I was just wondering if it was okay to put a CMMG bolt and carrier that has less than 300 rounds into another upper?  I am thinking that it is not completely broken in yet, so I shouldn't have any issues with the change, but I heard different advice from two different individuals that I asked.  Naturally, my fun shop who assembles ARs stated that it was okay and I know they do it all the time, yet a smith I know said not to do it.

Since I can't think of any reason not to do it on a fairly new CMMG bolt and carrier, I was wondering if anyone could tell me the cons of doing this?  Basically I am putting it in another upper, and it will stay there since I decided to do something different with the upper it is coming out of.

With that few of rounds I don't see a problem with a switch. Your opinion that the bolt has not fully mated with the old reciever extention may or may not be accurate, but the wear should be very slight and should not cause an issue. If you have head space gauges it is always a good ideal to check, But if you don't I would not be to overly concerned with it.

It is fine to swap the BCG into another upper.  If you are really worried then you can check the headspace.

Hmm, I think I will wait for the brown truck and just swap the carriers.  I was planning to go shooting later today, but, I can't get in touch with anyone that has 5.56 gages on this fine Sunday.

To take his question one step further...

Is there harm in taking a new bolt and putting in a used (3000rd) carrier?

What about that carrier in a new upper?

The important part is the bolt to barrel extension fit.

The carrier simply carries the bolt back and forth and cams the lugs in the extension. As long as the carrier is serviceable you should be fine.

IMHO if you do lots of parts swapping it is a good idea to invest in a go/no-go gauge set for your chamber.

Thanks - this is going into a new SDI barrel.