Will a 25 rimfire block work on a 22 lr ?

I also think it will work.

But if it doesn’t, here is what I did on a #4 Rolling block I’ve been picking away at.  It was a 32rf and I changed it to a 22.  I think John Taylor mentioned this method at one time, so it’s not my idea.  It also saves on changing breech blocks and don’t know why it wouldn’t work on the 44 1/2.

It does require a lathe at least, a mill is handy but a drill press may work using an end mill in it.

Mount the breech block in the mill with the breech face up and squared with the firing pin hole.

Mill by elongating the fp hole toward the center of the bore just enough to cover the rim of the 22.  This will leave a hole with straight sides and rounded ends.

Turn a new firing pin with a body the original size and the small pin large enough to fill the old hole diameter.  This leaves an over sized pin.

The pin then needs to be hand fitted to the new modified breech block hole.  The sides and center sections need to be trimmed to fit.  I did this in the mill but it could be done by hand.

This #4 has worked very well with this setup.
Bob