Other mistake: When Michael Pena is looking at the photo of Swagger after winning the Wimbledon Cup, it identified Swagger as "Sgt Major Bob Lee Swagger, USMC, from Gillette, WY." Two things are wrong: (1) Swagger is wearing civvies, if he's in the Corps, he should be wearing his uniform; and, (2) Swagger is supposed to be a Gunnery Sgt, not a Sgt Major. (00:44:20)
Other mistake: In the beginning of the film where the truck with the machine gunner and the driver is shot, the truck goes out of the road and falls over, but there is no machine gunner falling of the cargo area nor is there any driver falling out from the truck.
Other mistake: In a whole lot of scenes, Bob closes both his eyes when he fires his sniper rifle. This is a common and natural reaction when an inexperienced shooter fires a gun. An experienced hunter or marksman would never do that, and especially not a military sniper.
Other mistake: When Swagger is dialing in his sight to shoot the Denty Moore stew he is dialing in the windage but they show the elevation moving.
Other mistake: If you look at the license plate on the government car that mark Wahlburg is driving, they are not government plates but regular Pennsylvania license plates. The next scene the driver tells the FBI guy that they are government plates which they are not.
Other mistake: When Swagger shoots a driver through the windshield in the opening scenes, there is no hole in the windshield.
Answer: If you cut a slit into them and squeeze when the ball expands back it creates suction. So they could have been used to clear away any blood or pus when cleaning the wound before stitching.