First your self made gauge is meaningless as we have no idea what it's dimensions are. Second, your gun is marked for chamber length somewhere in the proof marks on the underside of the barrel. Tell us what those marks are.
Second, I doubt your Gamecock was made to handle steel shot so using steel or hevi shot would not be recommended by the manufacturer. The Gamecocks I have handled were not well suited to being made into long range guns as they were more the quick handling upland game type gun by weight and balance.
It would take a bit of measuring by a gunsmith to determine whether lengthing the chambers will reduce the safety at the margins of the new chamber.
Third, the extra range of hevi shot is not that significantly different from good lead shot and even if it is the difference is out where few shooters can make proficient use of the much more expensive ammo.
