I didn't quite understand the last bit, but guns are improved in probably every way possible when PaP. While they're ammo capacity increases by a lot or not, it makes sense.
In the occasion that guns decrease in this field, it's because it would be overpowered otherwise.
If anybody has problems with the ammo count, then they need to rethink strategies. By the time, ammo is that much of a problem, guns wouldn't even be that effective, unless they are Wonder Weapons. Usually people would just use any gun they can to try and get points, then buy traps, and eventually get lucky enough for a powerful weapon.