Not to have an e-peen contest here, but just how many guns have you fired in your life? I've fired literally over ten thousand rounds of various ammunition types from various types of guns, including a fully automatic weapon. A shotgun isn't some sort of magical damage monster that video games and movies make them out to be. A bunch of us at some gun forums had a good laugh when
this comic was published because it so perfectly describes shotguns in video games. In real life, when I shoot stuff with my 12 gauge (or my cousin's 10 gauge magnum (OWW MY SHOULDER)) it doesn't go flying around. It might knock over the smaller targets and the larger targets may move a bit, but it's not enough to be considered knockback in the way you want it.
As for muzzle energy:
A 12 gauge 00 buckshot shell delivers slightly less energy than a .308 rifle, which is a very common caliber for marksman/sniper/hunting applications. Why don't gauss weapons have any knockback?
As Sak said, it's a video game. I like the balance of the current shotgun and dislike knockback mechanics in general. Keep shotgun how it is.