[lgjhn]

"How easy is it to change the barrel on weapons like this thereby rendering ballistics records moot?"

Rest assured, it wouldn't take long. The barrel could be changed pretty quickly or the existing barrel could be polished a little which would change the profile, or the easiest way would be to just change the bullet weight that is being shot as it would have a different profile.
Most anti-gunners jump on the "bandwagon" without a clue about reality and that's their basic problem to start with.

[Dead Dog]

an AR can swap "uppers" in less then 5 seconds. Only the "lower" receiver is registerd. That is the aluminum housing for the trigger group. Everything else can be bought of the internet.