Hey as long as it goes "bang" every time one pulls the trigger instead of "click...oh dang it" like the M-16 and other variants of it's kind, that's all that matters during combat. When you have that much rounds (drum magazine) there's no need to aim like a sniper.
The Objective of a Drum is reduce reloads and give near LMG capacity.
The Problem is that by using an assault rifle as a LMG is that the Assault rifle is not designed for the added stresses of the repetitive firing in the same way. the Burst or Automatic mode of firing in a assault rifle is intended for occasional use and short seratic covering with the primary use being aimed fire. An LMG is intended for longer bursts of dedicated covering firing. every round fired wears the barrel and heats the gun and heating expands the barrel loosening accuracy.
Now in Theory a modern barrel has a life measured in tens of thousands of rounds but that's over the life of the barrel with time between shots too cool. But rapid long stretches can heat the weapon and cause warping and damage.
Proper LMG's feature barrels with ribbing and increased surface area to increase Radiated cooling they also tend to feature quick Change systems that allow a over heated barrel to be discarded in the field and replaced with a new barrel.
Assault rifles have fixed barrels to support bayonets, reduce weight and ease construction. They are intended to be used differently.
This is why I dislike QBB95, MG36, M27, L86 and the like although I do like the Ultramax. I prefer a proper LMG and if you need a Lighter weight LMG then you build something like the Stoner LMG
Anyway I think drum magazine are cumbersome anyway you put it. Here is a AR-15 version.
That's a bit subjective.
Beta C double drums have a bad habit of jamming on their own no matter the host weapon, and frankly it wouldn't take much for a AK drum to get troubled either. The size of the mag is also a issue. I mean the mags you are showing here are intended for 90 rounds and demand a complex snail shell shaped operation where the rounds have to to make there way around and up in a long movement that gives lots of possible spots for bunching.
Of course the other way around for this has it's own issues. I mean look at Surefire's High capacity magazines
I mean the magazine is protruding so far that fighting from the prone is almost impossible. And that is a Casket magazine.
but in a Bullpup you have a added issue which is that the mag well is in the rifle stock this means that aiming and moving the weapon places the drum in direct contact with the shooter, interfering with the grip and potential meaning that the magazine my be stressed by contact.
A conventional configuration means that only the support hand is interfered with although this can be alleviated by design. The design you posted places all the structure on one side demanding a large amount of space.
Of course it's not a total waste,Some double drums place it more in a center line allowing better spacing. And farther if you reduce the capacity like the Magpul D60 you also have a very manageable size and increased reliability if partnered with a system like the USMC's M27 it would offer a improvement.
However in the case of a shotgun Drum, we have a typically simplified operation. there need is based on the scale of the rounds as unlike Battle or Assault Rifle magazines which can be double stacked Shotgun shells demand single stack they also suffer as a magazine fitting 30 rounds in an Assault rifle would only fit 6 in a shotgun. meaning that a shooter using such with box magazines would need a large amount of bulky magazines. A Drum with 20 or 30 shells may be big but offers a lot of power.