Commercial .308 Win. Case Wall Thickness Uniformity

[Bart B.]

I've used the pressure ramp length (extractor groove to pressure ring max diameter) as an indicator of case wall thickness uniformity for decades. Here's an example of a once-fired Remington .308 BR (small rifle primer) case showing virtually zero ramp (left case in picture) on one side and maximum on the opposite side:

On cases I've cut the head off at the pressure ring and measured wall thickness, those with uniform ramp lengths all the way around the pressure ring have had quite uniform wall thickness; .002" maximum spread. On those with no ramp on one side (thick side) and a full, well pronounced ramp on the other side (thin side), wall thickness varied .003" or more. That variation also extended to the case mouth where its thick side was typically in line with the thick side at the pressure ring but the spread was about half as much.

In sorting a batch of cases to set aside those with non-uniform pressure ramps all the way around the pressure ring, here's the percentages of uniform pressure ramps in 200+ cases per make/lot of .308 Win. cases with headstamp in quotes:

99% - "WCC58" Western Cartridge Company match brass made for the US Army International Team with 200-gr. FMJBT match bullets.

98% - "PALMA 92" Winchester cases made in 1991 for the 1992 World Long Range/Palma Matches. The plant in East Alton, IL, had to retool/reset their case making machines and dies 3 or 4 times to get that uniformity in case wall thickness.

97% - "WCC60" Western Cartridge Company match brass made for the US Army International Team with 197-gr. BTHP match bullets.

95% - "R-P BR REM" Remington .308 Win cases, circa 1993.

93% - "FC 308 WIN" Federal match ammo with Sierra 168's and their hollow points swaged closed to be used as sniper ammo to meet the Geneva Conventions issue, military lot FC-81-xxxx xxxx


Question: What brand of cases have you found to be the most uniform in wall thickness?

 

[Johnboy]

If this counts in any way this has given me more reason not to weigh my brass. As I have said before one cannot make the same two pieces of brass every time. It all starts with the coil the cups are punched from. I like to look to compare it a pad of concrete. One cannot pour a pad for a driveway that is completely the same thickness, just as one cannot make a coil of brass. Then you move up to the different drawing stages then final shape. All of these steps with many different ones going at a speed of so many a minute just calls for different pieces of brass across the board.

thanks for sharing Bart you always give great food for thought

 

[Bart B.]

Johnboy, I think weighing brass into 1-grain spread lots is good enough. While some may want fraction spreads or even case capacity spreads, it's my opinion that these two are the least important of all variables in ammo to shoot one-holers (or smallest clusters) at any range.

Those same properties of cartridge brass are the same as jacket material. It wasn't until the late 1980's that bullet jacket copper was homogenous enough to make long, heavy 28 caliber match bullets. And 26 caliber ones had to wait until the 1990's before that happened. Shorter, light weight match bullets could be had sooner.

A good friend set the world on fire in 1970 shooting a 7mm Rem Mag at the Wimbledon Cup 1000 yard match at the Nationals using a borrowed rifle and ammo to set a record that stood for years. Sierra Bullets had one small batch of jacket material that was used to make a thousand or so of their first 168-gr. 28 caliber HPMK bullets (at about 90 per minute) that were used and they sometimes shot in the ones when tested in their 100-yard range. One of Sierra's machinist's (Arvie Martin?) chambered the barrel and fit it to that Winchester action that belonged to Sierra's ballistic technician (Matin Hull) who tested all their bullets for accuracy. It was 18 years later that they were able to again get jacket material that good. No wonder that most lots/boxes of their 7mm 168 HPMK's were not all that great and the 30 caliber magnums were the norm in long range matches.

 

[Johnboy]

Bart I agree with all you say. I too can compare the brass to plastic bottles, in a small way. I use to run a SYDAL spelling may be wrong, but when keeping it tuned in you had to make changes to keep the bottle in spec. The stretch rod was one of the changes you made many times a day during a shift to keep the bottle from having thin spots.( or you would change the push rod out for a new one and start over ) I can in many ways see this happening to brass while making a case from the cup. I know that I am not comparing apples to apples but see why the brass isn't always the same. I think that most problems in weight come from the thickness of the head of a case rather than any other part of the case. For this is why I will not weigh my brass, the head is thinner or thicker in my eyes. So I chalk it up to this causing the brass to weigh differently from case to case.( but in some small fraction this shouldn't change the inside volume of the case from case to case that much ) Even when one weighs his brass one cannot really say that they have 50 pcs of brass that weigh within the same amount. The trimming alone can cause different weights because one cannot trim the same every time over and over. Even the cutting of the primmer pockets will change from the different thicknesses of the head. As said before the coil is the beginning of the process, one cannot make a coil of several 100 feet the same thickness through an through. Just the pouring of the brass with the chemical mix will cause the thickness to change when poured into a large rectangle piece of stock. ( that will soon meet the rolling process to make a coil )

