So I'm a machinist and decided to check the inside diameter of my .54 cal. GPR against 2 new .54 cal GPR barrels. I have put approximately 1000 rounds through my first one and used gage pins from work to do my measuring. Both the old and the new barrels measure .543 inside diameter. I use a muzzle protector every time I load and I only shoot patched round balls. If this is any indication of how long a barrel will last with reasonable care using a muzzle protector, then I would think that most barrels should last a lifetime.