Weapon, but most of the stats stay the same for specific ammo.
For example, 9mm ammo has the same stats for most of the weapons that could use it.
M4A3 pistol, MP5 submachine gun have:
10/L
10 stands for damage, L stands for (low) penetration capability. The difference is that MP5 shots in bursts, which might be useful to quickly dispose those bugs that were stupid enough to come near Marine.
FN SCAR rifle has:
10/M
Which may seem the same as 10/L. But in fact M stands for medium penetration capability (I think :D ), and this translates to better handling when used against armored targets.
Also, each weapon has its own accuracy (hidden stat ;) ), which (along with skills, pain, etc) affects penetration capability and damage delivered.