If in any way the inside volume is what matters to a great load, but all so can see the case not being the same to help aid in the reaction to the explosion when the case is fired. If a case is abnormal in many ways through and through then the case will react differently from the next causing it to change the way the bullet reacts when entering the bore. This is why I link the two ( chamber to brass as to gas and air mix to the cylinder of a bock. Still not apples to apples but in some manner it does, because if one case is different enough then the load is different, and if a cylinder is off then one will out push the crank faster than the other to cause a bearing to wear more on one side.

Bart I am sorry for ranting but need too for I always put to much into my thinking, and when one post a post like you did, well lets say it brought out the thinker in me again. BUT I must say it keeps me going and learning more each day, Plus it helps me to break my thinking down and reason to an answer and move on.

 

[OldManTA]

Folks,
There is an excellent article by German A. Salazar about the Creighton Audette method of case wall thickness checking. There are illustrations showing the technique and the equipment used along with a technical discussion. Take a look at www.riflemansjournal.blogspot.com. There is a wealth of information there and I urge you to click on any ad you see to help keep the site going.
Yom Alves

 

[rminut]

Bart,
do you have much experience with the Remington BR brass on the 1000 yard line? Why didn't the palma team use it during the 90's?

 

[Bart B.]

rminut, I've shot a few hundred rounds of Remington's .308 BR cases in Palma matches. With Rem 7.5 primers, they shot as accurate with Sierra 155's as my WCC and 92 PALMA cases in warm weather. I've heard that in cold weather (temp's below 40 F) the Rem 7.5 primer ain't hot enough to give low muzzle velocity spreads. I had two hangfires using Rem BR cases in a Palma match shot at 28 F at the Whittington Center in early March; that convinced me to use large rifle primers all the time. Otherwise, in warm and hot weather, those Rem BR cases are as good as any as long as their wall thickness spread is small.

As one of the half dozen or so former US Palma Team members who worked up loads for Sierra's new Palma bullet in early 1991, I got some insight as to why Winchester was selected to make the cases. We all used new cases with weights about 165 grains as that was what Winchester's plant in East Alton, IL, felt would be what their dies would make, That plant still had the case making dies for their famous WCC58 and WCC60 cases used for the US International Team's free rifles. But they didn't want to make those thin-wall 150 to 155 grain cases for use by folks who would load them way over safe levels. Winchester was also the only ammo company that would make match cases with uniform case walls at a reasonable price. Plus, Winchester had a good record of making accurate ammo for their 30 caliber Model 70 match rifles, too; Remington did not. Federal and Remington was way too expensive in their quotes as I remember. Remngton's .308 Win. match ammo was never anything to write home about anyway and neither were their 30 caliber 40X centerfire match rifles. So Winchester made new coin, cup, draw, head, bunt and necking dies based on the design of their famous WCC ones decades ago; they did it right and well priced, too.

To my pleasure, I shot the high agg score over 4 days of long range matches in June, 1991, when those 92 PALMA cases with Sierra's then new 155's were first used in a match. Several other country's Palma Team members were there and we all agreed that ammo shot about 1/2 MOA at 600 yards in spite of a 3/10ths spread in IMR4895 charge weight and .0035" max bullet runout with brand new cases in different bore/groove/chamber dimensioned barrels. Those bullets mic'd .3084" and my thirty-inch Obermeyer 1:13 twist barrel had .2980" bore and .3070" groove diameters. Some of the overseas shooter's barrels had .3065" groove diameters.

 

[Bart B.]

Johnboy, I understand your concerns about case head dimensions. But for a given case weight, it has the same mass and will subtract from the chamber volume the same amount regardless of where the thick and thin parts are. This is why some of the .308 Win. shooting match winners and record holders for matches sort cases by weight and not volume. Besides, the more out of round a case is for a given case weight, the less water it will hold. There's no such thing as a perfectly round case. Well, maybe 1 in a few thousand.

There are a few who claim muzzle velocity is different for cases of the same weight but different volumes. And they measure case volume only after firing a case 2 or 3 times neck sizing only so it closely represents what it will be at peak pressure as the case is pressed hard against the chamber limits.

 

[rminut]

thanks Bart.