whats the deal with "full auto" bolt carrier groups?

M16 bolt carriers are perfectly legal to own and use in an AR; They don't make the gun full auto.

They have a little more mass, which lends to enhanced reliability. That's why they're more desireable.

Now, if you had the BCG, the FA selector, FA FCG and a receiver that can accept them, you are looking at constructive possession

The M16 or "full auto" BCG doesn't make it a full auto rifle and is not illegal. I think the main difference is the weight of the completed bolt and it's affect on bolt speed.

A heavy "Full Auto" carrier has several advantages,
1st the mass of the carrier keeps the weapon more stable during bolt cycle, the gun is less bouncy and stays on target.
2nd the fully shrouded firing pin tunnel prevents match type low mass hammers from catching in the firing pin slot which induces a malfunction of epic proportions.
3rd more metal in the carrier= less chance of an early part failure.
I have never seen a full auto type bolt carrier crack or break at the side wall, only seen them shear off the gas tube port assembly. I have seen semi auto carriers that have cracked or broke at the sides and I admit that these failures happened when the guns blew up due to faulty reloaded ammunition.

I have also seen several actual M16A1 rifles that blew up for varios reasons yet the carriers remained intact.

What's the price difference between the full auto bolt carrier and the semi auto version? Since everyone is saying the full auto is better is the higher price the reason why it isn't used in all ARs?

Here is a photo of the different bolts... Hope it helps

Back in the 80's it was still common to find all kinds of M16 surplus parts, including carriers, in mix & match semi auto AR:s. Nowadays the vast majority of parts in commercial rifles is dedicated semi auto spec, probably to avoid any kind of legal issues.

Now, if you had the BCG, the FA selector, FA FCG and a receiver that can accept them, you are looking at constructive possession

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The only full auto part that needs milling/ drilling of the receiver is the autosear or just milling for the DIAS.

You can even use a lightning link in most ar receivers unmodified, if you have the sp1 carrier.


The "can accept them" part of constructive possession could make it illegal for you to own a gun and string.

The weight difference between a SA and FA bolt carrier is 11 grams. That's a very small amount of reciprocating weight, .35oz to be specific. The difference between a standard carbine buffer and H buffer is .8oz.

The FA carrier is not stronger than the SA carrier. The extra steel in located at the tail of the carrier and bolt carriers fail at the firing pin retainer hole. That area is the same in both SA and FA carriers.

The SA vs FA carrier is way over hyped by the net IMHO. If you are going to be buying a new carrier, sure, get a FA mil-spec carrier. I would not waste money just to replace a working SA carrier.

The important part about carriers is they should be made for 8620 steel, correctly shot peened, heat treated and chrome lined.

The only full auto part that needs milling/ drilling of the receiver is the autosear or just milling for the DIAS.

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That was my point. If you have all the parts and a receiver that's been milled to take the auto sear, constructive possession is a slam-dunk.

I personally wouldn't want to be caught with an unmodified receiver and all the parts, because I have a vertical mill. I'm sure it would be an easy case to prosecute.

The "can accept them" part of constructive possession could make it illegal for you to own a gun and string.

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Well, there was the shoe string machine gun thing awhile back.

Part of them prosecuting a constructive possession case hangs on intent. Just because you own a shotgun and a metal cutting tool doesn't mean that you intend to make a short barreled shotgun, nor does owning a lathe and a chunk of round stock mean that you intend to construct a suppressor. But having all the necessary parts to assemble an NFA weapon when those parts would serve no other purpose can easily constitute constructive possession. It's not adviseable to have only a title I 870 express and a 12" barrel, have only an AR rifle and a < 16" upper, or to have all the parts necessary to make any gun you own FA.

You can even use a lightning link in most ar receivers unmodified, if you have the sp1 carrier.

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AFAIK, you needn't even own the rifle if you get caught with an unregistered LL, since that is the machine gun. I don't think the LL has the same legacy as the few unregistered DIAS that are legal to own so long as you don't also possess the rifle to use it.

I would not waste money just to replace a working SA carrier.

