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Sidelock
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Sidelock
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I am neither a physicist nor a Dr, but I think this is a rather complex thing of which there are varying opinions. Many years ago Burrard reported on this subject a series of tests done by, as I recall, one of the Major British Ammunition makers. Test bbls were made up with precisely fitted extension tubes so that all factors were identical except for choke or lack therof. The bbl was fitted for pressure checking (crusher), velocity recorded per standard British measurement of the time (IE observed vel over 20yds) & recoil "Measured" (NO Feeling depended on) in a pendulum gun.
Several differing loads were checked & results were extremely consistent & Amazingly while the full choke always showed a slight increase in observed velocity the recoil from the choke showed a very slight, probably imperceptile by "FEEL" reduction.
The only plausible explanation they could come up with was that there was a slight check as the charge hit the choke restriction, accounting for the reduction in recoil. The velocity gain of the choke was accounted for by the shot being held closer together acting somewhat as a solid projectile for a bit upon exiting the muzzle, thus individual pellets not taking on independant drag characteristics as quickly as from the Cyl bore.
While I understand the Venturi effect Rocketman describes (the reason the carburator on an old 2-cyl John Deere can pick up its fuel) from purely a laymans concept it seems reasonable to me that when the shot charge hits the choke it might not immedialy take on this attribute being more solid than either a liquid or gas, but would indeed receive a check in its velocity. As the individual pellets jocky for position from the reduced area they would then begin to take on this effect but not necessarily regain all that had been lost from the initial checking.
This then leads to my next question, in the 20s/30s just how close to the muzzle was Remington able to ontain accurate reliable velocity measurements. Only a very short distance would be required to account for the results the British obtained.
Miller/TN
I Didn't Say Everything I Said, Yogi Berra
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Sidelock
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2-p, I most surely wish I coulod find the article on the Remington tests (it was in a "major" publication). I was well enough scooled and experienced to be critical of their test equipment and method and found no fault.
Small problem with the "velocity check" idea. That is that even a small barrel obstruction will sometimes cause a gas hammer capable of bulging the tube. I'd have to hypothesize that any "velocity check" would do likewise.
Seems to me that each individual piece of shot will accelerate forward at increasing velocity (and lower shot swarm internal pressure) just as each gas/liquid atom/molecule does in true fluid flow. The counter intuitivity of the venturi principle is one of the reasons flight came about at least half a century later than science might have supported.
The report of systematically lower recoil from higher MV makes me suspect there was some test error. As I recall, the Remington test measured MV over only a few feet. 20 yd is a "fur piece" and could have introduced some measurement error, IMO.
Further, IMO, the reduced internal pressure of the shot swarm is the reason the pellets stay closer together. That is, they have less pellet-to-pellet pressure and less force to cause "spring-apart" and the entrapped air has less pressure to cause sideways velocity as the air expands on muzzle exit.
DDA
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Sidelock
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"Further, IMO, the reduced internal pressure of the shot swarm is the reason the pellets stay closer together. That is, they have less pellet-to-pellet pressure and less force to cause "spring-apart" and the entrapped air has less pressure to cause sideways velocity as the air expands on muzzle exit."
No - the pellets are radially accelerated towards bore center by the action of the choke. Rem, IIRC, also tracked pellet migration across the pattern by laborious assembly and recovery of multicolored pellets in designated sectors of the load. Read that when I was a kid so it was a LONG time ago. But a perfectly expected result considering the simple physics of the choke/charge situation.
have a day
Dr.WtS
Dr.WtS
Mysteries of the Cosmos Unlocked
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Sidelock
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What Rocketman recalls may be true for felt wads - no way to tell.
For plastic wads in identical loads in identical guns the answers would be: no, no, and no. Shotguns don't duplicate data very well anyway. The slop in the average load would overwhelm whatever diff there may be from the gun, if there was any.
