I once came across something called the Greenhill Formula, which is supposed to give you a rough idea of the twist rate necessary to stabilize a bullet. The formula is:
Twist rate = 150 * BD^2 / BL
(where BD = bullet diameter in inches, and BL = total bullet length in inches)
(Also note that "*" ="times", "^2" ="squared", and "/" = "divided by")
By plugging in various bullet lengths, you can figure out which length will stabilize with a 48" twist.
For the calibers (bullet diameters) you listed with a 1 in 48" twist, the longest bullet lengths that would stabilize are:
45 cal - 0.633 inches,
50 cal - 0.781 inches,
54 cal - 0.911 inches.
These are only rough guidelines to get you in the ballpark. I have found I can break this rule with some bullet designs, whereas other bullet designs that should work according to the math don't stabilize. Ultimately, you can only find out by trying the bullets in your gun to see if they actually work.
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