doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com 20 Gauge Pressures and Chamber lengths

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?

Somewhat different topic, but relevant information here
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=621506

SAAMI max. for 20g 2 3/4" and 3" is 12,000 psi

You are correct re: CIP 20g service pressure. The Max. Statistical Individual Pressure however is 950 BAR or 13,779 psi

THIS IS IMPORTANT: We have no data on the pressure rise when using longer than chamber 20g or 16g shells, which run higher pressures than 12g.
Bell's study can not be cited as evidence of the safety thereof.

So none of us have empirical evidence, but I'll give you my opinion, and other opinions are certainly valid:
I'd stick to 7/8 oz at 1200 fps in loads with cases measured at 67 mm
If there is no feathering of the case mouth, I wouldn't worry.

My answer would be yes, but you left out the measured chamber length of your gun. Not that the measured length would make much of a difference.

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

2mm is unlikely to matter.
67mm = 2.64" on L and 65mm = 2.56" on R in a nominal 2 1/2" chamber



Please let us know what the case mouth looks like the next time you shoot, or better yet post a pic, and good shooting!

For 12 gauge 70mm, 2 3/4" the SAAMI pressure level is 11,500
For 20 gauge 70MM, 2 3/4" the SAAMI pressure level is 12,000

You are mixing 12 and 20 levels. 20 gauge pressures run higher than 12 as do the 28's. And 12,000 is top end but still in the good range. Problem is they also run plus or minus on pressures. I have seen them run 1,000 plus psi with almost no difference in velocity.

This is a link to a good explanation and better chart.
https://randywakeman.com/shotgun_pressures.htm

You can not infer that American shells pressures are anything because we just don't get that information. Shell length does vary from maker to maker and lot to lot. I am in the middle of trimming down a lot of hulls, loading them for pressure and velocity testing to get a long term option for all my classic doubles. If things work as well as I expect I will start out with 5,000 shells loaded in five basic configurations for all my basic shooting needs. Then I will buy another 15,000-20,000 hulls to take care of those needs for the rest of my shooting lifetime. No more concerns about what is safe, what is available or what powders can I substitute for impossible to find powders.

KY John

Yes you are right. The pressure for 20 gauge is 12000 psi as per SAAMI. Plus a 1000 on individual shells. This falls within the max statistical individual pressure as prescribed by CIP of 950 bar or 13799 CIP. So still within the service pressure parameters. I am afraid I can’t reload where i live and often have no choice in cartridges available. I have plenty of 20 bore 2 1/2 inch ammo at the moment but was thinking of a “rainy day” use for my Winchester shells.

By the way

Given that 12 gauge proofs for CIP are lower than SAAMI how do pressures work for gauge reducers, 12 gauge to 20 gauge?

Thanks for your interest and info.

All the best

SkeeterBD

I won't shoot factory American 2 3/4" shells in 20's or 16's with short chambers (either 2 1/2" or 2 9/16"). But I will reload American 2 3/4" 20 or 16ga hulls--most of the 20's with 3/4 oz of shot and the 16's with 7/8 oz. If those loads are pressure tested in the 8-8500 psi range or lower in the 16 and under 9,000 in the 20, I don't worry. And I've never had any that have given any indication via inspection of the hulls that anything dangerous is occurring. Something you can do to offset some of the increase in pressure from the longer hull-shorter chamber combination is to have your forcing cones lengthened.

Sherman Bell published this in 2006, and I'm not aware of any other pressure data. Certainly not unexpected that pressures would be lower, with both a larger bore and some degree of gas escape if the wad does not adequately seal the ejecta.

In the 12g gun with 20g adapters:
A Winchester 20-gauge A-A Sporting Clays factory load of 7/8 ounces of #7 1/2 lead shot, fired in a Silver adapter, gave a velocity of 1319 ft/second and developed a pressure of 7700 psi.
The same 20-gauge load fired in a Go1d adapter gave a velocity of 1324 ft/second and developed a pressure of 7800 psi.
With all 0-rings removed from the Gold adapter, per your request, the velocity was 1302 ft/second and the pressure was 7900 psi.

Recent velocity report
https://www.shotgunworld.com/threads/gaugemate-insert-performance.558220/#post-4854142

Fitting the standard reducers designed for 2 3/4" chambers to shorter chambers does require some grinding

BD: please check your friend's empties for 'feathering' also

Thank you, Reverend.

All the best

Skeeter BD