I just acquired an H&A model 925 falling block rifle in 25 Rimfire.  When I went to the local gunshop, they didn't have any of that ammo. Imagine that!   

Seriously, I bought the rifle thinking of making an extra barrel in 22LR or perhaps rechambering the 25 rimfire barrel for another 25 caliber and converting the action to centerfire.  The rifle is a takedown, so making a second barrel would be relatively simple.  A takedown stud holds the barrel in place and the barrel is a slip fit into the action.   

But I really don't need another single shot .22.  Would anyone have any idea how strong the action is and if it is capable of safely handling 25 ACP or perhaps 25-20 ammo?

I have breech seated cast bullets and used low power 25 caliber concrete nail gun loads as shells. Works great in a favorite. Find the shells at the local lowes or home depot. Use a lighter bullet like the 25 rimfire was loaded with. Not a 120 grain one.