If you are planning on playing around with minies, you can get individual "plug" or pin gauges on amazon. I'd get several because you can interpolate your way to a usable measurement. Size range- .575. .577, .579, .581. Example, a musket you're checking out will let the .575 slip down but .577 won't, the bore is .576. Example, a musket you're checking out will just let the .577 enter but .575 drops to the bottom and .579 won't start, bore is .577.
Just these 4 gauges will cover most of the bore sizes you're likely to encounter. That said, I've seen and heard of repops that were .584 and larger. I view those as candidates for Bobby Hoyt to do his magic on.