>>Handgun rounds... have nowhere near enough force even to make someone stumble backward an inch...

You're aware the .45 ACP was designed specifically to knock opponents down? Relatively slow round (850fps out of a pistol, faster out of a longer barrel, 230 grain projectile, so slow/heavy - about like getting hit with a sledgehammer going 60MPH). It was needed during the Spanish American war because the then-current carbine round was just going through people - who weren't stopping.

As I understand it, the major issue with silencers (other than making a rifle long and awkward) is that they impede the flow of gasses needed to operate many automatic weapons (e.g. AR15) leading to that action jamming until it's been cleaned.

Oh, and that US Army credited image is of a prototype bullpup (from Sig?), designed to work with that particular silencing system. The weapon, rounds, and silencer are designed as a complete system, so the silencer can be 'tuned' for barrel length, propellant burn speed, and gas temp/pressure (matters for speed of sound) as well as gas volume - and the reload mechanism can accommodate the reflected gas volume (or operate on recoil).

Just my $0.02