Well, first, it appears that as a part of a potential M4/16 replacement, the army is once again looking at the 25mm smart grenade launcher, now called the XM25 Individual Air Burst Weapon, which grew from the abortive Objective Individual Combat Weapon (OICW).

Incorporating this into a standard carbine or rifle is, I thin, a losing proposition. You add weight and complexity when only one person on the fire team really needs to have the weapon under most scenarios.

As such I favor a solution analogous to the M203, which can be installed on the weapon as needed.

There are two issues with the M4 and one with the M-16, they both require a lot of care and cleaning, and the M4 has lethality issues beyond about 50 or so meters.

As to the former, much of the issue is the gas tube system, in which the gas which it tapped to drive the bolt directly, and there is a drop in kit, basically a new upper receiver assembly, which adds a piston, the H&K 416.

While people talk about going with a heavier round, the real problem with lethality with the M4 is that its barrel, at 14-1/2 inches is just too short, and so velocity, and hence energy, suffers. Seeing as how bullpup rifles are shorter with a barrel typically about 4 inches longer, that solves the problem.

As an interim solution, going with the upper receiver replacement makes sense, while one looks at something more extensive.

BTW, good article on the H&K G11, which used a 4.85mm caseless round, which would have been a world beater.

My understanding is that they fixed the cookoff issues, but the rounds, and the weapon, were too fracking expensive.

Here is some gun pr0n, with the pictures from here: