There are two main factors which affect the amount a gun recoils. One is the weight of the gun, and the other is the total weight of the "ejecta", which is the combined weight of the projectile and the propellant powder. The reason why your 870 recoils almost twice as much as a high-powered rifle in the .270-.30/'06 class is that it is relatively ligh in weight, uses about the same weight of powder, and shoots a slug or shot load which weights a lot more (1 oz, or 437.5 grains+ for the shotgun, vs 130-150 grains for the rifle)!! A lightweight shotgun firing slugs or heavy waterfowl loads has almost as much free recoil as some "elephant guns" in the .450/.470 Nitro Express class!!

Keep yore powder dry!!