I was a Mk-19 gunner in 2ACR in Bosnia.
We used the .50 ammo cans for keeping weapon cleaning supplies for the pistols, M-4 carbine and M-16. I painted the cover on mine so one could easily see it wasnt for ammo.
I had one 32 round, 40mm box used for my gunners box. This contained cleaning supplies for the Mk-19, an extra T&E mechanism, extra pins for the cradle, the M-60 machine gun adapter, and mounts for the PVS-4 and TVS-5.
For our ammunition load out, we were never issued a full combat load. I had 20 rounds of 9mm, 100 rounds of 5.56mm. On the truck, we had 3 cans of 7.62mm and three 48 round boxes of 40mm.
We had no AT-4 or LAW on the truck.
The seat behind the TC was folded down, and thats where we stacked our rucks. The seat behind the driver was open (this is where I would sleep when not on watch, patrol, etc).
We had 3 boxes of MREs, a dismount bag (radio, map, binos, PLGR (GPS), MELIOS (laser rangefinder), two tripods (one for the Mk-19, the other for the M-60), three 5gal water cans (one inside, two outside mounted either side of rear hatch), two fuel cans mounted on the tailgate. One roll of barbed wire, one coil of concertina wire (strapped to the hood), six T-posts, and a picket pounder.