How can you tell the gun type on top of the jeeps in Far Cry 2?

Solution 1:

I think this is how it works:

If you perform all buddy missions in the north, the trucks that spawn outside your safehouses come with a M2 .50 Cal rather than a regular M-249 SAW. (The M2 is probably what you're noticing.)

If you perform all buddy missions in the south, they come with a MK-19 grenade launcher.

I'm not sure there's any way you can reliably find a car with an M2 "in the wild".

Solution 2:

I don't think it has to do with your progress but you have to unlock the south area first i think then If it has a long barrel that is The M2.50 cal if the barrel is short that is the M-249 Saw (which can be purchased at gun shops. If the gun has a large magazine box (the green thing which houses the ammunition) that is the Mk-19 Grenade launcher. And you can not guarantee that it will be the one you choose to have.