OP,
I'm not familiar w/ your Baikal single shot rifle.
Cut the Bbl & thread the stub & Bbl.
Why not manufacture a dovetailed lug & dovetail the lug to the Bbl.
Purged-to keep the atmosphere from pulling carbon/temper out of the parent material...aka the barrel.
Purge the bore & TIG weld the stub/lug to the barrel.
Once fixtured/purged & 10 seconds later....Done.
Purging with a more stable gas without oxygen (nitrogen is usually good) only prevents surface scale formation (iron oxides mainly).
Any hardening of the barrel (most have almost none to speak of) WILL be eliminated in that area.
The temperature change and slow cooling without quenching alters the crystal structure of the steel.
It changes from body centered carbon to face centered (or the other way, I just don't remember) in the crystal structure.
Martensite vs. Austenite.
The typical low carbon concentration in stainless steels mean very few of them can be hardened at all.
It is hard enough (expensive) to control Chromium and nickel levels without hitting a Carbon number in the final material.
The extra steps to add Chromium and Nickel alter the Carbon level from what the source steel had.
Most of the techniques to 'harden' the bore surface (like Chrome plating) for increased wear resistance are not that
compatible with producing the 'best' (most accurate) barrel.
Under some conditions the loss in accuracy is still more than acceptable.
Minute of 'human adversary' is still a big target.
The military is not shooting at prairie dogs, ground squirrels, and ground hogs at extended range.