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Old January 1, 2009, 09:17 PM   #10
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 9,991
FALPhil,

Sorry to have missed this earlier. I picked it up today from your link in another post. I read your article with some interest. It reaches much the same conclusion about interchangeability that I have. I've certainly shot lots of match issue M118 and M852 in the past. There are a few caveats and fine points that could be addressed on the practical level, and that you might want to take into consideration if you do an edit?

First, Wogpotters table suggestion: I dug out a tiny Excel File I'd made awhile back. I'll put it up on my file repository so you can download it to use or ignore it as you see fit. It might serve Wogpotter's suggested purpose? You could easily expand on it to include other dimensions if you chose to? As it is, it is very short and was hardly worth bothering Excel with except as a convenient way to view it. Screen capture below:



You described the more compressive NATO EPVAT testing, but perhaps in somewhat overly glowing terms. There is nothing wrong with the protocol itself, AFAIK, but the implementation, except for external dimensions, would seem to be "more honor'd in the breach than the observance", except for the US made ammo. Read this rather distressing piece on disassembled NATO surplus ammo. In fairness, it is surplus, perhaps because of the very problems uncovered, but the differences seem a bit too wide spread for that to explain it all.

It has been my supposition that liability concerns have driven large scale commercial cartridge makers to load ammunition down below SAAMI maximums in most instances. U.S. military ammo seems to be loaded to spec. The US military ammo therefore recoils harder in most instances, flattens primers a bit more and generally leaves the impression that military loads are hotter than commercial loads. Well, in most instances they are, but not because the specifications call for it. That's the point of confusion: max SAAMI spec vs. what you actually buy. Even without measuring, I noticed some time back that military .45 ACP hardball had a tad more pop to it than commercial hardball. Military rounds are loaded to almost 400 ft-lbs, and a quick online survey of specifications claimed by big name commercial makers puts their standard pressure loads at about 350 ft-lbs.

You quoted Karl Kleimenhagen on Bramwell's curve fit. He is quite correct. Some time ago I noticed that Excel, possibly what Bramwell used for his regression, could not achieve anything but a linear least squares fit to his data. I realized that was probably because the data as he presented it in his Varmint Hunter article was random in order and the curve fitting algorithms were looking for pressure data numbers to be monotonic. I reorganized his data and tested to be sure his linear fit came out unchanged, then found two curve fits that returned higher R² test values than his linear fit did. Bramwells linear fit got R²=0.927. I got a power regression that returned R²=0.936, and an exponential fit that returned R²=0.939. A perfect fit would be R²=1, so none are fabulous fits, but the illustration with the formulae is attached, should you want a way to get a little closer?

Back in the late 1980's, when the U.S. started getting ready to host the 1992 Palma match, the Palma Committee got Sierra on board to design the 155 grains SMK bullet, and got Winchester on board to design a new .308 Winchester case with maximum internal capacity. Since case dimensions are only specified externally, that leaves a good bit of wiggle room to get clever. What Winchester came up with was a semi-balloon head design with about three grains more water capacity than standard .308 cases. This case’s weight averaged about 156 grains instead of the more usual 170 grains or so, and was headstamped "PALMA" (should anyone got confused). It is an notable, if meaningless, coincidence that the Winchester Palma case weight wound up close to the Sierra Palma bullet weight. Later, Winchester came to adopt this new design as their standard .308 Winchester case. Why not? It costs them less to make and it works fine. It is, however, unique among commercial .308’s for the head configuration on the inside and for total weight and case capacity. I treat it as a separate animal from other .308 commercial cases for that reason. I use it exclusively for long range, though I find I cull nearly 80% to get some within Lapua uniformity levels. I've also noticed Winchester has carried the semi-balloon head over to some other chamberings of late.

Most of the NATO brass I have (Lake City, IMI) seems to run in the 180-186 grain range, give or take, with IMI being the heavier ones. But if you do the math on the total weight, bullet weight and charge weight numbers for the pulled ammo tabulated in the story on NATO cartridges I linked to above, then subtract five grains for the primer, you'll see France made some cases that were only 162 grains. So, again, the generalizations get difficult to stand behind.

Case brass has a density of 8.53 times that of water, so when case external dimensions match, you can just divide that number into the brass weight difference to learn the water capacity difference. Figure the powder difference needed to match pressure and barrel time will be about 2/3 of the water capacity difference if the case is nearly filled wit powder. Otherwise, use 2/3 of the percentage change in total case water capacity (which means you'll have to measure one; my file repository has an Excel file to help with that which compensates for water density changes with temperature).

A hint for loose chambers. If you have a NATO chamber on the large side and are breaking in unfired new brass for it, you can reduce future casehead separations if you first load the brass as squib loads that fireform the case without applying enough pressure to stick the case to the chamber walls. This causes the case to back up and blow open more at the shoulder than at the pressure ring. I use a 100 grain plinker over 8 grains of Bullseye for this. Once initially fireformed, I keep in mind the rule of thumb that it only takes about 0.002" of shoulder setback to guarantee smooth feeding even in a self-loader, and I just don't try to set brass back farther than that, regardless of what size it came from the chamber. That marries the brass to the chamber, but together with annealing, it keeps cases working a long time.

Nick
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File Type: gif CUP vs. PSI.gif (22.9 KB, 83 views)
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