You might be surprised at what the accuracy might be. A late friend of mine built an insert barrel- chamber adapter from a 30-06 take off barrel. He shortened the chamber of the 30-06 to about 1.6”and made a wildcat round for it using 445 super magnum brass sized in a shortened 30-06 sizing die and just for a project ground the OD of the barrel with a cheap Harbor freight belt sander turned by hand until he could get it to fit in a 20 ga. Pardner SS shotgun. He did not use any O rings or glue to hold the adapter in he just figured the chamber end was bigger than the bore so where could it go ?
The shotgun barrel did have a scope base mounted on it and he had a 4 x scope to help with sighting it.
The funny thing was he wasn’t able to keep the barrel very round grinding it on a belt sander and he finally hammered it into the shotgun barrel and decided to shoot it like that. So what he had was an old junk 30-06 barrel with a cut down chamber, I did cut a rim recess for the rimmed brass on my lathe so the cartridge would chamber. The cartridge was shaped just like a short 30-06 with a rim formed on 445 SM brass and shooting 150 gr. jacketed bullets and to our surprise he was able to shoot several 1” 3 shot groups with it at 100 yards. It was really a project just to see if he could do it without a lathe and it did shoot well. There were noticeable gaps in the fit of the adapter to the shotguns bore but it was kinda beat in the final 1/16 th of an inch and was snug to where there wasn’t any movement.
As far as the 7.62 x 25 cartridge , I looked it up and it can be loaded as high as 36000 psi so I do believe you will need a small diameter firing pin on a break action gun or else the primer will flow back into the firing pin hole and cause the gun to be hard to break open after fired. Being a rimless case on the Tok I hope it not a PITA to extract the fired cases. Hope it’s a fun and accurate plinker for you.
Last fall about this time I seen Turkish made SS break action shotguns at a Dunhams sporting goods store near me that were on sale for $ 100. They had black synthetic stocks and matte finish on the metal but seemed to be well made . These were not the folding style shotguns which I see quite a few of online , I didn’t notice if they were hammerless or had a cocking hammer and don’t remember about the firing pin size ? I would have probably bought one for a rifle conversion project but I have 2 H&R ‘s and a Iver Johnson that I can use for such things already. I am always on the watch for good older break action shotguns a good prices and I know that they are hard to find without paying an arm and leg for but there are still some around .
Jedman