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Old 08-09-2018, 10:22 PM
22_boomer 22_boomer is offline
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Default need calculations for 38 adapter in 20ga shotgun.

I have the calculations for thrust for 38 Special and it's a hair over 1900 pounds and 357 magnum is 3948 pounds of bolt thrust but can't seem to google the thrust of a 20ga cartridge (2.75" shell) so I'm wondering if the adapter will be safe. So far I've built two adapters but all are in small caliber and I wanted to move up to a larger easy to find straight wall cartridge -- the 38 special or even the 357 magnum were what I wanted. The shotgun is a 1945 model H&R Topper M48 and there is a big difference between the shotgun frame and the rifle frame. The shotgun is cast iron and the rifle is steel. If I could find information on the thrust of the standard 20ga 2.75" shell I can figure out what I can build the adapter for. Does anybody here know what the thrust or bolt thrust would be for a 2.75" 20ga shell -- either shot or slug -- whichever is more.

Edit to add; I looked up the chamber pressure for a 20ga and it showed 18,275 psi for a Federal 20ga 2.27" unfired case. Bolt thrust for the shotgun shell comes to 6873 pounds. I sure hope the numbers I find in Google are correct. The bolt thrust numbers have to be calculated; bolt thrust = Maximum chamber pressure X inside area of case head. The math does not take into account the internal friction but who am I to question. Any help would still be appreciated.

Last edited by 22_boomer; 08-09-2018 at 10:52 PM.
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Old 09-08-2018, 06:25 PM
squirrel_slayer squirrel_slayer is offline
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Default

That could very well be a proof level load. My aliant data shows most 20ga data in the sub 12k psi realm for max. I wouldn't think the hull length would matter as it would likely offer minimal friction the rear brass is likely the only thing that would account in my opinion. yes the hull seals to the chamber walls but does it really offer much?

now as for the gun it's self. I want to say the M48 is on what most call the SB1 frame which is rated up to 44 magnum.

ETA: one thing I forgot to mention is the firing pin and hole can become a pressure limiting factor in these guns due to primer flow into the receiver face which can result in very difficult opening of the action or in extreme cases pierced primers.

Last edited by squirrel_slayer; 09-08-2018 at 06:34 PM.
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Old 09-08-2018, 08:09 PM
Eddie Harren Eddie Harren is offline
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Default 38

Don't let the part about being cast scare you. Ruger actions are cast and are some of the strongest actions on the market.
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