...but the steel is so thin walled to work with, thought it may not even work (?). kinda' on a whim with the .25 cal idea.
One big issue with .25 cal is the valve volume to barrel volume ratio. Seems once over .22 cal the fps efficiency really dumps off. However, with a 1377 valve dropped in (which requires a shorter piston, but still a longer stroke than a 1377 in the end) things pan out better.
A more doable alternative is to use a .25 cal liner and make a custom breech end to work.
"Well, I thought it was a rabbit but it turned out to be Bear Grylls in a rabbit hide."