Paul,
Go back one page and read my post in this thread of 10-13-10.
Think about it. Buring powder produces expanding gas that pushes against wad and shot. If a shooter restricts the path of that wad and shot by shooting a long shell in a short chamber the velocity of the expansion will be slowed but the powder will still be buring and gas will still be expanding. If the wad and shot cannot move forward the gun will move backward producing more felt recoil.
There are a lot of variables such as the thickness of the shell housing and angle of forcing cone but the theory if solid. If one restricts the rate of expanding gas a reaction will be created. All else being equal a 10 gauge produces less pressure than a 12 and a 12 more than a 20 and so on.
If you don't believe me load up some 2 and 3/4" and also 3" shells of the same shot weight and pressure. Then, shoot them is a 12 gauge with 2 & 3/4" chambers. If the gun has sharp forcing cones you will feel a big difference. It the gun has long forcing cones (1.5" or more) then you may not feel much difference in recoil. Chances are any old Parker will have sharp forcing cones.
You could also do the same for your 10 bore using correct length and longer cases.
Have fun!
Mark
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