Right now, every single shot with each weapon consumes exactly one item of the weapon's respective ammo type.

The thing is, this means that high-capacity, low-damage weapons like the Pistol and Machine Pistol (for Regular Bullets) or the Rifle (for Adv. Bullets) are relatively wasteful. The Machine Pistol has twice the capacity of the AR, sure, but it also has less than a quarter of the damage per bullet! Put another way, it takes only 4 shots with the AR to kill an unarmored player, but 14 shots to kill the same unarmored player with the MP! Meanwhile, the pistol only takes 3 shots to kill an unarmored player.

Because of this, guns with lower damage are effectively a waste of ammunition--each bullet is literally worth less damage when fired out of the MP or AR than it is when fired out of the Pistol.

The solution, I think, is to borrow something from the way TechGuns works. In TechGuns, each ammunition item represents either a full mag or a pile of bullets, and different guns get a different number of shots from the same ammo item--the Thompson, for instance, gets 20 shots out of an SMG mag, but the MAC10 gets 32. In this way, different guns in TechGuns can use the same ammunition item, and have different capacity and damage values, and still be useful relative to each other.

To that end, I propose a new stat for guns: Ammo Efficiency. Each time you reload, you use X bullets from your inventory, and get X*AmmoEfficiency shots in the mag. This would ideally come with a new model for the Regular Bullets and Advanced Bullets, since those two are the only ammo types that would really be affected by this change. Most likely, there would also be a change in the recipes as well. Here's a table of the Ammo Efficiency stats I'd suggest:

Regular Bullets

This would make the Machine Pistol much more ammo-efficient, twice as ammo-efficient as the AR, which is itself more ammo-efficient than the Minigun and the Pistol. Essentially, the Machine Pistol would become an economy choice for rapid-fire damage, while the AR would remain popular as a middle-ground choice with good capacity and damage, and the Pistol and Minigun would both become high-DPS, low ammo-efficiency choices.

Advanced Bullets

The Heavy Rifle does not have twice the damage of the Rifle, but because of how low their capacities are, this is as good as it gets. That said, a thoroughly-enchanted Heavy Rifle can get up to a 10-round capacity and 20+ damage per shot, so we'll call it the cost of being S-tier.

The Rocket Launcher, Grenade Launcher, and Shotgun do not need changes, as they do not share their ammunition with any other gun. Further, the RL and GL are single-shot anyway (even though the GL is clearly cylinder-fed...).