Well I had the barrel off a couple of these Martini actions, a BSA 310 Cadet & BSA 22LR 12/15 & thought bugger it I (when I say I, I mean my mate with the lathe) will make a barrel stub out of wood to see for sure, then ended up making it out of a piece of nylon rod instead, chambered close to enough with a boring bar & turned a rim groove .

Well the 38/55 will slide in no trouble even with the .22 block in the 12/15, when the lever is pushed fully down & will slide straight out to, with the lever pushed fully down !

I don't know if this is allowable (to load with the lever pushed down, may not be ?) but it seems to work so far !