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    20g chamber lengthening Archived Message

    Posted by Drew Hause on October 28, 2016, 8:14 am, in reply to "Re: 1919 20 gauge chamber "

    Lots of issues Phil; relevant primarily to small frame 20g and British light weight 12g game guns. U.S. maker's 12g doubles for the most part had thick walls at the breech.

    1. I've found no Hunter Arms' Specification Chart for wall thickness.
    2. Unlike Parker, Hunter Arms used the same FW frame for 16 & 20g
    3. I believe the gun makers designed the barrels in light of the pressures generated by the shells available and the tensile strength of the barrel material. c. 1900 20g 7/8 oz. 2 1/2 Dr. Eq. BULK Smokeless pressure was 8000-9000 psi; 7/8 oz. 2 1/2 Dr. Eq. DENSE Smokeless was about 11,000 psi.
    4. Actual tensile testing showed even Hunter Arms Armor steel to be similar in strength to modern AISI 4140 chrome moly at 100,000 psi. ONE report of Belgian sourced Ithaca tubes, likely from LLH and used by all the U.S. makers, was lower at 70,000 psi.
    5. Because of barrel eccentricity, using OD - ID does not accurately predict wall thickness.
    6. The critical dimension is the relationship between the forcing cone angle and the external taper of the barrel. As this illustration shows, usually angle of the cone is greater than the exterior taper and there is actually more wall thickness at the cone SO lengthening the forcing cone can be safely accomplished (and does not take a British gun out of proof) but may be dangerous in a 20g.
    7. Some 2 1/2" chamber U.S. 20s are cut short of 2 1/2". I had a Parker Trojan 20 made in 1924 with chambers close to 2 3/8".
    8. Factory original 20 and 16 gauge small frame guns may have a wall thickness of .090” at the end of the chamber, making chamber lengthening even more worrisome.



    Thoughts on wall thickness, by those much more qualified are here
    http://docs.google.com/document/d/1ZIo0y746UsSRZIgRuuxwAbZjSBHitO_EanvwLYc-kGA/edit

    I'd also add that most of us use low pressure loads in our vintage doubles. The low pressure frequently is attained using slow burning powders, which may actually generate higher pressures at the end of the chambers than fast burning powders.

    A old pressure-distance curve comparing Dense "Ballistite", Bulk "Schultze" and DuPont, FFFg, and progressive burning DuPont Oval



    I think the bottom line is that ANY barrel modification must be performed by an expert, and the smith must be MUCH more careful removing metal from small gauge/small frame barrels.

    BTW: it looked like a decent Monsoon in SE AZ. Have you seen any baby Mearns running around?

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