Just thought I'd give a report on this since I couldn't find anything written about it myself when I was contemplating trying one.
I ordered one from Sportsmans Guide and it came in the big brown truck the other day along with a box of .32 ACP so when it came I inspected it and found it to be machined from steel and coated with a type of tough black coating and I tried to fit it into my chamber.
It would not go in all the way and stuck out about a quarter of an inch. Not wanting to force it and risk getting it stuck I took it out and using fine grit emory cloth began to sand away and polish off the coating that was on the outside of the adaptor insert.
Fianlly had to insert a larger diameter borebrush into the adapter, and place the end of the brush into a drill and spin the adaptor while holding the emory cloth around the insert to finish the polishing.
Took me a while, but finally it slipped into the chamber with just a little nudge from the bolt and it locked up and ejected the way it was supposed to do.
That all done I headed out to my hunting property where I set up a target at twenty yards.
I sighted the rifle in with a couple shots, and it was impressive as the little cartridge put most all of my shots at that range easily into a spot I could cover with a nickel.
I then placed the target at forty yards and the grouping opened up a little to around four inches, with random grouping into the size of a softball.
I then set a large coffee can at fifty yards and put shot after shot through the can confirmed by the spinning can and sound of the round hitting it.
I then shot at some dirt clods out to a distance of eighty yards and hit around and sometimes did make contact with the one I was shooting at and the round didn't seem to drop any until I tried past that distance which I concluded would be beyond the practical even plinking range for the tiny cartridge.
All in all I was impressed, and it makes the old warhorse a viable survival weapon, as it can be used as a short range and not too loud 40 yard small game rifle, and by just ejecting the insert, you can then chamber a full house .303 round for taking down a large game animal.
Well worth the minimal cost of the insert, and the ammo although maybe not as cheap as .22 ammo I would still say it's not too expensive and pretty redily available.
I got where I could eject the insert, remove the empty case and load a fresh round without having to look at what I was doing, so it's a little slower than your typical single shot, but with practice it's almost as fast.