Are there any advantages/dis-advantages to running a
Full-Auto BCG in a Semi-Auto AR?

I don't think there is a difference. If I'm not mistaken the FA is in the fire control group not the bolt

The FA carrier has more mass, that's about it, plus these days I think they are more common.

As previously posted, they have a little more mass. Your carrier has a shrouded firing pin, so the only benefit is a little extra mass. Basically a non-issue with the S&W bolt carrier.

More mass = decreases the cyclic rate = one more variable to muck around with to achieve the holy grail of ejection patterns.

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-John

There are NO advantages to doing so.

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M&P's and K Frames Pleeze!

Yes and No. The FA is in both. You must have the FA bolt, as well as an M16 trigger, hammer and sear. Possession of an M16 trigger, hammer, sear, or any one or all three of these items is illegal unless you have a registered machine gun.

The F/A M16 bolt, by itself, is NOT illegal to possess. That extra 2 ounces of mass will make absolutely no difference in a stock AR-15. It truly is money better spent elsewhere. Now...

If you run an SBR, you most likely will need a FA bolt AND a heavier buffer. These short barrel rifles are overgassed, and the combination of the FA bolt and heavy buffer slows the cycle back down to a point of reliability (or you could get an adjustable gas block). In my suppressed 11.5's I run BCM bolts and BCM H2 buffers. In my 8" Suppressed Spikes SBR I use Spikes FA Bolt and H3 buffer.

Full auto bolt by itself in a stock 16" AR-15 is meaningless...

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M&P's and K Frames Pleeze!

Nope. The full auto is in both the trigger/sear AND the way in which the lower is machined to accept the full auto trigger/sear. A full-auto AR-15 can be run with a semi-auto BCG, but it's not recommended because of the increased cyclic rate.

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-John

I replaced a semi-auto BCG with a FA BCG, and replaced the buffer with a H buffer. Ejection changed from 2 O'Clock to 4 O'Clock.

Wrong! check your facts before you post:

"If you notice, the profile of the AR-15 bolt carrier isn’t completely cylindrical. There’s a cutout on the bottom, and a large chunk where there is no material on the top or bottom. That’s by design, both cutting down on the weight of the bolt carrier and allowing the hammer to properly impact the rear of the firing pin when locked in place. But the lug on the bottom of the full-auto bolt plays an important role in full auto fire. When the bolt is finally locked firmly in place that lug will contact the sear release in the full auto mechanism and automatically fire the gun. It’s a pretty nifty solution to an interesting problem."

That's why it's called a full-auto bolt carrier. A semi-auto bolt carrier will NOT trip an auto sear.

A quick search of the topic yielded a variety of results, all saying the same thing. This paragraph was taken from thetruthaboutguns.com

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M&P's and K Frames Pleeze!

I wonder why I even come here anymore.

I'm done. I'm out.

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-John

VI know. I just pop in here every once in a while now. For the most part I've gone back to the the revolver forums.

IIRC, the Buffer was re engineered after the M16's initial use in Vietnam. The purpose was to increase the mass and help lower the cyclic rate. This was to compensate for ammo containing powder with different burning rate than the original design. With the higher rate, and other problems, the M16 tended to stick cases in the chamber rendering the gun useless. (Edited to correct inaccuracy)

No the bolt carrier was not changed to lower cyclic rate, buffer was modified to do this. As stated above the bottom rear of the bolt carrier is what trips the auto sear during full auto. FA parts required in the lower are hammer, trigger, safety, disconnecter and autosear plus extra hole and pin.

Mack, my apologies. You are correct. It was late and I did not "RC". The buffer was changed to reduce the rate of fire with ball powder back closer to the design rate of 850.

The way the sear operates in FA seams petty simple but how did they get the 3 shot burst mode? That was my normal carry mode unless I need just one good shot.

Don

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USN Retired/VN VET
M&P X5

I hope you'll reconsider. I always appreciate the way you share your knowledge with new AR shooters and I look for your posts. No condescension and lots of patience.

It would be a shame if a few rude remarks eff'd that up.

All the best.

Same reason many of us gave up and moved on and don't post like we used to. I've been hands-on around the AR platform since 1969 (been around and handling/shooting weapons since the late 1950's). Like many, I got tired of the internet experts, Mall Ninja's, Trolls and what they've read over actual hands on experience. My biggest thrill is on the range when some one complains about there "junk" weapon has problems and can't shoot a decent group. 99% of the time it's maintenance right out of the box (reading the manual would be a good start), proper shooting techniques (like breathing, trigger control) and shooting cheap junk ammo. They buy a Corvette, never drove a clutch and use the cheapest fuel. Then complain it's a piece of junk because it won't race with the best.

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THIS WE'LL DEFEND ΜΟΛΩΝ ΛΑΒΕ

The 3 shot burst is done with a modified trigger and dis-connector. It has a geared wheel and ratchet setup that "counts the shots". If you fire 2 of the 3 shots when in burst mode, then squeeze and hold the trigger again expecting 3 shots, you only get one shot. Reset the trigger again and pull and hold, you will then get 3.

The 3 shot system sounds like a less than desirable setup, IMHO. Either semi only, or semi-full is a better solution.

No rude remarks at all, unless you count him "forcefully" telling me I was wrong when in fact I was right on.

If you're gonna get overly sensitive when you get corrected, well, don't let the door hit ya...you know the rest.

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M&P's and K Frames Pleeze!

Yeah, maybe contentious remarks would have been a better choice of words. I haven't been on this site long, but always considered it a pretty friendly place. (Certainly more so than some others a lot of others. The interwebz requires a thick skin.)

Interesting Thread.

A quick internet search revealed many illustrations of the marked differences in the BCG's.

The internet is a great place to get info as long as you don't believe everything you read or see. It used to be "I will believe it when I see it" Photo Shop put a end to that. Then along came video editing so you can't even believe You Tube.

Don

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USN Retired/VN VET
M&P X5

Off hand, I can think of 5 different variations of the BCG, and the number is likely double that.

I do prefer the original design, and will stick with the full auto version for the increased mass and tamer recoil pulse.

Did this on my sport with the same exact result. Felt recoil is less as well, just a little but less.

Many big name civilian AR makers (Smith and Wesson) will over gase their rifles (larger gas port) to make sure they shoot everything, like weak powered steal cased ammo. It is less calls they will get. A 16inch carbine will be even more over gassed because of the longer dwell time.

A full auto BCG is only .5 ounces heavier than a SA. I weighed my Sport SA BCG and my new BCM FA BCG. Adding a H buffer 3.8 ounces by replacing the carbine buffer 2.9 ounces adds a tad more weight as well. Over all the combo is less than two ounces but it tames the recoil a bit on over gassed AR's.

I personally would not go higher than a H buffer and while I do change out the spring with a chrome silicon based spring (for much longer life) I dont change the spring tension. Too heavy of a buffer and or spring will possibly cause FTE or FTL problems.