Over time, possibly per x time, ammo will appear, the stat could mean how many per x mins, say 5% = 5 per x.
Ammo Pick up:
You can pick up more ammo from the floor with this perk, possibly at a max of x2, requiring less ammo to be picked up and more reserve.
You can carry more reserve ammo for the gun this perk is on ( Or over all ammo if its shared ), either this could be used with the pick up, or make it impossible to get 2 stats on one skin.
Essentially, guns with have either less sound come out them, a muffled sound, or the dumbest one, the silenced pistol noise. ( Only possible in certain guns )
Health on kill:
Either on damage, or on kill you can receive a certain amount of health apon doing so, and varies according the percentage you have.
As a replacement or as well as the M1, shotguns can shoot one pellet, with more range and accuracy but less damage, and is only applyable to shotguns without increased bullets, and only the xm. Edit: or as a new stat, but you can only get increased bullets or this.
This probably should be on a hat, but per minute, the percentage on it is regenerated, but should be at a max of around 5, this shouldn’t be too over powered since it would have to replace something, like speed making you vulnerable but a tank, or health making you fast but weak.