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View Full Version : Can you use an auto BCG on a semi M4?


Vsanzbajo
10-10-2009, 5:31 PM
Can you use an auto BCG on a semi M4? Or you need to buy the semi? Differences?
Thanks

technique
10-10-2009, 5:34 PM
yes, that is a common question.

reidnez
10-10-2009, 5:40 PM
Yes. The only difference between the two is that the semi carrier is milled out further in the rear, thus it's an ounce or so lighter.

reidnez
10-10-2009, 6:29 PM
http://www.fototime.com/C466D2CD5713778/standard.jpg

TOP: Proprietary Colt design
MIDDLE: Auto carrier
BOTTOM: Semi-auto carrier

You will generally want to avoid the BCG style on the top, though they are pretty uncommon (maybe even extinct?) in new-manufacture carriers. The reason is that the firing pin is unprotected and rides over the hammer as the carrier moves backward, this is not so good for the longevity of the firing pin. When you hear people talk about a "fully shrouded firing pin," that is what they mean.

J_Rock
10-10-2009, 6:43 PM
No, they are definitely illegal.

Please send all your auto BCGs to me for immediate disposal.

reidnez
10-10-2009, 6:44 PM
No, they are definitely illegal.

Please send all your auto BCGs to me for immediate disposal.

Only if they are Colt, Noveske, or BCM.

DarkHorse
10-10-2009, 8:05 PM
Always wondered what "shrouded" meant. Thanks reidnez.

Cyclepath
10-10-2009, 8:13 PM
Run auto BCG on all my ARs.

missiontrails
10-10-2009, 9:29 PM
Yes, but even I quit doing that. It's all about the quality and setup of the bolt, not the carrier. Your carrier does not extract, it just gives the bolt a ride to do that. Although the gas key connection to the carrier is important.

Beelzy
10-11-2009, 8:00 AM
Why??

It's heavier at the back where the only good it does is slow the cycling that
ever so much. Plus you can't run a F/A carrier if the lower has a pinned sear
block. (Colt for example). JMHO

B Strong
10-11-2009, 12:15 PM
Can you use an auto BCG on a semi M4? Or you need to buy the semi? Differences?
Thanks

You're gtg with a full auto carrier.

Never use any M16 FCG parts in a semi-auto build though.

RECCE556
10-11-2009, 7:17 PM
Yes, but even I quit doing that. It's all about the quality and setup of the bolt, not the carrier. Your carrier does not extract, it just gives the bolt a ride to do that. Although the gas key connection to the carrier is important.
The quality of the carrier matters too. I've seen poor quality carriers eat gas rings. The other parts are also important. You have the best carrier and bolt just to have your firing pin break which makes a failed BCG (I just saw this happen in a carbine course). Don't skimp on anything.

missiontrails
10-11-2009, 7:24 PM
The quality of the carrier matters too. I've seen poor quality carriers eat gas rings. The other parts are also important. You have the best carrier and bolt just to have your firing pin break which makes a failed BCG (I just saw this happen in a carbine course). Don't skimp on anything.
True about quality, but S/A vs. F/A does not dictate quality.

RECCE556
10-11-2009, 8:08 PM
True about quality, but S/A vs. F/A does not dictate quality.
Absolutely...there are good and bad S/A carriers and F/A carriers. I was just commenting about your comment about how it's all about the quality of the bolt, not the carrier...basically, people should be buying quality everything. Anywhere you skimp, you will find fail. I see it happen all the time in carbine courses.

esskay
10-11-2009, 9:06 PM
BTW, one more thing to add on this topic. If you have an older Colt lower which has their goofy sear block in it, a FA bolt carrier will bind on it.

RECCE556
10-11-2009, 9:58 PM
BTW, one more thing to add on this topic. If you have an older Colt lower which has their goofy sear block in it, a FA bolt carrier will bind on it.
Good point! Yes, I had that issue. I just did it the "easy" way with a Dremel and cut down the protrusion on the block (as opposed to removing the whole block) and it's worked for thousands of rounds without a hiccup.