Corrected entry: The silencer devices on the rifles in the movie can only be used on a weapon that utilizes .45 calibre ammunition. The rifles utilize 5.56mm.
Corrected entry: I know being a bad shot isn't necessarily a plot hole, but several times the cops outside have a direct bead on Sgt. Roenick (as he runs in and out of the building) through their telescopic sights at almost point blank range, and miss completely. Likewise, a big deal is made in the laser site scene earlier. One essentially cannot miss with a laser sighted weapon, yet these cops cannot hit the broad side of a barn. This really stretches credulity to the nth degree.
Correction: My brother in law is a member of a police force. Yes it is possible to miss with a laser sighted gun. I can cite many news reports where a suspect has a shootout with police, dozens of shots are fired, and the suspect is only hit once or twice. And, as you said, a bad shot doesn't make a plot hole.
Corrected entry: When Roenick and the others emerge from the underground sewer, he uses one hand to effortlessly push aside the manhole cover. Large manhole covers weigh several hundred pounds - far more than can be moved with one hand.
Correction: As a child growing up in Detroit, I remember on many occasions I would be be able to stick my fingers in the holes of man-hole covers near my school and lift up the corners. If I could do this with a few fingers as a child, Roenick could with one hand. Saying they way several hundred pounds is crazy, how would ANYONE lift them up by themselves.
Corrected entry: It is snowing heavily during the drive in the bus near the start of the film, yet in the shot when you see out the front of the bus from the driver's view, there is no snow falling at all.
Correction: Not true. It's harder to see from inside the bus, since it's backlit by the streetlamps, but there is snow falling. In fact, the outward shot is probably of real snow falling, while the shot inward probably contains fake movie snow.
Corrected entry: During the first 3/4 of the movie, the wind is blowing very heavily (the snow is flying to the side), but when the snipers aim they aim directly at the different people. Real snipers would know you had to move a bit to the left or right to make up for how far the bullet would be blown off course. The directors chose not to have them aim off-center because it might be confusing, seeing someone shoot four feet to the left and hit them right in the head.
Correction: Presumably the sniper has to first locate the target, which is what we see, then correct for the wind. The director chose not to show us the correction which is not a mistake. In most of the scenes with a sniper the targets are moving or running so it's very difficult to say for sure if the sniper has corrected for the effect of the wind or not.
Corrected entry: The scope on Bishop's rifle retains the protective covers on for the length of the movie.
Correction: Bishop is not a marksman and would not use the sight, particularly since his shooting is from extremely close range.
Correction: Actually, suppressors are usable with high-powered rifles. The use would muffle the sound of discharge, but the bullets would make a nasty crack going by, due to moving faster than sound.