For your stated needs (500 yards) you'll want as much barrel as you can buy. For a self defense carbine out to 200 yards--16" is fine.
But at 500 yards you'll want the added velocity that the longer tube buys. Its not that you can't punch paper with a 16" tube. Its the wind and the ft lb's of energy to do work required at that range. Trust me on this. 16" barrels SUCK at 500 yards and really for much beyond 200.
You need to be running 70-77 grain match bullets and a LONG barrel.
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