doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Does Roll Crimping Solve Short Chamber Issues?

My Ideal with 65mm chambers will not go into battery with anything other than a 65mm shell; reloads must have a perfect crimp. The rising bite that engages the doll's head will stop short of coming all the way flush with the doll's head, with the result that the triggers will release, but the hammers are somehow impeded, resulting in light primer strikes. The first time I took the gun to Kansas for pheasants, my spaniel had worked hard tracking down a wily rooster. She put it up, and I swung on it, only to hear "click...click." My shootin' buddy declared, "I never did like French guns." Don't think my dog did that day either.

Mike

There are some guns--not many, and most are older with very short and sharply angled forcing cones--that either won't work or won't work well with shells that have a fired length longer than the chamber. It's rare to find guns with true 65mm chambers that won't accept 67/67.5 mm shells, because that's the FIRED length. They're shorter than 65mm unfired. The more typical problem is, they'll fire but you'll end up with blown ends on the shells. Charles Fergus, for one, reported that problem in Shooting Sportsman. The slightly longer hulls worked fine in a between the wars Brit gun with short chambers, but he got blown ends and also stated a louder report and more recoil in a 19th century gun. Chambers the same length; forcing cone different. He reported that true 2 1/2" shells worked fine in the older Brit gun.

Older SAAMI Specs for the 12 gauge 2 3/4" chmaber specified a minimum length of 2.614" length with a minimum diameter of .798" to a minimum cone of 5� per side. To a standard size bore this will give a cone length of about .400". At 2 3/4" this also gives a diameter in the cone of .774" or .024" smaller than the chamber end. These specs were in effect well beyond the introduction of the 1� oz 12ga "Short Magnum" load. This load should have of course been used only in those guns of later manufacture designed & proofed for the heavier loaded shells, but the problem with the older guns was not as a rule in the chamber dimensions themselves, but the overall strength of the gun.
"ANY" gun with its chamber ending in an abrupt step or cone of extremely short design should of course never be fired with a shell longer than its chamber. Certainly not one in which the crimp upon opening will extend up into the bore itself. Also no gun should be fired with a shell long enough in its "Loaded" length to actually enter the cone. As to the gun with 65 mm chambers which will not properly close with a 67/67.5 mm shell, you have some problem other that the length of the shell. As Larry correctly pointed out the loaded length of that shell will not extend to the end of the chamber, it will only do so after firing.

Had exactly the same situation here, with a Russian hunter to whom the late Geno told about Bell's experiments and that it's OK to use 70 mm shells in 65 mm chambers. The hunter experienced increase in recoil and blown heads; the shells were hand-loaded with roll crimp and were identical to the load which worked fine in 65 mm shells, except for the length and an extra fiber wad.

The use of a roll crimp in this case certainly exacerbated the problem. The roll crimp shell has a longer "Loaded" length than a fold crimp one having the same fired length. If in the case of this gun it had an extremely short cone the loaded shell may well have actually been pushed into the cone upon loading. This would in effect give a much stronger crimp strength, delaying its opening which can give drastically increased pressure.
The firing of a longer shell than the chamber length is dependent on there being clearance between the end of the loaded shell & end of chamber. It is further dependent on the end of the shell only lapping into a cone of normal length & not a very short cone which would allow the shell to reach either into or almost to the bore itself.

Interesting, all. I had thought, by looking at the spent hulls of slug loads, that roll crimped hulls were shorter. If they are not, then no, they would not properly expand into the 2 9/16 in. chambers. Thanks for the education, as usual...Steve

Steve;
Note that the length of a hull is determined by its New/Unloaded/fired length. Thus the type of crimp itself has no bearing on the length of the hull. Once loaded though the fold crimp uses more of the hull in crimping thus has a shorter loaded length than does a hull starting at the same length but roll crimped. In fact a 65mm (2 9/16") roll crimp & A 70mm (2 3/4") fold crimp shell will end up as loaded having very close to the same loaded length. Thus either of these shells will have a bit of clearance between the end of the shell (loaded) & end of chamber. How ever "IF" you take the 2 3/4" hull & roll crimp it the loaded length becomes longer & that chamber clearance is encroached upon.