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This. I only build new guns with M16 BCGs, but I'm not about to drop $140 to switch out the perfectlly good enhanced SA BCG in my Armalite M15.

'The weight difference beyween an FA and a SA bolt carrier is 11 grams, or .25 oz'
That isn't correct. 11 grams is equal to .388 oz and is also dependent on exactly what type of semi auto bolt carrier is being referred to.
A fully machined Colt type SA bolt carrier is closer to 2 ounces lighter than a full auto bolt carrier.

"Bolt carriers fail at the firing pin retainer hole"
That isn't entirely correct.
Bolt carriers fail at a number of locations including the bolt cam pin hole and the side walls.

"The important part is that bolt carriers should be made from 8620 steel"
While 8620 is the "Mil-Spec" standard again this statement is not entirely correct.
A number of manufacturers produce bolt carriers from steel mixes that actually exceed and outperform the old mil standard 8620 steel.

MachIVshooter said:

... I only build new guns with M16 BCGs, but I'm not about to drop $140 to switch out the perfectlly good enhanced SA BCG in my Armalite M15.

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Funny story, a couple years ago I did just that and bought a Daniel Defense BCG to put in my ArmaLite M15A4C figuring to keep the ArmaLite as a spare. Well a few months later I found a Black Friday special on a DD upper minus BCG so they got married and the ArmaLite got its BCG back.

Moral of the story, spare BCGs tend to morph into rifles!

Moral of the story, spare BCGs tend to morph into rifles!

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LOL. Yeah, so do barrels, receivers, stocks and even boxes of ammo for which you have no firearm so chambered!

After all, gotta make use of those "spare parts"!

1st the mass of the carrier keeps the weapon more stable during bolt cycle, the gun is less bouncy and stays on target.

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More reciprocating mass makes an AR pattern rifle move more during at least 2 of the momentum events that take place during cycling.

The last couple Colts I bought (AR6720 & LE6920) came from the factory with FA BCGs.

Onmilo said:

'The weight difference beyween an FA and a SA bolt carrier is 11 grams, or .25 oz'
That isn't correct. 11 grams is equal to .388 oz and is also dependent on exactly what type of semi auto bolt carrier is being referred to.
A fully machined Colt type SA bolt carrier is closer to 2 ounces lighter than a full auto bolt carrier.

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Correct .35, typo on my part. How long ago did Colt stop production of their SA carrier? 15-20 years?

"Bolt carriers fail at the firing pin retainer hole"
That isn't entirely correct.
Bolt carriers fail at a number of locations including the bolt cam pin hole and the side walls.

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Did I say that's the only possible point of failure?:banghead:

The most common carrier failure is cracking at the firing pin retainer hole, you well see wear at the cam pin hole, it would be unusual to see the carrier crack there, and even more unusual to see cracks develop on the side walls. A carrier can fail in multiple areas if exposed to a over pressure.

"The important part is that bolt carriers should be made from 8620 steel"
While 8620 is the "Mil-Spec" standard again this statement is not entirely correct.
A number of manufacturers produce bolt carriers from steel mixes that actually exceed and outperform the old mil standard 8620 steel.

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Documentation? The majority of carriers including carriers in the so called super bcg's are 8620 based.

Zak said:

Quote:
1st the mass of the carrier keeps the weapon more stable during bolt cycle, the gun is less bouncy and stays on target.

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More reciprocating mass makes an AR pattern rifle move more during at least 2 of the momentum events that take place during cycling.

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Precisely. If your main objective is to reduce movement then look at the JP low mass system, they work.

http://www.jprifles.com/1.4.7_bc.php

9130 bolts are harder, containing more nickel than 8620. I wonder if that translates into more brittle?

I can say with full honesty, I've not experienced nor personally known anyone whose reported a fissure or failure of any kind of an AR BCG other than firing pin retainer hole. Only once, and never has it happened to me. The bolt that experienced failure belonged to a factory Bushmaster with high round count.

The only "super bolts" that I know of are 9130. 8620 is up there with 158, nothing to sneeze at.

I was told it added a little mass which is supposed to slow down the ejection cycle when using short gas systems. YMMV.