You can beat a ballistics match with a coat hanger.
Throwing out a used weapon is paranoid and unnecessary.
Lands and groves in barrels give out just like anything else, which is why, like most anything else on a gun, can be replaced. Switching out barrels is so much easier. And there are even guns today that you can switch caliber with just a simple barrel swap.
If your worried about being identified, from a forensic point of view, Id honestly be more conserned about leaving prints on shell casings. Which is why in game, if you have to off someone, use caseless ammo or use rubber gloves when you load.