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Trying again, with the hope that we can stay on topic, which I believe is an important one for those of us using vintage guns.
The topic is NOT why does the chamber pressure rise when using long for chamber length shells, which was investigated 130 years ago

Sporting Guns and Gunpowders, “Tests Of Strain On Breech Actions”, 1892
1 1/4 oz. 3 1/2 Dram Bulk Smokeless Pressures in 2 3/4” case
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA86
Long Tons/ sq. inch converted to PSI by Burrard’s formula
(Proof) with 6 1/4 Drams “Tower Proof” Black Powder and 1 2/3 oz. shot – 4.51 tons = 14,034 psi
3 1/2 Drams Curtis & Harvey’s No. 4 T.S. Black Powder – 4.2 Tons = 12,992 psi
“Schultze” – 4.28 tons = 13,260 psi
(In a 3” case, with additional wadding the pressure for “Schultze” was 4.93 = 15,445 psi )
“E.C.” – 4.92 tons = 15,411 psi
C&H No. 4 T.S. 3 1/2 Dr. with 1 1/4 oz. in a 2 1/2” case “would increase the strain (pressure) about 40%; with the same charge in 2 3/4" cases, the increase would be about 50%."
Interesting that the 10% additional increase is similar to Bell's findings.

And all of us get that "feathering" of the case mouth is evidence of a potential problem

[Linked Image from photos.smugmug.com]


The question is "Can lengthening forcing cones mitigate the rise in breech pressure which occurs when using long for chamber shells?"

Sherman Bell's study suggested that it could, but his sample was small and he made clear that was not the point of his study; which is summarized here about 1/3 way down here, along with more vintage studies
https://docs.google.com/document/d/1ZIo0y746UsSRZIgRuuxwAbZjSBHitO_EanvwLYc-kGA/edit

and this one is interesting, from 1897:
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA280
A trial of 1 1/8 oz. 3 Dr. Eq. 2 1/2” shells in 2” chambers with "an exceedingly long and gradual cone" showed very little increase in pressure.
(1.72 Tons by Burrard's conversion is only 4,660 psi and most period reported numbers for 1 1/8 oz. 3 Drams Bulk Smokeless were 6000 - 8000 psi)

[Linked Image from photos.smugmug.com]

Miller's observation is here, which is an application of the inverse relationship between pressure and volume (when temperature remains constant)
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=37158&page=2
"Changing the forcing cone angle (lengthening) would give a very "Slight" increase of volume in the area, which would tend to slightly decrease pressure. A "Slight" change in friction could also accomplish the same purpose."

An interesting study NOT regarding cone lengthening on pressure but on pattern performance, with a short discussion at the beginning regarding the increase in volume issue. Comments are made by tim99 who is a mechanical engineer
https://www.trapshooters.com/threads/model-12-forcing-cone.286074/

Other opinions are certainly welcome.


IF the primary issue is frictional, one would think that the "chamberless" barrels would have revolutionized barrel design; which they did not

The chamberless Tolley "Altro" was introduced in the mid-1890s
https://books.google.com/books?id=BFRDAAAAIAAJ&pg=PA31&lpg
https://books.google.com/books?id=Bzo6EAAAQBAJ&pg=PT92&lpg
Country Life, June 25, 1921 ad
https://books.google.com/books?id=a0YxAQAAMAAJ&pg=RA8-PT8&lpg

Dr. Charles J. Heath chamberless patent
Forest & Stream July 1921
https://books.google.com/books?id=x1nlAAAAMAAJ&pg=PA308&lpg

1922 Sporting Firearms of Today in Use
https://books.google.com/books?id=9XMCAAAAYAAJ&pg=PA95&lpg

Field & Stream, August 1922
https://books.google.com/books?id=XQQ-AQAAMAAJ&pg=PA470&lpg


So in summary, Richard P. Feynman, Professor of Theoretical Physics at the California Institute of Technology
“It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it’s wrong.”

