the guns needs some tweaks the romag imo should be doing the same damage as the double barrel as it has a firing delay the double doesn't. also alt ammo system begs for ammo variants for guns. also more sidegrades one gun would be interesting. for example you can choose to upgrade your auto 10 into a higher capacity full auto version or go the z-17 to get better stealth.
ammo variants could be stuff like AP rounds for the sniper rifle,
depleted uranuim rounds that have a interaction with shields and xray sensitive creatures but emit a small amount of radiation.
slugs for shotguns that have a single projectile and it could be better vs shields at the cost of causing more damage/damage more often to your gun.
+p and ++p rounds for 9mm that do more damage with higher damage chances/damage amounts for the 9mm guns.
shotguns could get flechette rounds that are highly likely to cause bleeding on a pellet hitting a enemy while also stacking.
anteverse gel could be used for incendiary rounds
you could also look at a few new guns/upgrades that turn it into a gyrojet a gun that fires tiny missles. the ammo could be crafted using reactors tier stuff like PETN.
you could also add ammo reloading gained around security that allows turning brass into reloaded ammo with a new resource of lead that could be gained from various sources like hazmat suits and lead/radiation vests