The Red Army's elite sniper teams, when not busy killing Nazis, used their spare time to think up new and interesting methods of killing Nazis. In one of these epic brainstorming sessions, Zaitsev, probably after frantically sketching something in his notebook while making explosion noises with his mouth, came up with the idea to take a scope from a Sniper Rifle and attach it to a giant 14.5 mm PTRS-41 Anti-Tank Rifle. He wanted to use it to kill bunkers.
Just straight up murder a fortified concrete fortress.
Here's a PTRS in action:
The idea was to fire the huge explosive shells through the viewing slits on Nazi bunkers, exploding them from the inside out, which was roughly the equivalent of successfully performing eye surgery with a chainsaw. It's probably also worth mentioning that the PTRS-41 had a nasty habit of breaking the user's shoulder when they pulled the trigger, so we guess it's more like performing eye surgery with a double-sided chainsaw. Madly, awesomely, terrifyingly - it worked. In the laconic words of the 284th division's combat journal:
"Sniper Morozov managed to send an enemy bunker up in flames using an Anti-Tank Rifle"
"But it wasn't a thing; he does that s**t all the time."