Possibly someone with the interest could build on Bell's work and clearly establish that lengthening cones (and how long?) will substantially lessen (how many less psi is substantial?) the rise in pressure when using long for chamber shells.
I had a mechanical engineer friend who was enthusiastic about investigating pressure testing using a hydraulic device, but a real job and marriage has unfortunately gotten in the way.

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The red verse indicates a short shell will raise pressure 40%? Is this a misprint? My conclusion is "These are shotguns." Not dismissing your research, Reverend Drew, but a good shotgun is hard to blow up.

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Thanks Bill. I neglected to include the rest of the sentence, which I added. The point of the study was the danger of stuffing too much shot and powder in a hull, with an even greater increase in pressure using a long for chamber length shell. The letter from Wm. Ford questioned the use of 2 3/4" shells in 2 1/2" chambers.

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Short article but with advertisements for the thin brass cases designed for the "chamberless" guns
https://www.oldammo.com/january14.htm
The chamberless gun was developed in the 1920s by Dr. Charles J. Heath, a British surgeon and water-fowler. Dr. Heath determined that using a thin walled solid brass shell eliminated the need for the heavy forcing cone that was required on a gun that used paper shotgun shells. Removing this cone allowed the bore to be enlarged to about the same size as the inside diameter of the brass shell. Because these brass shells had such thin walls, they held more powder and shot than a paper shell of the same gauge, but also required that a larger wad be used. An added benefit to removing the forcing cone was that it significantly reduced the recoil of the gun. (or not wink )

I have not been able to find the Heath patent, but more on Heath from the IGC
https://www.internetgunclub.com/forum/viewtopic.php?t=63

Another thread which briefly touches on the chamberless concept
https://www.trapshooters.com/threads/don-currie-on-forcing-cones.894688/

And worth mentioning, Stan Baker's maximum 12g "Big Bore" was .800" which is essentially the end of the chamber dimension (Hunter Arms' engineering drawings showed .797" ), ergo no forcing cone.

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Did some dumpster diving at Ben Avery this morning after meeting up with Borderbill and my buddy Bill Henry.
Spent 12g Fiocchi Shooting Dynamics 7/8 oz (unavailable) and Federal Top Gun 1 1/8 oz. measured 2 11/16"
12g Federal Game Load 1 oz., Challenger 1 oz. Target Load, B&P Comp One 7/8 oz., and Estate 1 1/8 oz. each measured 2 3/4"

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Interesting that chamberless guns/backboring were invented in 1920, then reinvented in the prewar years (Becker) and then became popular with trapshooters in the 70's, yet they were ooriginally "de-invented" by Parker in the 1870-1880 period.

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my 1890 lefever pivot-lever 12ga has .749 -.750 bores...which appear to be original (and was told same by the seller - who had owned the gun since late 1970's). evidently, in the 1880's, quite a few 10's and 12's were bored and chambered for the use of (thin) brass shells...typically loaded with wads one gauge larger than the standard bore. at this time, i have not tried to "correctly" load my gun....but brass cases and circle fly wadding would seem to make it quite possible.

best regards,
tom


"it's a poor sort of memory that only works backwards."
lewis carroll, Alice in Wonderland
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I’ve been reloading a big box of new Remington Gun Club hulls with feathered case mouths shot by a guy at our club through his O/U. I doubt that this demonstrates a problem with chamber length, but may instead reflect degradation in materials quality used in the hulls. To me, a case shot in a short chamber would not open fully. Based on my experience I would not hang my hat on a feathered case mouth being an effect of short chambers. My $.02.

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greybeard: you are no doubt aware that the chambers of Parker Lifters designed for brass 12A shells usually had a 'step' between chamber and bore; not a forcing cone
https://parkerguns.org/pages/faq/12B.htm

Ed: another example of case mouth feathering

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Drew: no, the Gun Clubs do not demonstrate the radical distortion of those hulls. I think the GC hulls have thinner hull walls at the mouth, at least thinner than the GC I used to load in the beforetimes. Happy Thanksgiving. Ed

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