Measuring the outside dimensions is one way of trying to determine if the barrel has been stretched. Another way is to push a tight-fitting patch down the bore and pull it back out before and after firing. Any buldging or loose spots will certainly show up! Also, inspect the breech plug area and the touch hole or touch hole liner if a flintlock, and the drum & nipple seats on a caplock. I don't believe cracking or actually bursting is much of a concern, given the kind of steel most barrel makers are using these days-it is more likely to stretch or swell up if pressures are too high.

I am about to proof-test a .73-cal flintlock barrel. My first test shot will be with 200 grains of FFg powder and two tightly patched .715" round balls. Next, if the barrel survives that test intact, I plan to fire it a couple moretimes with 200 grains of FFg and one ball.

My standard loadings will never exceed 150 grains FFg and one ball, so if anything about the barrel was stressed but does not show up using the 200-grain/one ball load, I will consider the barrel O.K. for use.

Dixie Gun Works says to "use twice too much powder (whatever that is!!) and two balls/bullets" of the type that will be used in the barrel. BUT, they don't say what to check for after the shot! I think ypur question is a very valid and important one, since a barrel could survive one overload, but be damaged to the point of being unsafe.