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.
Drag/friction is always an issue when two masses are in contact and have relative motion one to the other. There is drag/friction between the wad-hull, wad-forcing cone, wad-barrel, and wad-choke. The various drags/frictions are manifest in the MV. Ergo, all drag/friction is accounted for as far as conservation of momentum is concerned, and conservation of momentum is relentless. Consider the gas pressure force on the wad as it reaches the choke. Muzzle pressure has been reported in the range of 200 - 500 psi. 12 bore has around 0.4 square inches of area, so the wad base has something like 80 - 200 pounds of force on it. A very simple, abbreviated test to see what force might required to push a wad through a full choke. Wad = CB 1118 1 1/8 oz white 12 bore and barrel = left side full choke of an Ithaca NID 4E trap gun @0.035" constriction and a cleaning rod. Test method = weigh cleaning rod, insert wad in barrel, insert rod behind wad, turn barrels vertical with the muzzle up and the rod sitting on a scale, push down on barrels and read the scale as the wad slides up the barrel. The barrel reading was 14 oz minus 7 oz rod weight = 7 oz required to move the wad. However, the choke maximum reading was 40 pounds (forget the 1/2 pound rod). So, if we have 80 pounds of gas pressure force with 40 pounds of that force used up in pushing (swedging) the wad, we still have 40 pounds of force available to accelerate the wad and shot. I'll agree that the acceleration would be relatively trivial, however I'm sure there would not be a check of the wad's velocity. 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.
Plausible, but strikes me as a pretty weak explaination.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.
I believe from the front to the back of the shot charge you could describe conditions by steady state flow - at any given amount of travel along the barrel or point along the choke,the shot column would have the same flow characteristics.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.
Why would the wad not continue to accelerate as long as the gas pressure force on the wad base is higher than the friction/drag?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.
I don't quite agree with this statement, but think you will agree that there is no further acceleration past muzzle exit.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.
Gas hammers happen verrrry quickly. The wad wouldn't have to slow/check much to cause a bulge.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.
I suspect that the "riveling bulges" seen immediately in front of choke sections stem from large shot bridging. I'm relatively sure all the choke specs were arrived at by "cut and try" as there was not science to support theory.
I agree that thisis a good discussion. Thanks for throwing on a couple of more logs. 
DDA
