Another update. Took the .303-.30/40 Krag ammo to the range. Here’s a picture of it, sandwiched between a piece of .303 brass (left) and a piece of .30-40 brass (right). As you can tell it’s a little shorter, but dimensions otherwise look good.
It chambered and extracted fine, although occasionally had trouble feeding. Dunno if it’s the case length, OAL length, bullet shape, or something mechanical. The 180 grain .313” rounds also seemed to work fine, and grouped close enough that I feel confident I can work up a load.
The only issue was the brass came out sooty. Too low pressure is my guess. 34.9 grains H380, the starting load from this manual:
This manual suggests a starting load of 42.0 grains:
Guess I’ll start with 36.0 grains and work up from there.