Looks like it is only good for single-loading, and the chamber insert is ejected with each bolt stroke.
As-is, the Mosin's extractor is not designed to grasp a x25 case.
So... you pop out the steel chamber insert (hopefully there is no cosmoline left in that chamber or it ain't ever coming out), take a stick and poke out the brass x25 case from the chamber insert, put a new x25 cartridge in there, cycle the bolt with the chamber insert in place, and shoot again.
And... only at sub-100 yard targets with any reasonable similarity to sight picture or sight settings.
I'd just as soon get 10 of these:
http://www.lothar-walther.de/209.php
.22LR adapter, with short rifled barrel to impart velocity and spin sufficient to sub-50 yard marksmanship practice. With 10 of them, you can load entire stripper clips and practice running the rifle for real.
And, even cheaper than the x25 adapter, after factoring in the cost of your first 500 rounds of ammunition.