Very easy. A few minutes with a Dremel tool and some emory paper, and you've changed the characteritics. With some rifles, you can buy and install a new barrel.
Another way is to steal a weapon. If the system actually matches a round to a weapon, it only leads to the legal owner, not the shooter.
The system doesn't handle the easiest method - use a bolt-action rifle and drop a casing from another weapon picked up at a local range.
Bottom line: gun-signature records will only work with stupid crminials.
I can't figure out why this guy hasn't been seen by someone. Assuming he isn't shooting from a van/car, why hasn't anyone seen a someone carrying a rifle? Is there a sniper-style rifle out there that folds up quickly?