Continuity mistake: When Atwood is sent back and continues his fall, he lands on a black police car with his head and body lying on the bonnet face down. From the overhead shot his body has completely disappeared, as he was wearing a light grey suit which would stand out on the black car.
Continuity mistake: At the end when they are hanging from the roof, the bad guy gets shot and falls off and Walker climbs back onto the roof. He then turns the light off on top of the gun, but when he walks away the light is back on.
Continuity mistake: At the end when they are fighting at the house, future Walker's hair keeps changing between wet and dry throughout the whole scene.
Answer: I'm pretty sure it was mp5 with muzzle brake.