Did you measure the chamber? a snap cap will not tell you much , I have a gauge, but you can easily check with a steel rule or bit of hardwood.
Just insert on side of chamber till you feel the transition to force cone, mark that spot and measure. It may , or may not be stamped on the barrel lug.
You could be right that it is 2 3/4, but over the years I see lots of old shot guns that are short chambered and the shooters don't know.
Canvasback is Correct, the weight and shape has lots to do with recoil.
I have a real nice Ithaca /SKB 500 o/u 20 ga, But I hated it , kicked more than all my 12 ga guns.
I will say, I have never shot slugs out of old shotguns, or newer ones, for that matter. That will have a tight choke I bet, Most are full.
If the gun is not pitted, and wood is solid, it would clean up easy, Doubles are a totally different ball game.