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Pressures, chamber lengths, cartridge lengths, once more. Please bear with me.

I have a number of Winchester Super-X 20 gauge 2 3/4 (70mm) cartridges. All the fired ones measure a tad over 67mm. Since my Rottweil Tiger cartridges are 67 mm on being fired and this is the allowed length for 2 1/2 the Winchester’s and Rottweils are comparable.

The CIP maximum service pressure for 20 gauges from 65 mm to 70mm chamber lengths is 830 Bar which is 12038 PSI.

The SAAMI pressure is 11500 PSI for 23/4 inch (70mm) cartridges.

Given the length of the fired cartridges not being long enough to enter the forcing cones and causing a spike in pressure there and given that max SAAMI pressures are below CIP max service pressures, would Winchester cartridges be usable in a 2 1/2 inch chambered, in proof, in excellent condition, British side by side 20 gauge?

Most vintage Brit 20s are proofed for 7/8 oz. loads. Most 16s are proofed for 1 oz. loads. I load for 2-1.2" 12, 16, and 20 bore Brit/Scottish guns.

I am often too lazy to trim hulls, reset loading machines, etc. to load true 2-1/2" hulls. Often I simply load my Win AA CF hulls with recommended 1-1/16th oz. (12 bore), 1 oz. (16 bore), or 7/8 (20 bore) loads. I just keep pressure under 8000 PSI or equivilent CUP and shoot away. My Atkin, Harkom and Robertson guns have never had a problem with this approach. IMO, Hull an additional 1/16" hull length is much less important than pressure and charge weight.

Thank you Reverend Hause for the extra info. Actually the 67mm fired Winchester case is not long for 2 1/2 inch chambers as per admissible lengths for them as permitted under CIP rules. Their pressure is below the recommended service pressure so should be usable as Eightbore concurs. A friend regularly does so with his British twenty without ill effects but I am hesitant. These are factory loads with one ounce of shot. But it is the max pressure that matters I presume.

Thanks again

All the best

SkeeterBD

I would say 'No'. British 20 bores have a standard load for the 2 1/2" cartridges of just 13/16th. ounce and are quite mild in pressure against U.S. made ammo. Most U.S. ammo is designed to fire in semi-autos and the pressure needs to be higher to cycle the action. You would just be straining the gun beyond what it was designed for. In the 1970's Winchester cartridges were imported from the States and it was found that the popular Winchester AA clay round with 1 1/8th. ounce actually approximated what was only suitable in English guns proof tested to the 2 3/4 inch Magnum proof. Our Winchester stuff is now made in Italy to CIP standards. I still have a few boxes of the old U.S. made Winchester 'Upland' but will only use it in continental guns to 900Kgs. proof. Your option but not what I would do myself. Lagopus.....