Don't mix your brass!
The 7x57 mm Mauser cartridge has been among our most satisfactory handloader's rounds for more than 50 years, but it will serve to illustrate the point very nicely. A characteristic of the old 7x57 has always been extreme variations in cases, depending upon the maker, and everybody and his brother have manufactured 7 mm Mauser cases at one time or another. Back in 1958, when I first began playing with this cartridge, i was using a custom-barreled rifle with a pretty tight chamber. Using Winchester cases, I developed some good loads and became aware of the minimum chamber. Then I got hold of a lot of Norma brass. Knowing it was heavier--and thus thicker-walled, with reduced internal capacity--I reduced my starting charges by a full ten percent in the first rounds made up in it, confident that such conservatism was unnecessary.
To my astonishment, the first shot fired blew the primer! So did the second, foolishly traggered in the hope that the first one had been in a soft case or one with a loose primer pocket. There was no third shot, you can bet. Subsequent trials demonstrated that that particular lot of Norma brass required a reduction in maximum loads of a full six to seven grains of slow-burning powders for safety.
I was reminded of that 25-year-old experience recently when developing hunting loads for my wife's custom 7x57 mm. We currently have three rifles in the chambering around the house, and I'd decided to keep brass lots separate by using different case brands exclusively in each arm. Any W-W case in my possession will have been fired only in my Ruger M-77, whereas any and all federal cases are for my wife's customized, Douglas-barreled Mexican Mauser, and so forth. Since the differnt guns take different maximum loads and require different bullet-seating depths, that's a convenient way to avoid mix-ups.
However, in the course of assembling some loads, I noted the powder level seemed to be standing unusually high in some new Federal cases, so I weighted tne of them, comparing them to W-W and R-P brass. We usually expect Winchester brass to be lighter than other brands, of course, but in this case the difference in average weight was a full ten grains! I do not recall having encountered so great a variation in so small a case before. And, needless to say, such a variation means a radical difference in pressure levels with teh same loads, very likely a hazardous one, if ignored. This is nothing against Federal brass, of course; it's of excellent quality . . . just heavier.
another problem with the 7x57 mm is that one runs into rather extreme differences in rim thickness and extractor cannelure design, so that cases of different brands may actually require different shell-holders. The only other round with which I can recall a similar situation is the .35 Remington, and that was a long time ago.
Obviously, the solution of both problems is simply to standardize on one case brand. But, if you're like me, you can't resist a chance to scrounge a good batch of brass, no matter what is headstamp. If so, take great care in keeping lots separate and when substituting in established loads Paul Mauser's excellent little 7 mm ... or any other cartridge, for that matter. LEE PRECISION AUTO DISK
Lee Precision, Inc. has introduced a new powder measure called the "Auto Disk" for use with their turret press and dies, and claims it to be the most accurate pistol charging system in the world. The measure screws into the top of the Lee expander die and is automatically actuated by the case itself at the top of the handle stroke. The device comes with four interchangeable precision plastic disks, each of which has six different cavities. Virtually any charge of any powder suitable for cartridges up through the large revolver rounds nand small rifle cartridges to about the .222 Rem.) can be thrown with one of these cavities, according to a chart furnished. A quick test with three different powders--a ball, a flake, and an extruded type--proved Auto Disk charges to be extremely uniform indeed, but not necessarily exactly what the chart indicated. A check against a powder scale is essential (and recommended by the manufacturer), and sample charges should be taken during normal operation of the equipment.
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|Title Annotation:||cartridge reloading|
|Publication:||Guns & Ammo|
|Date:||Apr 1, 1984|
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