After four years of fiddling around with this rifle, I finally got the for-end repaired and strengthened and have proceeded to start reloading for it. Not much data around, but I did find recommendations for SR4759, Unique, 2400, and IMR4227. I'm using a 180 grain bullet out of an Accurate Arms mold. Drops WW bullet at .3525". Bore is .351". I'm using my own 50/50 lube but I also have my lube sizer filled with SPG.
I chronograph 10 rounds of each powder type I use to make sure I am not breaking the velocity limits for this blow-back operated rifle which seems to be around 1400 fps with a 180 grain cast bullet. I finally broke 1,017 fps this afternoon using IMR 4227 powder. Average fps for 6 rounds was 966 fps. With all four of the powders I have tried, Extreme Spread was quite high. For this load, it was 118. I weigh each powder charge which I drop from a Belding and Mull Visible powder measure. I am using a .38 Special cartridge case that I reduce the rim diameter on. I have not shot the rifle for accuracy yet. The bore is like new. Mechanically the rifle is like new. (the for-end was abused). It has two "Life Saver" shaped buffers (looks like compressed fiber of some kind, almost a plastic look too them) that keep the metal recoiling parts from impacting steel on steel. Those buffers are like new, and I have 6 replacements if I need them. The bolt and its long counter-weight is not locked, but kept shut and regulated to open by very strong recoil/return springs. I'm beginning to think that there is no such thing as an "extreme spread" number due to the irregular opening and closing of this blow back system. This situation has occurred with any loading I have tried so far. Never chronographed a blow-back operated rifle before as they are quite uncommon. Any suggestions? Thanks for your input.