Okay, so... BiTz says he can't do weapon attachments because it would simply be too much work to make all the necessary graphics. I can understand that. But... Maybe...
... We could be able to do stat upgrades in a different way. By changing and adding in internal parts! Basically, every gun would have a certain amount of slots. Expensive guns, like BAR, might only have one or two. But, a gun like handgun or starter would have 5 or so slots. In each slot, you can add an internal part that can modify the gun's stats! Every internal can be used in every gun. They behave mostly the same in different guns. Only one of an internal may be used in the same gun.
Some examples
Adv. Rifling- +1 damage.
Low power sights- implies the use of less practical sights or scope. More spread.
Low-pull trigger- less sleep.
Incendiary Rounds- -1 damage, deals 3 damage over 1.5 sec. Every .5 sec.
Light Metal- less freeze.