I put this question up on the AR discussions forum but havent gotten much traction with it.  Maybe some of you local guys can help.

Can you guys give me a little info on the different BCG's? I am building a M4 clone and am down to the last two pieces I need. The BCG and Butt stock. What I am having a hard time with is what is the difference with the BCG's.  When they say M16,, M16 full auto,heavy ,national match ect? How do they effect the shot? I am using factory ammo nothing fancy but want whats the better BCG for my rifle hence the custom build.
thanks for any help

There is only 0.3 ounces difference in weight.  Don't get your knickers in a bunch if you have a S/A carrier.  They work fine.  Leaking and/or loosening carrier keys are a much bigger issue.

I agree that the weight doesn't make much of a difference, but I still personally go with what the weapon system was originally designed to use. The full weight select fire carrier.

Your biggest concern should be the quality of the bolt and carrier your going to purchase. There is some real junk out there.

Colt
L.M.T.
BCM
Young Manufacturing
CMT/Stag

In no particular order, all make a solid product.

Thank you very very much guys. Thats exactly what i was looking for.  Whats this about the carrier keys?  By going with a major name does that help? Or is it an idiosyncrasy of the weapon itself?

Mostly check the carrier key is staked properly. Any bolt can be missed or not staked well, stuff happens ya know.