I think the frustration is due to how quickly these players can close the gap to get the OHK vs how long it takes a player to kill the rusher with their primary. As long as it takes the average primary longer to kill a player than it takes the shotgunner to get in position for the OHK then shotguns will always be complained about.
This is why I mentioned it.
But nerfing the movements would not be good for the classes either.
I think an AR buff (Maybe pulses) for under that 12 - 15 foot mark may help. You need to be careful that you don't give them too much range again but if the short game is the problem then it could work.
Pulse rifles are fine in regards to their range and an argument can be made that their range should be toned down a bit. However, the point of my comment is that unless players feel as though they can win against a shotgun with a primary they are going to complain. I'll refer to your comment in regards to snipers being fine the way they are now, people feel that way because you'll win the gun fight more often than not if you hit the sniper first. In the case of a shotgun you can see/ shoot them first and chances are the shotgun player is just going to charge through your bullets and OHK you.