CC, I can't help you with a 12-bore, but I've got an 8 Jeffery B&S with invisible rifling.

I can feel it, and can measure it with precision tools, and can even see that it is there with a bit of fouling, but would guess the number of grooves as at least 12-15 and have no idea on the twist at all.

My gun was supposedly made for 1250 grain paradox-style conicals, but I honestly can't imagine such VERY shallow rifling imparting any significant spin to a such a heavy bullet. It feels almost like shallow polygonal rifling.

I've of course been wrong before and haven't shot mine past 50, but would love to know if it really had any advantage over a smooth bore. The barrels are a work of art and part of me thinks they never would have gone through such trouble if they really thought it was just advertising hype.

Best Regards,
Bob