I have owned several 10ga's.

The tightest bores were on an English hammergun. As I remember they were .755". I had the wall thickness measured when I bought it but didn't get around to measuring the bores until I borrowed a friends bore gauge. Shot plenty of felt and plastic wad loads through it before I knew the bore diameters and did the same afterword.

I assume the current owner is doing the same.

Mike