im pretty sure it will work in a 7.62x54 what im not sure of is what keeps it from splitting were its only .013 thick or so were it duplicated the neck of a round. Im thinling the addapter needs to be a pretty snug fit so that if it expands it is supported by the barrel. Im also conserned that it might get stuck and not easly come out. most of the adddapters ive seen are made to semi perminatly loctite into the chamber. the 32 auto adapters for the 308 and 30-06 that sports mans guide sells how ever are made to extract like a normal round after each firing. I have a couple of them and they dont fit all my guns BTW three on the tight side. on a 7.62x25 to 54r addapter I have to worry about it trying to fire down the barrel ans breaking the rim as well. that round head spaces on the rim not the shoulder so when the round goes off the adapter will be seeing pressure on the rim. an ideal addapter would seat against the shoulder and the rim simotaniously. that pretty tough to do when every chamber is differant. now if the addapter was made with no rim and loctighted in it would be simpler just leave the rim off and make it fit to the shoulder.
I see the case has a rounded face. Why? is the bolt face flat like a normal rifle? I will want the most material i can have there and if the addapter was made flat across the face it would be stronger. having a barrel stub that I could split down the middle would help. Of course I suspect all these chambers are not very close in specs