I came in here because I know a fair bit about guns, but then you started talking about something completely different than what I know about.
Pretty much this...
Is he RPing or something?
He is creating a sci-fi universe for a story he is writing.
Huh.
To the OP: 8x69mm for an SMG is not very believable.
Additionally, both the AR and Sniper are using what is essentially a .50cal slug (the only difference between the two you mentioned are the cartridge length) which is pretty fucking ridiculous.
If they're decked out in some big kick ass armor, then it's believable. It is a Sci-Fi universe. And in WH40k Space Marines use 19.05mm projectiles for their standard rifles.
Yeah, but nothing in 40k is supposed to be practical or even remotely believable.
I can dream, okay?
Actually this would cut down on recoil as the round picks up speed as it travels and doesn't have to have all it's energy imparted to it instantaneously. Potentially allowing an unassisted man to fire larger rounds. Design was found lacking but who knows, future developments could see a reevaluation. Definitely not for close quarters though.
But face it, at this point we're just talking about babyrockets, why not go balls deep and hand the dudes full auto rpgs.
The rocket launchers are designed to take down tanks with self repairing armour. And are very bulky.
The EXPL rounds are laced with nano-explosives (Nanotechnology is
an excuse for me to do cool shit used a lot by humans to make stuff like self repairing armour, the aforementioned nano explosives and even some experiments as to a swarm weapon of millions of nanos. This technology is common and used extensively to also increase efficiency of organs and do all kinda of crazy SPARTAN style shit.
Tl;dr the bullets are just as accurate as cluster bombs as their regular counterpart.
Because some people here know more about guns than me