This has turned into a nice little fireside chat, so let me chunk another log or two on the fire lest it die out. Also I have no Axe to grind here with anyone, just seeking knowledge.
Concerning either an airfoil or ventury motion is required for effect. Take the airfoil, as long as it is sitting idle it is simply surrounded on all sides by atmospheric pressure. As it begins to move through the air the increased distance across its top, in comparsion with the bottom causes the air to flow faster across the top, resulting in lowered pressure which gives it its lift. There is however a third component which cannot be ignored in wing design, IE the "DRAG". The faster it is pushed the more drag & if this is not accounted for in its building it will simply yield & fold back against the fuselage & plummet to the ground.
Likewise I do not feel it at all unreasonable to believe there is also a "DRAG" component as a shot charge is funneled through a choke. I believe this was in fact brought out in the British tests I mentioned previously. Note here the method of recording the velocity was not really a matter of concern, it was as stated measured as the average over 20yds & not at the muzzle. These tests did show a higher V for the choke, which does not in any way contradict Remington's tests. They did however record the recoil on a pendulum gun, (either 50 or 75 lbs) as I recall. Fact was the recoil measurement did not show an increase proportional to the Velocity increase, but in fact actually fell off a bit. This testing was not done behind some shade tree but was a seriously conducted test by people in the buisness who knew what they were doing & cannot be simply ignored. This loss of recoil has to be accounted for somewhere.
So let's look at some possibilities. They suggested the shot from the choke left the muzzle a bit slower than from the cyl (No in between chokes were tested, only Full vs Cyl) but regained the lead very quickly due to the shot from the cyl attaining individual drag more quickly. This does seem to be somewhat plausible.
Another factor here which seperates the shotgun barrel from most other venturies is we do not have a steady flow, but a mass charge of extremely short duration. Likewise the pressure of the burning/expanding gases are not directly impinged upon the shot but upon the wad behind them. The Wad of course does not flow through the choke like a fluid. I have no way to prove it but I seriously doubt if as the shot are going through the choke the wad speeds up beyond its presently reached rate of acceleration, which by this point has slowed considerably. In Fact I suspect the wad itself at least is checked. This would seem to borne out by pics shown taken by high speed photography of one nature or the other showing more space between the wad & shot just outside the muzzle from a choke bore than from a cyl bore. I personally believe this to be more from the wad being checked than from the shot gaining velocity & out-running it.
As to the question of bulging from this checking it must be born in mind the bulging is dependant upon the amount of checking. Consider for instance a line of cars running down the interstate bubper to bumper at 70MPH. The driver in the lead vehicle Slams on his brake, there is going to be one massive pile-up of mangled cars. On the other hand if he just lets up a bit on the gas there may not even be a collision or if it does occur it will be further down the road. In the case of the choke this is more of a slight check than a sudden braking & so before the pressure can build to the point of a bulge the wad clears the muzzle releasing the pressure.
It is however also noted that in the early days of choke boring it was discovered that if bbls were made with extremmely thin walls at the muzzle as some muzzleloaders had been bult a bulge in the choke often resulted so the bbls were beefed up a bit. I have measured bbl dias on a good number of guns & found the smallest dia is quite often behind the choke with the bbls sweeling as the choke is approached particularly on lighter guns. I highly suspect the reason for this was found by experence & not by theory only.
