You could adopt a similar method to mine if it helps.
I've got a two point sling on my M4, both clasps going through the sling plate by the buffer tube (granted your sling mount won't be in the exact same place).
Then I have a separate sling mount on the front of the gun and a separate sling clasp hanging from the bottom left hand of my vest.
The gun is fine and dandy on the regular sling, but when I switch to using the pistol for more than a brief move around a corner, I sling the front of the M4 to my vest and it stays put really solidly.
I used to hate having it swinging around like a donkey's Wotsit whenever I was trying to use the pistol successfully.
You could adopt something similar with your shotgun, but in reverse, unclipping it when you want it free.