>>63986666
38-40 has a 33mm long case, and is moderately bottlenecked, with a full rim.
To magnum-ify 38-40 would be to make a case with those same features, but slightly longer at the neck, and loaded for more pressure. A lot more pressure, since there would be so much case capacity to utilize with modern propellant. The projectile would be the only similarity to the 10mm Auto cartridge, while the 38-40 magnum would shoot at a much higher velocity, and be far worse for feeding through double stack box magazines due to the rim. But also it would be bad for revolvers due to the combination of high pressure and bottleneck. So the 38-40 magnum would be a rather powerful round that isn't particularly well suited for any feed system or long range, but it would work alright in a tube mag or small box mag, and be backwards compatible with 38-40 ammo you have on hand, so there would probably be three, four, or hell, maybe even eight fudds in the world who would like it.
