so, i have been reading some about full auto bcg. what have the DIY'ers been using?
are the "slowness" of the heavier full bcg providing more fps or ft-lbs out of the rounds?

People say that having a heavier carrier produces a smoother action, because the heavier carrier means that it will go backward slower.  The "heavier" part is only a few grams, perhaps half an ounce at most.  That really doesn't do much in terms of slowing down the carrier, but some people still swear by it.  I'm very doubtful about how useful it is.

This can have absolutely NO affect on the velocity your ammunition produces.  None, zip, no.

I've got some of both, I don't really notice the difference in between a carbine length with either a semi or full auto BCG.  I have a full auto BCG in a rifle length gas system and it functions great.

FA carriers are suppose to keep the casing in battery longer than the SA, but on a rifle with a longer gas system, it probably doesn't make much difference. On a pistol a FA carrier probably has more impact because of the short gas system. Using an adjustable gas block can negate the need for a heavier FA carrier and buffers.

As noted, on a short (midlength, carbine, pistol) gas system a full auto bolt carrier is seen as a free (FA bolt carriers typically cost the same as semiauto bolt carriers) way to add a little mass to help the effort to slow / delay extraction until the pressure level drops a little.  This has to do with the timing of the system: if the bolt starts to extract while the pressure is significantly higher than (the original rifle) design and the case is still gripping the chamber wall, it can lead to malfunctions (like a torn rim or the extractor jumping over the rim an failing to extract) and premature wear.  I'm comfortable with the theory, but I wouldn't bother to spend the money to buy a FA carrier to replace a  SA carrier if that's all I had as I doubt that there is little practical difference.

For some (many?), the appeal of the FA carrier is that it's closer to what is "correct" for an AR.  Remember, the SA carrier (and its variants) only exist because "back in the day" certain powers that be thought it would be a good idea to make certain parts incompatible with other full auto parts.  The SA carriers were never designed to actually be an improvement over the FA carrier, only to render "civilian" ARs incompatible with military FA parts.

ETA:  A heavier bolt carrier does not have an effect on the velocity of the bullet because there is no significant venting of gas into the carrier or upper receiver.

To put it another way, there's no reason to select a SA carrier over a FA carrier.  I don't know why some companies (LMT and Rock River Arms, for example) continue to ship their rifles with SA carriers as there is no technical merit to the SA carrier.

from an article i read about the two, the SA was a push (a technical solution) to keep full auto away from the public. the FA bcg was/is not under NFA. so from technical side, FA is more correct, and, would allow you to get the item stamped and use 3rnd burst trigger group.

but i also thought the FA was slower to move, thus a minimal delay in extraction. doest slower extraction mean more case time in the chamber, thus allowing more psi down the barrel (perhaps not even measurable, but on paper seems to be right).

Yup. There is a sligh felt difference in the recoil impulse. Especially in a light carbine like a Colt 6520. Swapping carriers back and forth it can be barely felt that the M16 BC produces a smoother recoil than a lighter half circle AR carrier. The difference can be quantified in split times though.

I'll agree on the "semi" carrier being a "solution" to prevent accidental full auto use, or to make it harder to convert a Title I rifle to an illegal Title II rifle.  However, since 1986 you CANNOT get a new (or newly converted) machine gun registered (stamped), so no, you can't "get the item stamped and use a 3rnd trigger group."

Considering the very small amount of mass difference, as I said, it's not likely the use of an "un-cut" carrier would have a measurable effect on chamber time.  By the time the bolt is unlocked, the bullet has been out of the barrel for a while and barrel (and chamber) pressure is rapidly going toward zero.  What it MIGHT do is act as an added buffer weight, which will slow down the carrier as it moves backward - just enough to "soften" the subjective experience of the overall rifle's recoil.

yeah, 3rnd is off the table.

No, the bullet has already left the barrel before the gas starts to unlock the bolt.  If it wasn't, the pressure would still be too high to extract the case, and the case couldn't hold that much pressure, unsupported by the chamber.  

A full auto BCG can't increase the bullet's speed or energy.

doesnt it start as soon as the bullet passes the gas port? i suspect a delay in bc movement due to inertia, and by that time the bullet is gone. that said, anyone have data or links that show the timing of bc movement after the rnd fires?

Here is an old Army video that shows the sequence of operation:  https://www.youtube.com/watch?v=NtwhZj1_TlI

The part you want to see starts at 10:18.

The carrier's cylinder begins to be pressurized as the bullet passes the gas port.  It takes long enough for the cylinder to begin moving the carrier backward that the bullet has passed the muzzle before the carrier's cam slot has started touching the cam pin.  The carrier is essentially pushed backward by a small amount of HIGHLY pressurized gas, with inertia taking the carrier all the way back.

Watch the video linked above and you'll see how it all relates.