At last!  After eliminating quite a number of 8mm bullets that all keyholed at close range and barely hit the berm at even moderate distance, the purpose of the 76mm twist has been revealed: OO Buckshot over shotgun powder works.

I can't speak for any but the one I have, but the C.I.P. drawing calls out a 0.318 groove diameter and a 196 grain round nose jacketed soft point bullet. Mine has a 0.323 bore and the chamber neck is very generous indeed, requiring some resizing to get any purchase on a 0.323 cast bullet. I've tested numerous cast bullets from 170 grains to 223 grains. All with very poor to zero accuracy, but no leading. It is possible that my ersatz brass is at least a contributing factor, the final length is 54mm instead of 58.5mm in the C.I.P. drawing. Also, my wall thickness is 0.0095" rather than 0.013" that appears to be required. A CH4d sizing die reduces the neck to take a 0.318" bullet in the parallel neck. An M-style expander allows seating a 0.323" bullet far enough out to reach the lands but there was no meaningful improvement in results.

What's really perplexing is that the 76cm twist bore is very finely done with six lands and grooves and a perfect bore.  00 buck (0.330") finger seated over enough Unique to leave a clean bore returned groups about 6 MOA at 25m in a freshening crosswind. Greenhill says a maximum 100 grain bullet should stabilize. Buffalo Arms catalogs Sauer brass reformed from 303 British that are out of stock. I won't attempt swaging the web of 303 to fit then thinning the rims, but may buy a few to see if it's worth going further or not.