LadyMuck
Striker
I've not found *that* much difference in the guns so far and I'm pretty far in. Still like collecting them though.
Find the intel and select the gun crates option and it will show you where they are for pickup on the map. You also get special guns for taking out each boss, but you cannot mod these ones (which is a pain if you like to be sneaky as you cannot put a silencer on them (or can you???)).
How far you through so far? I've only redded out 8 people. Nearly 9.