Ka'imiloa,

After a bit of browsing, I came across this thread http://forums.nitroexpress.com/showflat.php?Cat=0&Number=128153&an=0&page=0#Post128153

If you scroll down a bit, you will see a copy of some 12 bore data, along with their pressure figures. And after a lot of reading on some other forums and stuff, I've heard of guys loading up to 150 grains of black powder without any problems. You can even go higher, but recoil is insane I'm told.

Anyway, check that out, as it certainly appeased my curiosity. Cheers.