I have a M&P bcg and a colt upper reciever. Will the M&P function correctly and safely in the colt upper? Sorry if its a dumb question, fairly new to all this. Thanks

Function- Yes.
Check your headspace before firing to ensure safety from using mismatched parts.

+1, and welcome to the forums!

If you have new parts or parts with relatively few miles on them then there is no need to check head space. It won't hurt anything if you do, however. Generally there is no problem swapping parts as long as they are in spec which you will find out in short order. Enjoy.

So - anyone gonna tell him HOW to check headspace?

I didn't know M&P made BCGs. I didn't realize M&P made any AR-15 parts.

It has been said that M&P ARs were made and serviced by Stag/CMT.

I just read the OP again.

OP: Are you talking about a stripped upper receiver or a complete Colt barreled upper?

I have a question. I am kinda new to all of this too. If I have a F/A bcg, do I need a heavier buffer? It is installed in a 16" car with a mid length gas system.

Its a bcg out of an M&P 15. and it is a colt upper with gas tube and barrel....

You should be fine.  If you're worried about headspace, just order a set of gauges from Brownell's or Midway.

You should be well served with an H buffer.



The best option for checking headspace is the 5.56 NATO FIELD gage made by Bill Ricca:  http://www.billricca.com/collectors_corner.htm

About a quarter of the way down the page, its noted as "M16 Maximum Headspace Gage"

Out of the millions of civilian AR-15s and the millions of military M-16s there are only a hand full of accounts of "supposed" headspace issues. Unless you have several thousand rounds on that BCG you are nowhere close to having a headspace issue. I garantee your bolt lugs will break before you ever get to that point! In the military bolts get swapped all of the time during batch cleaning. I've never seen a M-16 upper or bolt with matching numbers. If it were an issue they would be marked like a bolt action rifle.han

What part of tight tolerance and total interchangiblity are we missing. The only issue arises with a chamber being improperly reamed (which is quite rare). That is what the headspace gage is for, not the bolt. Never heard of a Colt chamber being out of spec from the factory. After millions of uppers produced you'd think they'd have it right. Ever hear of a bad Colt, Spikes Tactical, Sabre, BCM or FNMI?

Check your PM's   (upper left)


Heavier than what ?
What ammo ?
There is a combination of elements that determine the best buffer for your particular rig.

Sorry about the hijack, OP.

Yes