Part 1 - Background
Lots of words follow. Please join me in mourning the electrons that have given their all in their production.
Short version of the whole thing: Hornady has published contradictory information about 450B trim length, and has published a very confusing statement relative to trimming.
Case length and trimming to length can be significant factors in reloading the 450B. The cartridge headspaces on the case mouth. The length of the case from base to mouth determines headspace relative to chamber length. If you get case length wrong, you can have trouble (… with a capital T and it rhymes with P and it stands for Poof ! - or Kaboom!)
In a recent thread, a forum member made a statement about trim length as described by Hornady. The statement contained a typo, but even corrected it didn't make much sense otherwise, and I replied to that effect. Since writing that reply, I came across the likely source of the confusion of his statement. I've apologized to the forum member 450 Bushy help needed.
Let's locate the contradiction first, starting with the information in the 8th edition of Hornady's Handbook. The information about the 450B begins with a list of some critical measurements. Here's a scanned image of these.
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The 450B max case length is 1.700", trim is 1.690", with a 0.010" difference. We note this difference of 0.010" is the usual specification for most cartridges, rifle and pistol, in the Handbook.
However, there are quite a few exceptions, mostly straight-walled cartridges that headspace on the mouth of the case. For such cartridges, trim length is 0.005" less than max. These include the 380 Auto, 9mm Luger, 38ACP, 38 Super, 9x18, 40 S&W, 10mm Auto, 41 AE, 44 Auto Mag, 45 Auto, 45 Win Mag, and 50 AE.
This makes obvious sense. Trimming affects how these cartridges headspace in the chamber, and too much trimming produces excessive headspace, resulting in various problems.
So, we can make a general rule from the Handbook information. Cartridges that headspace on rims, belts, or shoulders will have trim lengths 0.010" less than max. Cartridges that headspace on the mouth will have trim lengths 0.005" less than max.
Alas, there are exceptions to our general rule. The Handbook lists four straight-walled cartridges that headspace on the mouth and have 0.010" trim differences, not the expected 0.005". These include the 25 auto, 32 auto, 30 Carbine, and the 450 Bushmaster.
(For the nit-pickers, the 30 Carb ctge doesn't have straight walls, altho the chamber walls are straight. There are still other exceptions to the general rule. I counted six, but these cartridges are odd in other ways.)
A possible simple conclusion: The 450Bushmaster falls with the 25 auto, 30 carbine, and 32 auto in the exceptional group of cartridges that headspace on the case mouth, but have a Hornady-specified trim length 0.010" shorter than the max.
So what's the big deal? See next post.