doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com 2.75" shells in 2.5" chambers

Note the above posted pressure curves. Note these would not in reality start at 0" as the shell would have a base wad, but note how high the pressure rises before hardly any movement of the charge itself takes place. Also note this initial movement of the charge is a very critical factor. Too soon & pressure will not rise quick enough to insure adequate combustion & a "Squib" may be the result. Too late & pressure can Spike to a dangerous level. Way too late as in putting a stiff charge of a fast powder behind a heavy shot wt & the charge might not leave the bbl at all. In this case the shooter would experience hardly any recoil at all, but most likely would note the "Blinding Flash" as the chamber top lifted off in front of his face. (Very High Pressure)
The question boils down to exactly how "All" factors interact to affect the acceleration of the charge down the bbl, & exactly how that acceleration affects what the shooter feels. I am very willing to recognise that "IF" that rate of acceleration of the gun can be showed to vary to a significant degree, that even though total recoil developed remains the same, there can be a difference in how it is absorbed by the shooter. In this case I Would be in the camp who believed the "Slower" rate of acceleration would be the most comfortable to fire.
I am at still however, this point thoroughly convinced that within the parameters of currently available shotshell powders suitable for consistent reliability for a given load that not enough variation will exist for a shooter to seperate two loads into "Mild" vs "Brutal" by the firing of a few shots. Possibly there might be a slight difference in "Accumalated" fatigue by the firing of a large number of shots over a relatively short time. In this case extreme care will have to be excerised to insure one is actually testing the different loads & not "Themselves" over different sessions as alluded to by another poster, several pages back.
This is esentially my stand on this & I am "Always" open to any "Meaningful Input".

Everyone has given their opinion and some have given a lot of research and great information. The whole thing still comes down to that no one has answered his original question. So Mark I will give you mine. Your gun has 2 1/2" chambers and you are reloading low pressure-low velocity loads in 2 1/2". I would say that 7,000 psi loads and up to 1100 fps would be fine.
I don't know why with all the available shell sizes out there now and the low pressure-low velocity selection why anyone would want to shoot shells that were not made for their gun. Years ago ingnorace was bliss, but not today with all the information out there.

In gettng back to Sherman Bells article on "Finding Out For Myself" I believe he fired one proof round in each barrel and that was it. Since no one knows what modern shells actually produce in shorter chambers and you will be firing more than one why take a chance.
Just my opinion.

Miller,
Isacc Newton's 300 yr old laws of motion address your uncertainty about acceleration of the gun vs. the payload. They basically say: if you push harder, the object accelerates harder, and; if there is a force it has an equal and opposite reaction. Pressure is the origin of the force as it pertains to guns. Put all this together and you can see why I conclude that the gun accelerates differently with different pressures.

What a person can 'feel' or what fatigues a particular person more than another is very subjective. So if a person says a particular load is less comfortable to them than another, what basis would I have to discount his 'feeling'?

Hard to argue with that.

David,
I believe Bell did fire more than one shell for each test, I just don't recall how many. He did indeed find somewhere between 0 and up to about 15% increase in pressure on the testing he did. I've fired a number of shells in an instrumented short chamber but not in comparison to a 2 3/4" chamber. The pressures were what I found to be acceptable (6500 psi) for my use.

That would be because larry can’t give you any facts or reasons. He blindly quotes Thomas and gets defensive whenever anyone questions him. for him to accuse someone else of unflinching devotion to a writer is the apogee of conceit. I adhere very closely to what Burrard wrote because it all holds up under scrutiny and everything adds up.

I don’t doubt Thomas performed some test and I don’t doubt he achieved the results he claimed. The problem is without documentation they’re trash. Here’s the problem: step one of the scientific process is “formulate an hypothesis”. Every subsequent step is specifically designed to prove said hypothesis. I’ve spent enough decades in a research world to have seen what people do when data doesn’t match their pre-conceived notions: they come up with a reason to discount that data. Gough Thomas performed some test which proved his preconceived notion was right because the test was arranged to accomplish that end.

