Here's an extract from your SuperMag's instruction manual. Remington says:
TO LOAD THE CHAMBER AND MAGAZINE:
1. Point the firearm in a safe direction.
2. Engage the safety mechanism. RED BAND WILL NOT SHOW.
3. Push upward on the action bar lock.
4. Pull the fore-end fully rearward to open the action.
5. Put one shell of the correct gauge and length through the ejection port.
6. Push the fore-end forward to close the action.
THE FIREARM IS NOW LOADED.
7. Press shell against the carrier.
8. Push three (3) 3-1/2" (870 Super Magnum™ only) shells or four
(4) 3" or 2-3/4" shells of the correct gauge and length, one at a time, fully forward into the magazine.
You gun will shoot 2-3/4, 3 and 3-1/2" shells. It the rifled deer barrel didn't shoot 2-3/4" slugs the Remington legal department would be going ballistic. And, the barrel would be marked with something like "NOT FOR 2-3/4" SLUGS" -- what exactly does it say on your deer barrel?
One caveat I think most seem to forget - the length of the hull is the FIRED length - I have watched many a person slip a 2-3/4 into a gun chambered for 2-1/2 and think it was OK.........