3BM42M is a prototype. Cartridge Length is 735mm. This design eventually evolved into

3BM59 Svinets-1 and 3BM60 Svinets-2. 

3BM42-2 is the next Russian Standard Service round. Cartridge Length is 685mm which makes 

it compatible with unupgraded autoloaders. It

looks completely different to 3BM42M.