Should have just posted here in this forum for the higher visibility but...hindsight is 20/20. Anyway, post recreated below for convenience sake:

www.ar15.com/forums/topic.html?b=3&f=4&t=316662


I had a question that was recently fielded to me, and I've never built an AR before, so I'm kind of curious of myself.

"Are all AR15 uppers and lowers interchangeable?"

I know for the most part the answer is yes, but are there any exceptions? I remember reading somewhere that Colt's civilian rifles weren't made milspec and therefore there might be some compatibility problems with milspec parts?

Finally, can a gas piston upper work with a lower that was originally part of a direct impingement rifle?

Thanks.

For the most part yes all lowers and uppers fit together fine. Colt is the only exception to the rule. Colt made some uppers and lowers with a larger front pivot pin. Of course they don't do that anymore, just on some of the earlier AR-15's. And yes you can add a gas piston upper to a standard lower with no problems.

Thanks GunDisaster. That's what I thought, but I wanted to be 100% sure before I passed on any false info.

You WILL find that some receivers are "tighter" than others, starting with RRA.  They did this on purpose; their lowers are a hair narrower in the pivot ears and their uppers are a bit wider in the pivot lug.  The takedown lug is also just a bit bigger.  All of this gives you an incredibly tight rifle when it's put together, but it can be a bit of a chore to get it together in the first place.

no, if you try it you will effectivly kill yourself. you can send all your AR stuff to mee for disposal






grins greedily

Quoted: no, if you try it you will effectivly kill yourself. you can send all your AR stuff to mee for disposal

Heh...I'm sure I'll be fine.

Also, some of the parts for a Bushmaster Carbon Fiber AR15 are not mil-spec, and therefore cannot be easily interchanged with some others. Old Professional Ordinance CAR-15 carbon fiber rifles have very few mil-spec parts on them. Uppers won't match mil-spec lowers and vice-versa (the takedown pins are not located in the same position) also the barrel nut is different on some of these rifles. gk