Making 450B brass from 7.5x55 brass284 brass has been used to make 7.5x55 Swiss for decades, ever since the 284 was introduced. The extractor rim of the 284 is smaller in diameter than the rim of the 7.5x55, but it seems to work adequately in Swiss rifles. What's proposed in this thread is making reverse conversion, from 7.5x55 to 284, and then lopping off the shoulder and neck to end up with a case 1.700 inches in length.
To check on some dimensions, I dug out my 7.5x55 brass and made a couple of measurements. (I used to shoot my long M1911 in old military matches; it wasn't as accurate as the later K31, but with my eyes the long sight radius made up for that.) I have some 7.5x55 brass made by Norma, and some with a Grafs headstamp, probably made by Prvi Partisan (PPY). I found the following diameters, in inches:
Norma: Rim=0.496 Head=0.490
Grafs: Rim=0.496 Head=0.495
Nominally, these two dimensions of the 7.5x55 are supposed to be: Rim=0.498, Head=0.493. These dimensions are maximal; most cartridges have acceptable tolerances of
-0.010" for rim diameters, and
-0.008 for body diameters. So, minimum acceptable dimensions are Rim=0.488, Head 0.485.
The rim diameters of the Grafs and Norma brass are within specification. The head of the Norma brass is also OK. The Graf brass head is 0.002 too large. Fortunately the chamber on my Swiss rifle is generous.
Here's the case head problem with using the 7.5x55 brass for the 450B, illustrated:
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As Hoot warns, to use the 7.5x55 brass for forming 450B brass, it will be necessary to turn about 0.020 off the rim diameter of each piece of 7.5x55 brass to obtain a diameter of 0.473. The extractor groove will likely be ok.
The head of the 7.5x55 brass will bulge a bit when fire formed in a 450B chamber. The head of the Grafs brass is larger than specifications, and that makes it a little bit better for the conversion to 450B. There will be a notable bulge near the case head, above the web, when the 7.5x55 case is fire-formed.
The 7.5x55 brass uses a LR primer, which may or may not cause problems, depending on the bullet and powder combinations.
More on this conversion follows in the next post.
--Bob
edited to repair php code & incomplete sentences
