ar15 bcg's

[Soybomb]

I've built my lower and have been trying to decide what to do for an upper for a while. I think I'm going to have a larue rail put on so I think I'll just order my upper from cmmg. To my understanding thought they use the cmt carrier and most people aren't especially fond of its staking. I'm thinking about leaving the bcg off the upper order and ordering a carrier (lmt or denny's) elsewhere along with another spare mpi bolt to put in my grip.

My questions are whats the deal with headspacing, an upper, and different bolts? Will ordering the assembled upper and the bcg seperately cause any problems or are they more of a matched set that cmmg should be putting together all at once for me? Do the bolt and carrier wear together or do you just toss in a new bolt when the old one breaks and call it good?

Also are both LMT & CMT's MPI testing actually done well or just so they can stamp MP on the parts? I hear colt fires a proof load with them first so the test is actually somewhat more useful.

 

[RockyMtnTactical]

I have no problems with CMT BCG's, but if the staking was not done properly, it'd be a cinch to fix.

 

[vanfunk]

The headspace is set when the upper is assembled, so that should not be a concern. I have the most confidence in Colt bcg's as I feel they are the best for "hard use" but I wouln't turn down an LMT bolt. I have had 2 CMT bolts in the past and the staking was pathetic, but they were never a problem once restaked with authority. In sum, if Colt is not an option I'd choose LMT over CMT. Whatever your choice, if you buy from Denny he'll take care of you.

HTH,
vanfunk

 

[Soybomb]

Where do you even get the colt bcg? All the online shops I've looked at seem to just have rra, cmt, lmt, etc.

 

[Bartholomew Roberts]

You can buy a Colt bolt online from Specialized Armament Warehouse; but they aren't cheap - they run about $163 just for the bolt. Given the rate that non-MPI bolts fail (which is very rarely), you might be better off just buying a CMT bolt and spare.

Headspace is set when the barrel extension is set on the barrel. As long as this was done correctly and the bolt is in spec (and in 99.9% of the cases they are - I've come across one case of excessive headspace in new parts in 18 years), the headspace will be correct on new parts. Generally you only have to worry about headspace when you start swapping old, worn barrels and bolts back and forth.

 

[Soybomb]

Wow $363 for the colt carrier and bolt!

Whats the deal with a spare bolt, if the original ever breaks can I toss it in and run forever on it? Does the carrier mate to the bolt or something that would require it to be changed eventually too if the bolt did get swapped?

Is a spare carrier a decent idea too, or is it just usually the bolt?

Thanks for all the help guys, I think I'm learning and getting my ideal ar put together!

 

[Bartholomew Roberts]

Whats the deal with a spare bolt, if the original ever breaks can I toss it in and run forever on it?

You can just replace a broken bolt with a new bolt (assuming both in spec) and be good to go. In fact, if you are having problems with an AR that is a fast way to get it back into action since you address bolt lugs, ejector and extractor all at once by swapping the bolt.

Bolts won't last forever; but even with hardcore use you will get 3-6k out of them and for semi-auto shooting I would think 15k or more is a realistic expectation. So far the longest I have personally run a bolt is 10k... I was planning to run it to failure but a vendor trying to impress me with service replaced the BCG for me with a new one.

Is a spare carrier a decent idea too, or is it just usually the bolt?

Unnecessary; but if you are adding spares it isn't a bad idea to have a spare cam pin, firing pin, and two or three firing pin retaining pins.