I just got a new BCM complete bolt carrier group.  Do I need to worry about head spacing?  IF not I am just going to slap it in and be good to go. It is being used on a rifle that has maybe 300 rounds on it.

Should be fine.

I've got the same question although I have a new barrel(CMMG) using a RRA BCG that has about 300 rounds on it.

I was under the impression that if the stuff(like the BCG and or barrel) is not pretty much worn out then the head space with any bolt should be good if the parts are in spec.

Reading a thread here about the barrel extensions and how head space is set, I hope this is true because I would never attempt to do it myself.

For purposes of making sure, which gauge should a person buy?  The Colt field gauge which is 1.4736 or a Forster that measures 1.470   Is there an M16 field gage available someplace with the 1.4730 measurement?

My head is spinning from all this..........

For complete peace of mind get the Go, No Go gage set.

300 rounds is "Break in" level wear.

Why would I need the go gauge?  I'm not trying to be a smart alec, I really want to know.  Since I'm not setting the headspace, I wouldn't think the go would matter to me.  I know the rifle chambers factory 5.56 Winchester ammo.  

I'd really like a GI Field gauge.  If anybody has a source????

The Colt field gauge seems right at the edge, but maybe that is ok too.

Quoted: Why would I need the go gauge?  I'm not trying to be a smart alec, I really want to know.  Since I'm not setting the headspace, I wouldn't think the go would matter to me.  I know the rifle chambers factory 5.56 Winchester ammo.   I'd really like a GI Field gauge.  If anybody has a source???? The Colt field gauge seems right at the edge, but maybe that is ok too.

Headspace

For complete peace of mind get the Go, No Go gage set.