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spring Offline OP
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I’m looking at the nitro proof on a 20 gauge I recently bought and am curious about what loads it can—and cannot—handle. With a 20, of course, you have a variety of load options, and I often use 2 3/4” shells with 1 ounce of shot. What’s the best way to know if that’s good to go with this gun?


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The gun was re-proofed to 2&3/4" after the time that shot charge was part of the proofs. Originally a 2&1/2" gun I'm sure but an inquiry with H&H would confirm. I would shoot light loads in it most of the time myself, though I'm sure it would be fine with occasional 1 oz loads. CIP 2&3/4" cartridges operate at a fair bit lower pressure than our SAAMI cartridges.
Steve


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The 1954 Rules of Proof “Highest Mean Service Pressure” equivalent transducer values as converted from LUP by Burrard’s formula:
3 tons = 8,960 psi = 618 BAR
3 1/4 tons = 9,800 psi = 676 BAR

Major Sir Gerald Burrard, The Modern Shotgun, Volume II, “The Cartridge”, 1955 3rd Revised Edition listed standard service load pressures:
20g 2 1/2” 3/4 oz. 2 1/4 Dr. Eq. - 9,632 psi
20g 2 3/4” 7/8 oz. 2 1/3 Dr. Eq. - 9,968 psi

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What does the gun weigh?

The accepted formula is that you need 6 pounds of gun weight for every ounce of shot in the cartridge.

If you match the load to the weight of the gun, it will be kinder to both you and the gun if you use that as a maximum shot weight.

Remember, less can be more and a lighter load may keep you on target if you are not being knocked cross-eyed by the recoil of too heavy a load.

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I use 8,000psi as my default British proof 20 bore loads in both 2 1/2” & 2 3/4”. Remington 20 hulls are 2 9/16” so I no longer trim them to 2 1/2” but I do for the rest. It all comes down to being certain that every shell I have is safe to shoot I every gun. I could go up to 9,500psi and still be safe but I do not for two reasons. Old wood cracking is more of a concern than barrel failure. The second concern is shooting 2 3/4” shells in 2 1/2 chambers would result in slightly increased chamber pressures so I err on the safe side.

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Vic,

All of the European ammunition and most shotgun ammunition manufactured by US companies here are CIP compliant and each box will have the mark of the proof house that tested the load.

Look for the "inspection of ammunition" proof mark on the box - they start on page 7 of this document https://www.anarma.org/media/kunena/attachments/1233/genco_en.pdf . The most common I see on US ammunition boxes are Birmingham, Liege and Munich but I have seen others.

I would have no hesitation shooting US made 20 gauge cartridges with 7/8 ounce of shot at 1200 fps or less in a nitro proofed gun. My only concern shooting 1 ounce CIP compliant loads would be recoil damaging the stock not a pressure issue; recall that recoil is mass of ejecta and velocity related and has nothing to do with pressure.

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When a British shell box is labeled:

These cartridges are suitable for use in:
70mm case length: Guns with a chamber length of 2 3/4” or longer, nitro proofed to a service pressure of 3 1/4 tons per square inch (900 kg per square cm)
67 mm case length: Guns with a chamber length of 2 1/2” or longer, nitro proofed to a service pressure of 3 tons per square inch (850 kg per square cm)

The CIP transducer “Maximal Statistical Individual Pressure” of 850 BAR is for a “Maximal Average (Service) Pressure” of 740 BAR = 10,733 PSI
900 BAR is for a “Maximal Average (Service) Pressure” of 780 BAR = 11,313 PSI

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Bob,
We need to chat so you can give me a lesson in this stuff!

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Try the 3/4 oz loads that the 3/4 - 20 gauge pattern control wad make easy to load. They are pleasent to shoot and efective.
bill

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Originally Posted by spring
Bob,
We need to chat so you can give me a lesson in this stuff!

Vic,

Give me a call 803 530 5380. I'm not available tomorrow but should have time Friday after about 9:30 until about 4:00 and almost all day Saturday. If I don't answer leave a message with a good time to call you.

Bob

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