I would not recommend an overload in an antique gun. I would recommend taking it to a competent gunsmith, with a good working knowledge of muzzleloaders, and have him check it out. Quality standards of workmanship, and questionable quality of materials used, back in the day, could result in injury to you or your gun, or both. Much better safe than sorry.
Hungry Horse
Its impossible to inspect and insure its safe. Just not possible.
Many years ago I bought a side by side rifle-shotgun that had been bored smooth on the rifle side.
Pulled the breeches and it looked OK reinstalled and fired both barrel remotely. The lower rib blew down at the breech. No real damage anywhere, don't think I ever found the leak. But there was one.
So unless it will stand a proof load I am not going to put it next to my head and set it off.
And even then.....
Its either sound or its not. The SIMPLE way to avoid this is to do as I do, tell everyone. "No old gun is safe to shoot". So bring me an antique and I will tell you not too shoot it.
This includes breechloaders as well. Like 1873 Win and 1893 Marlins etc. Why? Because I am not going to tell someone that an old gun is safe because its impossible to determine EVEN WITH PROVING. A look at the flaws in some old Winchester or Marlin (especially the ones marked "Blackpowder") bores should be enough but some people don't understand the significance of "forging lines" in a barrel. Proving finds gross flaws but will not prevent bad metallurgy from causing failures at some later time. This has been known for a long time. W. Greener in "The Gun" 1835 mentions proved guns failing in service and thought it was the fault of the proof process when in reality its metallurgy.
So proving an old gun (or a new one) will find serious flaws but it will not assure the gun is truly safe if it has purity issues or the steel is too brittle when pressure is applied.
So people shoot old guns at their own risk. I will not OK this as safe and any gunsmith that does needs to talk to a lawyer who understands old guns about liability.
Dan