I carried my Maurshin M500 in its stock-less form for both CQC7 and in the field, and it was a pain. Depolying it was not so bad, but putting it back/ out of the way was too much of a hassle. That and I found the extra weight wasn't as useful as say a pistol.
When I did have it, I had the top of the shotgun secured in an Eagle Weapon Catch and had the muzzle in an Eagle MBITR pouch (chose that one since its padded and has a wide opening with makes getting back in easier). I also had a sling attached to the pistol grip stud that was then secured to my shoulder strap on my PC. It allowed a full range of movement with it, but kept it high enough to where it wouldn't bash me in the knees if I had to let it go.
http://www.optactical.com/weaponscatch.html (granted this is the belt version, but its the same idea)
http://www.optactical.com/eaprc148mbra.html
Don't get the chinky shotgun scabbards, Switch had one and the lining went tits up after 5 uses.
Before you get something to big, try running with it slung on your back and see if its comfortable enough for you to have that added weight on your back.