The crew only had access to the venue for four hours. The entire combat scene was filmed in less than an hour, running and gunning. The final product looks like it took days to do.
The helicopter featured in the film was a UH-1 Huey, which is rarely used anymore. It was the only military helicopter footage that the film maker had which could be cut into the story and look good.
99% of the gun shoots and bombs were done with digital effects. This included the sounds, muzzle flashes an expended shells. There was only one, single gunshot in the film that used a blank round.
There where only three real guns in the scene, and of those, only one fired a single round. The rest were air soft mock ups.
The "soldiers" where mostly air-soft players and they provided most of their own gear for the shoot.