Any idiot can stand up and say “tests have shown blahblah ….” unless a comment like that is accompanied by documentation of every particular, every facet, and preferably the raw data as well, of the test, any conclusions drawn are plain and simply garbage.

Don’t believe it? try selling a $75 million fighter to USGOV and saying “tests have shown it meets your criteria. Trust us.” I guarantee you they won’t. they will want back up.

Roger

Chuck;
I am indeed following what you are saying. However, think very seriously about this question before replying.
Again looking an an old Alliant guide, I find two loads both with 1 1/8oz shot at a listed 1200fps. All components are "Identical" Except the wad. one used a Rem Fig 8 @ 10,700 psi, the other a Rem RXP12 @ 9,800psi. Now again think this over "Real CarefuL", can you "Positively" state the load using the Fig 8 accelerated faster, because it showed the highest pressure. "OR"?? did the Fig 8 wad take a tighter hold on the walls of that particular brand of case for whatever reason & Actually "Retard" the acceleration, creating the higher pressure. Remember "All" factors have to enter the equation. I certainly do not find it true that acceleration can be figured from the max pressure recorded in a pressure gun, nor for that matter, even from the entire pressure curve. Certainly a factor, But "Not" the only one.
I also think since the majority of plastic wads are rather loose in a bore & depend upon obturation for a seal, if not given a quick enough blow to ensure obturation, might quite well move down the bore with faster acceleration than if a faster powder was used giving that tight obturation, resulting in greater friction & keeping the pressure up but retarding movement. Many Many factors to be considered. I really think the only true answer lies with an accurate Accelerometer test of the gun's movement itself, while firing different loads.
Actually having followed your posts here for some time, I must say I am quite surprised you have seemingly failed to consider any factor other than just the pressure. This does not seem compatable to your usual thouroughness of a subject.

Miller, do you often look in the mirror and argue . . . with yourself? Why should I have to give you any reason why a faster-burning powder seems to recoil less, when:
a. You agree the IMI tests were carried out;
b. You agree the results were as stated (the shooters unanimously observed less recoil with the faster powder); and
c. You're stating that Thomas did not lie about the tests.

Your only complaint--which is quite accurate, I readily admit--is that Thomas did not print complete results including powder types involved, etc. True--just like Burrard did not publish complete results for his conclusion that long shells, loaded to proper pressure parameters, are perfectly safe in short chambers. And in that latter case, Thomas did in fact publish complete, comparative results.

Looks to me like a tie between two dead Brits.

2-p and LB, thorns and barbs aside, you two have done a commendable job of debating the issue from the Thomas-Burrard points of view. Thanks.

2-p, the acceleration will follow the wad base force, as generated by the pressure, minus friction drag. The amount of friction drag will be a function of sliding coefficient of friction between the plastic wad's obturation section and the hull wall followed by the steel barrel (think pretty low) and the wad's velocity. 10,000 psi is going to exert about 4200 force pounds on the wad base (0.42 square inches). Suppose that about 0.2" of the wad skirt is in contact with the hull wall/barrel and has a cirumference of 2.3" for a contact area of 0.46 square inches. So, the coefficient of friction would have to be 0.91 (0.42/0.46) to stop acceleration. Another way to look at this issue would be the possibility of the wad holding the force on its base with friction. Suppose the thickness of the wad skirt is 0.050" and has a circumference of 2.3". The area available to resist the 4200 lbf load is 0.115 square inches. 4200 lbf over 0.115 sq in requires a tensile strength of 36,500 psi. Both the case of a friction coefficient of 0.91 and a tensil strength of 36,500 psi are implausible. Somebody please check these numbers. Unless I calculate wrong, the acceleration is going to follow the pressure curve pretty close for the first part of the shot.

Rocketman:

The coefficient for clean steel should be around 0.6-0.8 but I'm not sure of the value for plastic on steel(plexiglas on steel is about 0.5 and plexiglas on plexiglas is near 0.8). Would the forcing cone surface have a higher coefficient of friction? Also, I don't see it but did you include the weight of the load along with the wad in your friction force(Normal)?

Kind Regards,

Raimey
rse