I own 2 seemingly identical 12 gauge Mossberg 500 shotguns.
They were manufactured 12 years apart. The newer one is guaranteed to bruise you with every shot and the older one has no more kick than a Ruger 10/22.
The only thing that appears different is the color of the stock. The new one is darker and the older one is light.
I’ve never figured out why one kicks and the other doesn’t/
“I own 2 seemingly identical 12 gauge Mossberg 500 shotguns.
... The newer one is guaranteed to bruise you with every shot and the older one has no more kick than a Ruger 10/22.
The only thing that appears different is the color of the stock. The new one is darker and the older one is light... never figured out why one kicks and the other doesn’t.” [Buffalo Jack, post 9]
The two guns may look the same, but probably are not.
The angle of the stock in relation to the bore axis may be different: left or right, up or down, surface of butt plate in relation to bore. These dimensions are critical to shootability, but mass-market gunmakers rarely bother with them; it may not be obvious if they are different, to a casual observer. Custom gunmakers (such as James Purdey & Son in UK) expend much effort in measuring their customers, and shape the buttstock accordingly. Felt recoil is affected.
Shotguns are made with very loose tolerances of bore size and chamber diameter (compared to rifles). A larger bore or a looser chamber can cause lower recoil; tighter choke dimensions do it too, even between chokes of the same nominal constriction. Some shotgun barrels are back-bored, creating a very long oversized area, lowering recoil. Small changes in forcing cone angle or length will affect recoil also.
Differences in butt stock color may mean the use of different wood (birch is common in budget guns and much lighter in tone than walnut; its density is different also). Something as apparently insignificant as butt stock thickness will affect recoil: a narrower stock will transmit the recoil force to a smaller surface area, and the impact will feel worse.