Their site states $95/250 rounds for large box full of "entropic" contents.
Most QUALITY lead game loads fall in 450bar to 650bar range. Hot German 3&1/4dreq 1&1/4oz 70mm
shell develops about 600bar (pretty much like one of those crazzzy live pigeon loads). I can't see those "subsonic" Winnie loads developing >500bar.
PS. Short chambers & forcing cones plus longer shells make for very poor combo. That will result in extra slap und......
I see those guns you mentioned were made before late 20s, so you might want to stick with 65mm cases.