doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com 2 5/8 Chambers

Just reading the info on Parkers on another thread and it says chamber length 2 5/8 and shell 2 3/4. Why would they not have chambered to 2 3/4 so to avoid the 1/8 of the shell in the forcing cone? I know the 2 5/8 chambering came into existence before 2 3/4 shells were standardized but that should have been by 1910. Thereafter I would have expected chambers to be 2 3/4.

Not sure where I read this, probably on this forum. As I understand it, in the days before the development of modern plastic one-piece wads with shotcup and an effective gas seal, making the chambers a bit shorter than the length of the fired shell was done deliberately as it was thought that having the end of the hull forced tightly against the inside of the forcing cone helped ballistics by preventing escape of gas around the shot column, since the card or fiber wads did not produce a very good seal.

Tamid, American gunmakers chambered 12 ga. guns to 2 5/8" as that was the standard 12 ga. cartridge length until industry standards changed to 2 3/4" in 1929.. L. C. Smith was the only maker, to my knowledge, that chambered their twelve bores to 2 3/4" earlier, circa 1910. Everyone else chambered at 2 5/8" until 1929. As an aside, the so-called 2 1/2" chambered English guns, almost always actually measure 2 5/8". Sandlapper

L.C. Smith, Maker, Syracuse Hammer 1883-1890 & Hammerless 1886-1890 12g had 2 3/4" chambers, as did the Hunter Arms Co. 12s thereafter, unless otherwise requested and so marked on the barrel flats



“Mr. Griffith on Shotgun Patterns”, 1897
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA234
p. 243 “Turnover” – case longer than the chamber in reference to vintage
roll-crimped shells
No. 9 2 1/2” case with 1/8” turnover
No. 10 3/8” turnover
No. 11 & No. 12 with 2 3/4” and 3” cases in 2 1/2” chambers = “…patchy
patterns, clustering, and frequent balling…” Pattern examples on p.244
p. 245 “balling or clustering”
p. 247 “Summary of patterns”, Field, March 5, 1898
No. 9 - 1/8” turnover better pattern % than 10, 11 & 12
Pressures with 3 Dr. “Schultze” with 1 1/8 oz.
No. 9 – 2.13 tons = 6,040 psi (converted by Burrard’s formula)
No. 10 – 3.03 = 9,060 psi
No. 11 – 3.22 = 9,700 psi
No. 12 – 3.71 = 11,345 psi
“When long cases are used in short chambers, the paper overlaps the cone and causes greater resistance to passage of the shot and wads. The pressure then goes up considerably, while muzzle velocity and recoil are both increased.”

Charles Askins “The Cone” Field & Stream, Nov. 1921
https://books.google.com/books?id=UPtAAQAAMAAJ&pg=PA700&lpg
W.A. King for Parker Bros.
Referring to the cone, we ordinarily permit our shells to lap into it 1/8-inch, but as a matter of fact we have found that the majority of guns will shoot a better pattern with even more lap than this. In fact, we can secure extra good patterns by shooting extra long shells, permitting the lap to extent to the point where we begin to tear off the ends of the paper. At that point we do not seek to give more lap. In user’s hands we do not advocate the use of longer shells than those which give 1/8 lap.
As regards the shape of the chamber cones, we have tried very long taper cones, short, abrupt tapers, and now use on of a certain standard length, which is nothing be two reverse cures, easily run from the end of the chamber and as easily curved into the bore proper. When the tools are kept up so that these curves are correct, we consider that we have a properly shaped cone, and one that does no harm whatever to the shooting or patterning of the gun.

Shortening chambers was common to give a better gas seal with fiber wads and paper hulls. Plastic wads seal so much better that we fail to understand why short chambers were done. Improved patterns and less gas blow by. Trap shooters were almost obsessed with it. I am sure live bird shooters would be interested in anything that could give them an edge. But I understand they went after longer chambers to allow for more and better wadding.

so does it follow, that opening 2 1/2 and 2 5/8" chambers to 2 3/4" would better accommodate modern loads with plastic wads and hullls?

Winchester Model 21's were introduced in the early 30's and, to the best of my knowledge, all 12 gauges had 2 3/4" chambers (excepting 3" guns). Some of the chambers were a bit short of 2 3/4", most likely for better ballistics for period ammunition.
Any comments from M21 gurus?

Another advantage of the slightly longer shell/shorter chamber was that it would help cushion the shot when making its initial contact with the forcing cone. As noted above, trap shooters wanted that advantage to deliver better patterns. Somewhere I've got a 2 part article from the American Rifleman back in the 30's stressing the advantage of the longer shell/shorter chamber combination. The author of the article ran some tests to demonstrate that it worked.

Accommodate Ed? Why would removing extra metal, in the exact place chamber pressures are greatest, make things better when you could just use the right shells? Do not forget our factory shell can run up to 11,500 which is a 1,000 psi above the service pressure for the gun you just reamed more metal out of the chambers. And guns made for 1900 ammo were not designed for modern SAAMI pressures that came out 20-50 years later.

Stop trying to blur the lines. Modern shells can have too high pressures for safe use in vintage guns. And our shell makers never share pressure data and are not going to held responsible for any damage that those shells cause. They will claim they passed SAAMI pressure standards. It always comes back to using shells with the right pressure for what the gun was designed to shoot. Built to a standard, built to handle the loads for that era, not built for future loads based on the chamber length basis.

Your constant attempt to say ammo ought to be OK is a bad thing. People searching for information might grab onto your post and decide that WallyWorld ammo is fine and shoot it in a gun designed for ammo at 2/3 that pressure. Will it fail? I do not know. But I do know if it does fail, major injury could result. Not that long ago K-80's were blowing up and they had 3" chambers from the factory. Not safe with the ammo they were using because they blew up. But that was factory ammo, 2 3/4" factory ammo. Chamber length and pressures all seemed fine but the little issue of failure ruined a few people's day. If you are shooting a proofed gun like a British 2 1/2" gun use proper ammo. Do not ream out the chambers for convenience or thinking longer chambers can handle more pressure. Most 3" gun barrels are built thicker in the camber areas. Making a chamber longer will not add extra metal to the outside of the barrel.

many, many pre 1926 made guns have chambers opened to 2 3/4"...

guess what, the bores are not bulging in front of the chambers and they are certainly not blowing up...

listen to your gunsmith...and follow his safety guide lines...re barrel wall thickness and recommended loads for your particular gun...