Can someone post the "formula" for sizing a magazine box to the proper dimensions for a specific cartridge? I remember some lengthy discussions on this issue awhile back, where someone cut out the triginometry and posted an actual formula, but with the search function gone I can't find it any longer.

FYI, I'm working on a P14 Enfield that has been rebarreled in 300 H&H, using one of the aftermarket (Numrich Arms?) "magnum" magazine boxes. Given the taper of this round I'm wondering if I need to open it up in the back a bit.

Tape 4 rounds together staggard and see how they fit, then allow for about thirty thousands clearance...but on your project, as I recall, you just need some rail work, the magazine is OK for the 300 H&H, if not it just needs to be opened in the rear a bit...

I believe what you're inquiring about is held by most to be a black art; possibly requiring pure water, a goat, and maybe a virgin or two. From what I've read, only the best of the riflesmiths know how to properly and mathmatically construct a properly fitting and functioning magazine box and I don't believe I have yet encountered where anyone has given the secret away yet. -I'd sure like to know myself...-

This aught to get you started in your search: Magazine Geometry.

Good luck and happy hunting!

From the thread linked above, Systeme 98 posted this formula:

The formula for optimum magazine width is:
(Cosine(30degrees) X base diameter) + base diameter

For a .30-06
(.866 x .473) + .473 = .882 inches

For a .300 Win Mag
(.866 X .532) + .532 = .993 inches.

This is for the magazine width at the rear. I think you have to also calculate the width at the shoulder to do it the way Mr. Mauser intended.

Some of the material on the history of the Mauser which I have read indicates good old Peter Paul Mauser himself developed and used that "cosign rule" in the manufacture of original mauser magazines and rails...for both the base and the shoulder areas of the various cartridges for which they were built.

Supposedly it was one of his secrets for the flawless feeding of the various cartridges used in his military rifles for various countries with various cartridge base diameters, shoulder diameters, and lengths.

Whether that is true or not, I can't say because I am not 135 or so years old, and probably would have never met Mr. Mauser anyway. But it IS a thing I've read in more than one place.