This is a concept that is actualy useful for very limited circumstances, and is already solved. They use such ammunition for penetration of body armor which is standard for most modern soldiers. The smaller the diameter the less actualy foot pounds required to penetrate. So tiny bottle necked cartridges achieve this, but tend to in general have horrible terminal ballistics and stopping power.
This means they must arrive in large numbers to compensate, so even as they are hitting the target, the target can return fire. That makes it mediocre for suppression (which requires a bit more deterrent).
The quick solution rather than spending tons of money and creating new contracts for a limited use weapon? Do the same thing with a 12gauge firing flechetes. Buckshot is spherical and a poor penetrator of armor which makes it unsuitable for most general combat roles (other than killing civilians/insurgents.) If you figure around 20 flechettes per round, and an AA-12 fires 300 RPM that means you are getting 6000 rounds per minute of armor piercing firepower which can be delivered from 20 round drums. That is 400 rounds delivered at 6000 rounds per minute which already exists, requires no research and development, and can be fielded at minimal cost for the rare situations it is useful to have high rates of fire with ammunition with low terminal effects that can still penetrate soft body armor.
So something already exists that fires pathetic rounds suitable for the role at 6000 RPM with a 400 round capactiy. You just don't see it that way as the firearm is listed at 300 RPM yet can fire 20 projectiles with similar capabilites to the gun hose per round.
in regards to the AA-12, how about we take that one step further and make a belt fed 12guage. you could do it off of a M2 machine gun, shorten the distance the bolt travels, modify the links and change the barrel.
Something better already exists. It is the belt fed grenade launchers in standard use. You can simply use flechette rounds or shot and the thing acts like a giant shotgun far more powerful than a standard shotgun.
These could use the SCMITR flechettes previously designed with no change in logistics and achieve the "gun hose" objectives with no additional effort or costs required. Think 20MM-40MM shotgun. Sure beats a 12 gauge.
So bottom line is there is no use or practical purpose for the "gun hose" that cannot already be filled by gear that already exists and already is fielded.
Israel uses flechette rounds for urban use (often out of a tanks main gun), and the USA played with them in vietnam in artillery pieces etc.
Yet people often want better stopping power than the .223 is delivering and I personaly would want to have a 7.62 or better in combat. So switching to small diameter rounds with less knock down power than pistol rounds, and high velocity for added penetration of armor is a fad that is not battle proven IMHO and will pass. PS90 could stand for "piece of _____ 90", which is what you will think about it after seeing people returning fire after taking multiple hits from the thing. I would much rather have something like a G3K variant for a compact firearm that defeats low level body armor, which is what weapon platforms like the P90 and MP7 are designed for. To provide non frontline personel a way to fight against soldiers when necessary, and store and wield the firearm in cramped spaces like the inside of a tank. My point is the type of ammunition the gun hose concept would use is not very effective and more powerful compact firearms do a better job at the one role they currently fill.