/ar lower. It's the trigger assembly/sear that are different. And the M16 bcg is different too, but will work in all.

 

[Robert]

Yahoo Answers

Really? Yahoo Answers, really? Wow that has to be the worst source of information since the creation of the internet.

Next time come to us first. We can help better than Yahoo Answers.

 

[Zerodefect]

Should work fine. May need a different buffer as noted above.