I came into an interesting double, it was built as a 500 Jeff with claw extractors. Then some prior owner converted it to what appears to be a unique wildcat rimmed version of the same ctg using 577 brass, presumably to ensure more robust extraction. I can see where the bores in the extractors for the spring loaded claws were plugged.

Trying to regulate said beast. Proof marks indicate 535gr bullet (35 grams), 12,7x70mm, NP. No load given (German proof marks).

Started trying to find a practice load using a 425gr cast bullet and 4198 worked up to 50gr with plenty of dacron. Easy to shoot but all shot low and all shot wide apart with the right low. Chrony showed wide variations in velocity left vs right and I did not like that. Gave up on that idea to try some jacketed at higher velocities.

Currently using 485gr Woodleigh softs over H414. Up to 2400 fps at 117.5gr but still shooting apart and recoil is brutal even though this is a 12lb double. Don't want to go any higher as the recoil is the limiting factor and the rifle was built in 1927. I am also getting wide variations in velocity left and right, so much so I am doubting my chrony but if it's real, I feel I am rolling the dice here. Extraction is fine but the brass is showing every reamer mark in the chambers. Since this is 577 brass, it's not as thick as 500 Jeff brass so that may not be unusual.

So I want to try a faster powder next, ideally 4320 as I have some and it's about as fast as I want to go. But I cannot find any load data for this powder in this caliber for Woodleighs.

Anyone have data with faster powders in this cal with this bullet?