I like having a couple oddballs, but not so odd that the local big box store might not have some. Right now my main oddball is a .450 Bushmaster.

Have had good luck buying all the .38Spl, .44Spl, and .44Mag I want. .357 is a little tougher.

Piles and piles of .45ACP in some stores near me. 9mm...good luck! Seems everyone went out and bought a 9mm plastic gun.

I don't think Tappan's methods are quite as valid anymore. He lived and wrote during the heyday of milsurp when there was a really good argument for piling up loads of cheap military calibers. Now the only cheap ammo is Russian steel case, of which I get 7.62x39 and little else. As noted above it's more useful to spread out into as many common calibers as you can tolerate with a couple uncommon but current production calibers like .41Mag, .450 Bushmaster, 7mm-08, etc.

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Psalm 27:2