I bought a brand new colt bolt and carrier for my BM M4.How do I tell if it is a M4 by chance.Looking down the lower where the mag goes I can see the firing pin when the bolt is forward.My BM V match I can not see the firing pin.What is up ?? Dice

?????Last I heard, the M4 uses the same bolt/carrier as every other AR/M16.  What you may be referring to here is the difference between different assemblies used over the years by all the different mnakers of AR type civilian models.

Colt started it all when they brought out the very first civilian type AR15 in the '60s.  tobe be politically correct, they cut that slot in under the firing pin,put a notched hammer in, and cut back the bottom rear shoulder about half an inch so it wouldn't trip the auto sear.

In reality, all they had to do to keep the feds happy would have been the last, and apparently they finally figured that out a year or so ago. So they started just cutting back that bottom shoulder----and the rest of the market, which had slavishly followed Colt's cutaway under the firing pin style for 30 years, now has slavishly followed Colt's return to common sense. And who says Colt doesn't set the standard for the AR market[:D] Rock River even has the nerve to call it a "new improved piece". Yeah, the new improved 1963 model piece.

From your post, it sounds like you've got a Bushie with the "new" old style, and the Colt piece you acquired is the "old" new style. So far as I know, they'll both work equally well in an M4gery, though my own attitude for the past 35 years has been to use nothing but M16 carriers with the shoulder cut back about 1/4 inch.  My belief has always been that he closer you get to original M16 weight in the assmbly, the more positive and trouble free your operation will be. I've niether seen nor heard anything over all those years to change my mind; and these "new,improved" jobbies are just one more confirmation that I got it right in 1974.  Too bad it took Colt, and the other guys so damn long to figure out what some of us always knew.

I want to thank you very much for your info.Now I understand.