And for a person to perceive the difference, of which there is none, it would require a nervous system so advanced beyond the current model that it won't be a problem for some considerable time, if ever. Of course that doesn't mean that there will not be people that will swear to the converse of any or all of the above. There are also people who attest to the tonal difference if the phase is reversed in a single speaker. I'm sure they also ride their flying pig to work.
HTH
have a day
Dr.WtS Amen Wonko, +1, sanity spoken, given two of the same model shotgun, the same weight with a stock that fits you and shooting the same load it would be impossible to tell the difference INMO (for what itīs worth)
Last edited by Mike Bailey; 09/09/12 03:24 AM.
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Sidelock
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Only way to tell as far as perceived/felt recoil would be via a blind test, 2 identical guns, identical factory shells, only difference being cyl vs full. Would "slop in the average load" compensate for any velocity gain in favor of the full choke? Guys with their own chronos might comment on variations they've seen in factory loads. I had Tom Armbrust check a couple factory loads (both British) for pressure and velocity. Gamebore Traditional Game 16ga showed a whopping max difference of 3 fps, high to low. Not much slop there. Eley Blue 12ga was 20 fps. Maybe enough there to offset a full choke velocity advantage . . . or maybe not.
Always better to check theories via real world testing. After all, there was once a theory that a curve ball is an optical illusion . . . to which a major league pitcher (I think maybe Dizzy Dean) suggested they try hiding the theorist behind a sheet of wood 60'6" away and he'd beat him to death with illusions.
Last edited by L. Brown; 09/09/12 06:06 AM.
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Sidelock
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Greetings, Dr. WTS, "Further, IMO, the reduced internal pressure of the shot swarm is the reason the pellets stay closer together. That is, they have less pellet-to-pellet pressure and less force to cause "spring-apart" and the entrapped air has less pressure to cause sideways velocity as the air expands on muzzle exit."
No - the pellets are radially accelerated towards bore center by the action of the choke. Rem, IIRC, also tracked pellet migration across the pattern by laborious assembly and recovery of multicolored pellets in designated sectors of the load.
I read the same test and my take away was that enough of the pellets swapped sides to indicate random sideways directional velocity as the swarm separated (something like an equal number of rights on the left and lefts on the right), but in no sense a wholesale swap. IIRC, the author made a kinda half-hearted, weak case in support of his pre-determined conclusion.
Read that when I was a kid so it was a LONG time ago. But a perfectly expected result considering the simple physics of the choke/charge situation.
Unfortunately, as I indicated above, this is not at all "simple, horse sense" physics; Journee was wrong about inward momentum. The venturi principle is counter-intuitive; however, it is true science.
have a day
Dr.WtS
And a good day to you, sir. DDA
Last edited by Rocketman; 09/09/12 02:10 PM.
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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.
Miller/TN
I Didn't Say Everything I Said, Yogi Berra
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Sidelock
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"I read the same test and my take away was that enough of the pellets swapped sides to indicate random sideways directional velocity as the swarm separated (something like an equal number of rights on the left and lefts on the right), but in no sense a wholesale swap. IIRC, the author made a kinda half-hearted, weak case in support of his pre-determined conclusion. "
Certainly an interesting statement. I really like the "random sideways directional velocity" part.
I think I need to do something important.
over and out
have a day
Dr.WtS
Dr.WtS
Mysteries of the Cosmos Unlocked
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Sidelock
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With all the chatter above, does ANYONE disagree with my SHORT concise answers?  Mike They were 1. No 2. Not much, lost in the numbers of statistics 3. A. Some , B. A tad bit less Remember the volumn of the void as the wad leaves the hull and travels down the barrel is larger in a overbored shotgun, but not much by percentage.
Last edited by skeettx; 09/10/12 01:37 PM.
USAF RET 1971-95 ![[Linked Image from jpgbox.com]](http://www.jpgbox.com/jpg/57865_80x60.jpg)
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Sidelock
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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
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