We will start with the Lenin era. That is 1922-1924.
The first and most obvious guns in use were traditional muzzle loaders. While not modern they were still effective then as they are now. With the rise or rather, re-emergence of traditional blackpowder hunting in the 21st century, I feel it is worth mentioning.
Most of these were probably snaplock guns, a predecessor to flintlocks and a successor to wheellocks. These guns were of course pretty common, as the technology has been around for 350 years by this point in time.
Matter of fact these guns were so common that even in the 30's they were still in use, as seen in the sketch.
Other guns included rifles converted to smoothbore. A process achieved by simply boring out the rifleing.
The first options were converted blackpowder breach loaders.
Such as the Krnka:
(NOTE: This one isn't converted. It's the only really good image of Krnka I could find, even in it's rifle form.)
And the Berdan:
(NOTE 2: This is a conversation.)
16 Gauge was the most popular cartridge to convert too, as 16 was extremely popular globally.
(NOTE 3: 16 Gauge shotshells.)
Other options included converted military rifles of the time.
The most common were Frolov Guns. These were single shotguns just like the ones above. Manufactur of these guns started in 1921. Any modern bolt action rifle appears to have been the basis of these conversions, usually in 32 Gauge, one of the smallest shotgun cartridges available.
(NOTE 4: From top to bottom; A Mosin conversion, a Mauser conversion, and another Mauser conversion. )
(NOTE 5: 32 Gauge shotshells. Image from ShotgunLife)
I have also read that even Winchester 1895 lever actions underwent this process, but I have not seen any pictoral evidence.
