doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com DGJ Article on Chamber Length/Pressure

Hello Larry,

This topic will again bring out those who only read a part of all the studies and come
out crying "bloody murder". They are out there and they will show up. ;-)

I have decided, after all the information I have seen, to use my Manufrance Ideal 16bore
with 65mm chambers marked 1100kilos with 67mm shells loaded to CIP pressure standards.

Comments appreciated.

JC

Jaycee, I do it all the time and don't even think about it.

Thanks King. I don't either, but it is always good to hear well versed opinions.

Best,

JC

I am just an ole Tennesee Redneck Hill-Billy & had absolutely no problems at all understanding what Burrard wrote on the subject of Chamber lengths, loads etc. Seems neither Thomas or Bell, both supposedly very intelligent people, could sort it out. I just chalk it up to the fact I have no EGO problem, in thinking "I" have to be the one to "Discover" it. Burard stated the reason for the investigation to begin with was the fear of 2½" cases closed with the fold crimp being mistaken for 2" shells & used in the guns designed for much lighter loads. This started in the late 30's. A very thorough investigation was carried out & the resulting conclusion seems to have been that virtually "ALL" factory loaded British shells of a "Nominal" 2½" length closed with a fold crimp, from that day forward have had fired lengths longer than the nominal chamber length. They in fact had a "Loaded" length of approximate equal to a roll crimp 2½" shell. One simply has to wonder why Thomas thought it "Started" in the 60's when had already been going on for some 20 years.

Just remember that 70mm shells are typically loaded up to the old 2 3/4" British standard service pressure of 3 1/4 tons per square inch rather than the 2 1/2" standard 3 tons.

Do not confuse the idea that a home-loaded shell with the same pressure in a 65mm case as loaded into a 70mm case is the same thing as putting a factory loaded 70mm shell into a gun chambered and proofed for 2 /12". You will be over-stressing your gun.

If you see the ends of your plastic cases 'ragging' or splitting after use - your case is too long for your chamber and/or your forcing cone is too tight.

This is quite common. The solution is to ease the forcing cone or use shorter cases. If you lengthen your chambers,the gun will be rendered out of proof.

All kinds of things are possible. not all of them are agood idea. My advice is to use ammo of the correct length for the chambers in your gun.

Miller,
I'm wondering if Bell's approach was simply to do an article (a source of income) of a subject covered by a previous writing likely long forgotten by most. I know we've all seen writers in periodicals repeat subjects over the yrs, this would not be a surprise. Certainly, a writing half a century old has had little exposure lately, save a few readers of historical writings.

JC, you're WAY on the safe side with that gun and 67MM CIP loads.

Miller, I think you need to reread Thomas. I can't find any quote from him about the whole discussion "starting" in the 60's. In fact, he says: "I should have thought that all these doubts and misunderstandings had been cleared up by now, but this is evidently not the case . . . " And although the Thomas article in question dates from 1964, the test he conducted with Eley shells goes back "some years ago". Also, as far as I know, he's the first one to show actual pressure and velocity figures. Burrard certainly said that the shells were safe, but he did not provide the results of a test, as Thomas did.

What Thomas is doing in his article is not disputing what Burrard found, but rather correcting the conclusions of people who misread Burrard. Both Thomas and Burrard point out that you don't want to grab just any 2 3/4" shell and fire it in a gun with 2 1/2" chambers.

Bell makes a significant contribution to the discussion by dealing not only with factory shells loaded to CIP pressure standards, but by testing various reloads as well. In addition, he also tests the effects of lengthened forcing cones on pressure, concluding that there is usually some reduction in pressure with the longer cones.

It has been some time since I read Thomas's writing on this, was simply quoting this previous post

I do very distinctly though remember in reading Thomas he stated " Burrard Should Have Said" that the load played a part as well as just chamber length. Now that is exactly what Burrard did say. Thomas simply took parts of Burrard which were written when no 2½" loads were put up in a 2 3/4" case & it is just as true today as it was some 75 yrs ago when Burrard first wrote it, A 2 3/4" case loaded to a higher level than the gun was designed for "SHOULD NOT BE FIRED IN A 2½" CHAMBERED GUN".
If you go back & read Bell's article you will see he stated that neither Burrard nor Thomas came to a definitive conclusion, both left doubts as to whether firing a longer case with proper load, in a shorter chamber was safe.
Larry I am not totally ignorant, I can read what people say. You know I stopped & put some gasoline in my vechicle today. In doing this I did not prove an internal combustion engine can burn gasoline, that was done well over 100 yrs ago. It was proved in the late thirties & early fourties that the proper load for a 2½" chambered gun could be put up in a fold crimp 2 3/4" case & fired with no discernable difference in either pressure or ballistics. Burrard did recommend "Against" firing any shell whose loaded length allowed it to extend into the cone. I might note in the loading manual of a major powder maker the statement was made that any pressure desired could be obtained by simply varying Crimp Pressure. If the loaded shell is crammed into the cone the force needed to open it is increased which can increase chamber pressure. I do not recall Thomas referencing this & know that Bell did not in the articles we have been referencing here. It is of paramont importance that the "Loaded Shell" have clearence between it & the cone & then that the load inside is proper for the gun it is to be fired in. If these conditions are met there need be no fear if the end of the case happens to lap into the cone upon opening. This was what was proved some 65 yrs ago, I have seen nothing of importance which has been proved since.

What Bell wrote about Burrard and Thomas was indeed out of context--and incorrect. He has Thomas disagreeing with Burrard and offering proof that long shells don't cause a problem in short chambers. Last part, of course, is correct--and Thomas did offer the proof.

And while you're not ignorant, Miller--even though a self-described "Redneck Hillbilly" :)--your "distinct recollection" is wrong. That's why I said you ought to reread Thomas. Sometimes our "recollecting" goes astray. Nowhere does he write "Burrard should have said". And in fact, Burrard does say that case length is one of the issues with increased pressure:

"When a true 2 3/4" cartridge is fired in a 2 1/2" chamber the pressure is increased for two separate and distinct reasons: (1) the length of the loaded cartridge is exactly the same as that of the actual chamber and so, when the turnover is opened on firing the mouth of the case is prevented from opening completely by the chamber cone; . . . " Thomas is correct in straightening out that issue: " . . . the main danger arises, not from the constriction when the cartridge is fired, but from the fact that the longer-cased cartridges he had in mind invariably carried heavier loads; . . . "

Burrard eventually gets around to straightening out what he wrote earlier, but he could have used a better copy editor.

In the modern world, the use of thin plastic instead of thick paper for cases makes the additional 1/4" intrusion into the largest part of the forcing cone a non-issue as far as pressure is concerned. As always, loading the case full of powder and shot is still an issue, regardless of case length.