In the last part of the Metro on the way to the alien spore thingy, I found 2 Hammer pistols shortly before the wagon where 3 of the alien lice thingies are picking up organic material (a little further down you observe a train crash). When picking up one of the Hammer guns, the amount of ammo in the gun was shown as -9 and -32, both numbers in red (the second gun was immediately auto-picked because of the ammo). When getting some ammo in a crate a little bit further down the numbers changed to 9 in the gun and 0 in reserve. Emptying the gun resulted in no ammo in gun and still no ammo in reserve.

I bought the game on Steam but suppose that shouldn't make a difference.

Yes. Happened to me with the SCAR, which really svcked. I kept playing, and after I got out of the Hive the problem went away

lol, negative ammo.

Someone on the inCrysis forums have also had this bug with the Scar.

this is what happens when Hargreeve divides by zero

i once had A glitch, it said the ammo was unlimted/unlimited. god i wanted that to work, sooo badly... but it just said ammo empty.. happend with the SCAR and SCARAB with me, god unlimted ammo..
Ceph"argh hes shooting us"
40 shotss later
alcatraz*bluff* i need to reload
Ceph, mwahahahah! attack
BRRRRT. dead.

ive also ran into problems starting the game into a checkpoint and none of my jaws will fire even though i got 4 in my loadout it wont relaod. an im stuck running and hiding in cloak throwing 6 bricks of C4 at the giant ceph bi pod. total momentum slower in the game