20 Magna is wildcat centerfire rifle cartridge based upon the 20 BR -a necked down .22 BR Remington -and is the smallest of the Magna family of cartridges which includes 6mm Magna, 7mm Magna and 10 mm Magna. The 20 magna was designed by budding gunsmith Stewart Gamble in an attempt to reach 4400fps with a 39 grain sierra bullet. Having a shorter neck length than its parent case and a blown out shoulder angle of 42 degrees ensures the 20 Magna has 15% higher powder capacity too. The shortened neck allows the bullets to self seat in the barrel ensuring perfect alignment and best possible conditions for accuracy. The 20 Magna is very fast with a muzzle velocity of 4450 fps with a 39 grain bullet fired from a 28" archer barrel with 1-10" twist. Barrel life is low, with low shot strings a predicted 500 rounds, but this can be increased by having the barrel salt bath nitrided which would raise velocities too.
Drawbacks include; very low availability of rifles in this chambering and reloading dies virtually non existent. Reamer dimensions are not published thereby each attempt at recreating this cartridge is different.
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