Continuity mistake: This is a two-for-one. When the astronauts are boarding their shuttles, the gantry for the crew of the Freedom (i.e. Bruce Willis' ship) is adorned with the red triangular mission logo for the "Independence" shuttle (i.e. Ben Affleck's ship). The Freedom logo, worn by the nearby technician, is bluer and more circular. Also, the Independence logo on the gantry wall has "STS-90" written as the mission designation, while the patches for the same logo on the crew uniforms say "STS-98." (01:05:55)
Deliberate mistake: According to the mission patches, both the Independence and Freedom are assigned the mission ID of STS-98 (possibly referencing the film's 1998 release date, and is around where the actual STS numbering was at the time). Two problems with this: First, they are two separate shuttle flights, so each should receive its own mission number. Second, STS was the coding for NASA's civilian shuttle. The shuttles in the film are Air Force vehicles and would probably not receive STS designations.
Answer: If you look at the deleted scenes on the special edition DVD, you will see a deleted scene in which A.J. asks what they needed a gun for, and Max explains that it's for debris elimination, in order to take out small rocks in the way.