The choke tube function is related to how much constriction it has from the backbored barrel. So, if the barrel is bored to .740" and the choke measures .710, the constriction is .030" while if that same tube was put on a barrel that measured .730", the constriction would only be 20 thousandths. Choke size must be related to the bore size....breakemall....Bob Dodd