I'd appreciate some advice from you guys on the single player part of this game, I'll try to explain my problem without including spoilers. That said there may be light spoilers.

I'm currently at the mission at the dam, I've played through the mission up to the point where I'm at a complex with a large satellite dish. I died at the satellite dish and I've had to restart right from the binning of the mission, getting to and then through the dam followed by the fights en route to the satellite. My question is, am I missing some check points here? I'm replaying over an hour worth of campaign each time I die.

My second problem is I cannot use the SCAR gun, my ammo for that weapon is in negative figures, by that I mean I have -140 ammo when I pick it up. This leaves me playing through the game with either a shotgun or the special guns and my bow.

These 2 things combined are changing what was a truly amazing campaign into a bit of a pain in the butt. I most likely wouldn't mind the lack of checkpoints if I could use my damn SCAR gun but without it it's a pain.