The concept is a Mosin-Nagant rifle with the barrel cut down to 4-8 inches and the buttstock lopped off, to make a concealable weapon in leiu of a proper pistol. Mosin rifles were fairly common and handguns much more difficult to obtain, so the obrez was a way for folks who needed a sneaky gun (be they criminals, partisans, or revolutionaries) to conceal under a coat. Recoil is pretty hefty, and accuracy non-existent, but they work well enough in close quarter.
Bureau of Sabotage