*IF* you are being 'Futuristic' then consider some of the current weapons in production for testing...
Rounds that penetrate, then explode into shrapnel, that can be nearly instantly programmed via the ranging optics to explode over/behind targets or after impact with a door or wall...
Rounds that self correct in flight, artillery that can be fired up to a mile off target and GPS guide themselves to the target coordinates.
Some current military weapons systems can be fired at high angle (more flight time) and barrel declined, fired again, ect, and can deliver several rounds on target at the SAME TIME.
Reducing angle reduces flight time, so rounds impact at the same time.
Some long range, and I do mean long range, like two miles, use a computer controlled weapons mount with Doppler radar to figure wind, air density, ect to have first round hits...
Right now, its a weapons mount that takes a pickup truck to haul around, but you know the DDP (Defense Department Procurement, or as we called them, Death & Distruction People) they won't stop until its man portable...
Since its fiction/'Future', you can do anything you want...
I keep telling people the best 'Zombie Gun' is your fingers,
Just go 'Bang, Bang, Bang'...
If you want a 'Silencer' then go 'Pssst'...
Since zombies are fictional, your fictional gun never needs ammo, never misses, and is always with you!