If I might add a little to the thoughts. In one of the appendices to The Modern shotgun bu Burrard he reported on some tests which had been conducted in England. A heavy pendulum gun qa fitted to record max pressure, velocity & recoil. Two bbl extensions were very carefully fitted & lapped to identical diameters, one cylinder bore & one full choke. Loads of several varieties of powder were tested from the heavier of the bulk smokeless down through a fairly light dense powder & If my memory is correct I believe black as well.
Bottom line was that though small, statistically significant readings showed the full choke to give higher velocity & lower Recoil readings. Velocity was of course the "Observed" velocity over 20yds. Load in all cases was the normal 1 1/16oz with 3 dram measure or equivelent depending upon powder. The conclusion drawn at the time was that the choke "Checked" the velocity but the shot remained closer together as the exited the muzzle
& drag was less for a short distance allowing them to more than regain the loss.
Now, if we can take it the Remington tests recorded velocities close enough to the muzzle to eliminate the possibility of the results being skewed by outside influence, then obviously their conclusion, though reasonable, was not factual.
But, Something caused the lowered recoil & it wasn't the "Speeding Up" of the shot. So what's left. Note the comparsions are strictly between the F choke & Cyl for Each load, everything else being identical. Maybe, just Maybe we need to look to the wadding. Could the wadding have been checked enough to slow the ejection of the gases just enough to give the lowered recoil??
I am going to go out on a limb & "Predict" you will find a point of a slight pressure spike when you run your tests. You may have to move the strain gage around a few times to find the right place, but it'll be there. Normally this will not be enough of a spike to bulge a bbl as the check is slight, & the load is moving so rapidly at the time it takes a bit for the gases to "Catch up". I do seem to recall seeing high speed pics of the charge as it leaves the muzzle & the wad was snugger against the shot from a cylinder than from a full choke, which indicates the wad was delayed by the choke.
From further back in this thread I recall it being mentioned the proof house had quit using shells supplied by the trade & started loading their own. Maybe its not the shot at all, maybe they are using stiffer & heavier wads & these are making enough difference to create the bulge in sme cases.
