To further that point...

No two barrels shoot the same.
You could have two barrels made by the best barrel maker in the world, out of the same material, on the same tools, done at the same time, and have two barrels with completely different character.

Funny that way, but that's just the way it is.
He who regulates a double rifle works to massage the pair of barrels to shoot to the same place with over the counter ammo so the gun will continue to do that all over the world with same said ammo.

--Tinker