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Old November 10, 2018, 02:08 PM   #14
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 15,378
If you're not married to the idea of using shotshells, then Houndawg's suggestion is a good one, assuming you haven't bought bullets yet. I say that because the groove diameters of the seven different 40-70's vary from 0.403" (40-70 Ballard) to 0.417" (40-70 Maynard), but none fall right on 0.410". 40-70 Winchester has the most case capacity.

All those rounds should handle 45-70 Trapdoor Springfield pressures (22,000 psi or so) in guns normally chambered in them. The Cowboy Action Shooters have revived a lot of these formerly-obsolete rounds, though there are no SAAMI standards for the 40-70's as they preceded standardization but have not regained enough popularity to interest large-scale ammunition makers. The CIP, however, has standards for the 40-65 (about 30,000 psi) and the slightly bottlenecked 40-82 Winchester (about 24,000 psi), which is why I think you will be fine with Trapdoor pressures in the 40-70's and can probably go higher.

I'll add what I think is a better suggestion: If you just want a straight cartridge in the 410 shell size range, go to the 405 Winchester, the most powerful rimmed straight wall cartridge ever made. It is old (1904), but has been revived with enough modern popularity to garner a SAAMI standard (while absent from the 1992 SAAMI standard, appears in the 2015 SAAMI standard). It has a 2.583" case and a 0.412" groove diameter, which could shoot a .410 jacketed bullet if you wanted it to (if not quite as accurately as a 0.411-0.412" jacketed bullet). It has a 46,000 psi MAP, so it will contain all the pressure you need to hit your velocity range (it will do over 2,200 fps with a 300-grain bullet). It should be very happy with wide meplat flat nose hard cast bullets, which are terrifically effective on game, and which you could get a mold for or buy. You could even get a .404" bullet and paper-patch it if that interests you. The 405 Winchester (aka 405 WCF) simply gives you more options than the other choices we've discussed.
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