HA! - now I am thinking I made a poor guess. I just checked 3 breeches 1 threaded 1/2" x? and 2 at 5/8x18 breeches. The 1/2", looks like a replacement TC Seneca breech was .388", and the two 5/8x18 threaded breeches, one an English breech and the other a Hawken breech, were both about 5/16"- something just around .313".
So - perhaps nothing is standard, but it is an important # to know.