If I may -
Regarding DavidAR10's excellent historical documentation of a discarded solution.
Having made the transition from "tanker" to "leg", I can tell you that mechanized logistics are exactly the opposite of LPC packing. I learned to hump a ruck as a dependent, unaware of human limitations. Later, I was always adding batteries and gadgets, as well as poggie bait to my load. Somehow, I was never around when the load weights were checked!
The mech guys load it if they think it might be useful. Better to have it and not need it... They don't check weights. Fully tracked vehicles are not effected by a few extra pounds.
So, if fed with a 90 round drum, the higher firing rate might be useful. The Bradley's were necessary, because there was no way an M-113 would keep up with an Abrams. Our understanding of Soviet tactics, in the 1980's, was that infantry would closely follow armor. That is: close enough to run a Sagger missile system. Therefore, a situation where a task force breaks through the "hard outer shell" of the "godless commie hoardes" and is set upon by masses of infantry was to be expected. The Soviet philosophy was to overwhelm with numbers. Hense, the firing ports.
The M321 is an interesting device. Alas, pitched battles are not expected any time soon. Nor has time occurred to fully explore the intended function - fortunately. The wisdom behind such solutions should be maintained, if only for intelligence value.
IMHO, and from the pics, the M321 does have a buffer, of sorts. It is on the opposite end of the springs. It also fires from the open bolt. Engineering such a solution into an existing outer shell is something that could only come out the office building with the extra side on the Patomac. The factors involved remove the M321 from a discussion of "assault rifles".
Back to the thread, leaving the buffer out is crazy. Folks here have addressed feed problem solutions. Messing with the rate of fire is not to engaged in lightly. Those of you who do need wisdom and luck. Thanks for taking the risk!