Advantage of full-auto BCG?  Weight or just bragging rights?  My ARs have semi carriers, but my spare is FA - will it drop in, or do I have to worry about spring/buffer weights?  Thx:

additional weight to help with a reliability (additional weight helps delay the unlocking time).

Yes it will just drop in (as long as the bolt is new)

no need to change spring/buffer weight unless you are having reliability issues.

If it ain't broke, don't fix it...

The weight difference is fairly small.  Over all it really doesn't matter unless you want to use it with an RDIAS/registered lower someday. (Or take "Will Not Comply" literally and do what you want)



Personally I run almost all FA carriers.  I've got a buddy who's an SOT and I've shot some of my uppers on his machine guns.  Someday I'd like to buy a RR, so if I ever get the money together I want everything to work with it.

       
 

I bought an inexpensive M16 BCG as a spare for my two mid-90's ARs on Black Friday from PSA.  I built both of my ARs 20 years ago from 'mil-spec' (FN) parts kits, and both have all SA parts - there were only a couple of AR parts suppliers back then, and they all went with the theory that ANY FA parts in a SA rifle was an NFA violation.  Of course, we now know that isn't true, as it seems that almost all of the aftermarket BCGs are FA these days.

similar thread a few days ago.  I weighed mine then on an electronic scale.  If I remember right there was a quarter ounce difference in weight.  

There are different styles of semi-auto bolt carriers and the weight will vary, depending on which style of bolt carrier it is.  All semi-auto bolt carriers have reduced length full-circle diameter at the rear (or completely removed in some cases) and do not have the ability to trip the auto sear (that is installed in a full auto lower receiver).  Some semi-auto bolt carriers have shruided firing pins, and some have unshrouded firing pins.

Unshrouded semi-auto bolt carriers have the bottom of the bolt carrier machined away so the hammer end of the firing pin is exposed.  This was meant to be an extra safety feature by Colt, as the step in the bottom of the bolt carrier on the unshrouded bolt carrier is supposed to "catch" the hammer in the case of a malfunction where the hammer is not cocked, and tries to follow the carrier (and fire the gun out of battery).

In truth, what happens is the hammer is cocked by contact with the firing pin instead of the bolt carrier, and the side force can cause damage to the firing pin and the firing pin retainer pin.  Changing to a full-auto bolt carrier is a upgrade in reliability for this reason.

The slightly higher weight of a auto bolt carrier will also delay unlocking, which can help smooth functioning, and reduce perceived recoil.

Changing to a auto bolt carrier from a semi-auto bolt carrier is "plug and play" with no down side.

the difference is nil.....just stick with a reputable company and move on with your life......not all BCG's are created equal, but there are several that will serve you well for years!

Not worried about it - just curious.

Back in the 90's the ATF considered the installation of one or all of a number of parts used in full auto guns to be the assembly of an illegal machine gun when installed in a semi-auto gun, even if the gun did not fire full-auto.  Later, (I think in the 2000s), Colt sought a change of status of the full auto bolt carrier from the ATF, and the ATF agreed that the installation of a full auto bolt carrier in a semi-auto gun would not be illegal or be considered the manufacture of an illegal machine gun.

There's a copy of the Determination Letter to Colt from the ATF somewhere here on Calguns.

I built my Bushmaster ARs in the mid-90s and was forced to use semi-auto bolt carriers at the time.  I swapped to PSA logo bolt carriers around three years ago.  No change in function other than a slight zero change, and no more bent firing pin retainer pins.

I got you!

There are very few serious questions that are stupid. This would not qualify as one that is. Most people on the site are very helpful and enjoy doing so.

I wouldn't bother. You'll never know the differences. I bet there are still more SA carriers out there, than FA.

If buying new, I would want FA carriers, but I wouldn't bother replacing a working SA carrier that I currently owned.

How did his zero change when he replaced the semi carrier with a FA BCG?

I would guess that by delaying the unlock time that it changed the node at the time of bullet exit from the muzzle. (changed barrel harmonics)

The guns that I changed BCGs on are both 95 Bushmasters.  Both with heavy barrels (one 16" and one 20").  Both are very accurate (will both five shots groups under 3/4" at 100 yards off the bench).  My brother's similar age Bushmaster 16" carbine can group five-ten shots under a dime (I've seen my little brother do this several times).

Changing BCGs from Bushmaster semi-auto to PSA premium logo auto changed point of impact slightly.  Not a lot, but enough to notice.  I suspect that it as the previous Poster wrote: that it has something to do with changing/delaying the unlock time, and changed barrel harmonics.  I don't know exactly why, I can only tell you what I observed.

These was a noticeable difference in the weight of the bolt carriers with the auto bolt carriers weighing more.  I also don't know if it makes a difference, but the gas relief holes in the PSA bolt carriers are also significantly larger than the gas relief holes in the Bushmaster bolt carriers.