My gun wont allow anymore upgrades but there is still room to upgrade what do I do?

You've probably reached the limit of upgrades you can buy for the gun with the golden coins you've earned in the Spec Ops, Primary Missions etc.

You will need blue "battle" tokens to add any more upgrades (which will just add to the weapon damage, whether it's the muzzle, ammo etc.).

You get the blue tokens by playing in the pvp arena, where you have to use a sniper rifle.

My advice would be not to waste the blue tokebs on sniper rifles for cities/levels 1-20, and put them towards upgrading the DOA Hunter rifle after you reach and finish Rohz - Eh Tow (unless you have paid for Premium and have other weapons) - this is because you have to eventually purchase a sniper rifle every level to complete it - extra weapon damage won't help when you can't add the extra range needed on the next/higher levels.

But you can use the blue battle tokens to upgrade the weapon damage on your assault rifles (or shotguns) - you can often use an upgraded assault rifle a couple of tiers/levels above your current position so you don't have to buy a new assault weapon every level like the sniper rifles.