I'll second the recommendations for Nuts! and .45 Adventure, both great in different ways.
Nuts! is more realistic, when people are shot they either fall down screaming or die, people spending most of the time ducked in cover or sneaking about trying to find good places to give supporting fire from. Works well with 10 to 25ish models per side.
.45 Adventure has far more detail on the characters, people can be shot dozens of times and keep going by taking arm or leg wounds, good rules for traps and other non-combat things. Works well with 3 to 6 models per side, 12 or more if using the "Hordes of Mooks" rules available on the website.
Also good for this kinda thing is "Legends of the Old West" from Warhammer Historical, it's a nice system and include most of the weapons you'll need, treat a burst of SMG fire as Fanning a Sixgun, when people are shot they usually end up running for cover. Good advance scheme and works well with about 10 models a side. A balance between Nuts! and .45.