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Stoneypoint Headspace gauge

 
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  #15  
Old 04-19-2006, 05:58 AM
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Re: Stoneypoint Headspace gauge

[ QUOTE ]
Bench rest accepted sizing practice has also moved about quite a bit from what I can establish and the records seem pretty awesome throughout this.

[/ QUOTE ]I'm not one who uses benchrest records to base their accuracy on. The smallest ones usually have 5 shots and a few have 10 shots. Don't forget that all the other groups fired in the same discipline and type of match are larger; much, much larger. And rarely does the same rifle used to establish a record better it setting a new one.

Benchrest records that most realistically reflect accuracy attained are the aggregate ones. Several 5- or 10-shot groups are fired, then measured to establish their averge size for the aggregate size. But whatever that average group size is, don't forget that some of the individual groups are larger. How much larger is not often published.

All of which means to me it's best to consider accuracy as about the largest group fired. That's what can be counted on the majority of the time.

The best analogy of the "smallest group syndrome" I've heard was from a ballistics engineer at Lake City Army Ammunition Plant. I'd called them to ask about some 7.62mm NATO M118 match grade ammo our unit had got that was pretty bad. We talked about their accuracy standards and test processes. He related an incident where they had just tested a very good lot to be sent to the Nationals. They had shot about 300 rounds into one group from a test barrel at 600 yards. After getting the target with it's two widest shots a little over 6 inches apart and calculating the mean radius for each shot from group center to be about 2 inches, one of the engineers looked at it and said: "Wow, look at all those half-inch 5-shot groups! There's dozens of 'em!" Another replied by saying: "Yes, there's a bunch of them. Too bad they're not all in the same place."

Which is why I shoot at least 15 shots per group and 20 is better; anything less starts getting less and less meaningful. If one shoots enough few-shot groups using any reloading technique they'll eventually shoot the smallest one again. Then that becomes their "record group." The smaller that record group gets the harder it'll be to equal or better it.
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  #16  
Old 04-20-2006, 05:12 AM
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Re: Stoneypoint Headspace gauge

Hired Gun, here's some comments and answers.....

Your comments: “It is normal accepted practice and my personal experience that fire formed brass consistently presents the bullets centered in the bore and the case head in close proximity to the bolt face for consistent primer ignition.”

Seating bullets out to engage the rifling when the bolt’s closed on the cartridge will also center them before they’re fired. Besides, when a rimless bottleneck cartridge whose case head space is several thousandths of an inch shorter than chamber headspace is fired, it gets driven hard into the chamber shoulder by the firing pin and that centers the whole front end of the cartridge. With the firing pin set to protrude .055- to .060-inch in front of the bolt face when stopped, that’s more than enough to get very consistent primer ignition even with .005-inch or more headspace clearance on the chambered round. With belted cases having only .003-in. difference between GO (.220-in.) and NO GO (.223-in.) belt headspacing gages and case belt headspace in the normal .216- to .218-in. range, there’ll be no more than .002- and .004-inch headspace slop when fired in a proper .220-in. GO or “zero headspace” regardless of how far back the shoulder is. Besides, the firing pin is speeding up all the way to the primer. So, the further away the primers are, the harder they’re hit which keeps their ignition qualities quite uniform.

Your comments: “In bench rest they even cut chambers and necks undersized to tighten the fits up even more. This theory applies to all bottleneck cases.”

I know that’s what they do. But there are other bullet shooters who get the smallest groups when the cartridge case fits the chamber like “a rat in a fiddle case” or “a turd in a punch bowl.” Those are phrases originally coined by Martin J. Hull, Sierra Bullets’ first ballistic and test range person who proved to many folks that proper full-length sizing with just a bit of shoulder set back produces the smallest groups. That’s still how Sierra sizes their cases used to test bullets for quality as well as developing reloading data in all kinds of rifles. I don’t think anybody shoots their bullets as accurate as they do. Their best match bullets shoot in the ones (.100 to .119 MOA range) from full-length sized cases; even the 30 caliber ones. And they just throw charges from a measure; none are weighed.

Your comments: “Now I have to ask, why is a belted case is not going to benefit from the same time proven methods?”

They will benefit from that time proven method you mention when full-length sizing sets a rimless case shoulder back way too far or a belted isn’t sized all the way to the belt. Such full-length sizing techniques are very common so it’s normal that people get better accuracy by neck-only fired case sizing.

One thing neck sizing does that improves accuracy over this method is the case headspace is long enough to keep the tiny step in front of the belt away from the chamber's belt shoulder. The extractor pushes the case head sideways so the case body bears against the chamber wall opposite the extractor. That's the point where that tiny step slams into the belt step that causes accuracy problems. When that tiny step isn't there, the entire case belt stops against the chamber's belt shoulder all the way around and accuracy isn't degraded.

Full-length sizing means reducing body and neck diameters. How much depends on chamber and die diameters combined with the peak pressure when the round’s fired. When full-length sizing, you don’t have to reduce fired case body diameters by more than about .003-inch. Any more than that is more than needed and it can work the brass right in front of the extractor groove or belt too much and will work harden the pressure ring area pretty fast which leads to incipient through partial and sometimes complete case head separation. With most folks full-length sizing cases fired in factory chambers typically larger in diameters than SAAMI minimum specs, they’re going to reduce diameters quite a bit. And without setting their die using a case headspace gage to ensure it’s not setting the shoulder back too far, most folks do indeed set the shoulder back too far. All of which means short case life. The more the fired case headspace is shortened, the shorter case life will be. Folks then size only the fired case neck because they don’t know what else to do and that method does give much better accuracy than improperly full-length sized fired cases. With so many people doing it so often, it’s only natural neck sizing has become a ‘time proven method.’

Another ‘time proven method’ has been used by match winning and record setting highpower rifle competitors for over 50 years. But because their targets’ scoring rings are rather big and they shoot from positions supporting rifles with quivering muscles, their scores for 20 shots are great but shot hole groups often go to the 2, 3, and 4 MOA range from 200 to 1000 yards. That doesn’t impress the benchrest crowd nor hunters striving to get half MOA groups at short ranges. But some of these folks clamp their rifles in machine rests to test them and their ammo. They learned over 40 years ago that best accuracy was attained with either new or proper full-length sized fired cases. The smallest 600-yard test groups fired in these box magazine rifles using full-length sized cases have yet to be equaled by any benchrest rig using cases sized any way. And using new belted cased, they get better accuracy than neck sizing or conventional full-length sizing. Only when belted cases are double sized reducing the body diameter at the belt do they equal new cases. All the military long range rifle teams learned this some 40 years ago; they quit reloading and used new cases all the time. These folks use chambers with more case clearance at the neck than benchrest tight neck chambers have and equal or better their accuracy.

Testing reloads for accuracy, in my opinion, needs to be done such that whatever the group size is can be counted on at least 80% of the time. The smallest of several few-shot groups is very popular to use. It’s also the worst method to use. When all the other groups are larger than the smallest one, that proves the smallest one is the worst one to base accuracy on and tell the world how that stuff shoots. The largest few-shot group is about what one can count on most of the time and that’s what should be used to base accuracy on. Especially when the smallest few-shot group produced gets harder to equal or better each time accuracy is checked. 15-shot test groups are about 10 times more meaningful than 3-shot ones and about 5 times more meaningful than 5-shot ones. How many shots per test group to you shoot?

“Please also explain how you get any case life at all if you keep pushing your belted cases back so far that they have to head space on the belt alone.”

Back to what I mentioned above, the shoulder doesn’t have to be pushed back very far for belt headspacing to happen. And even with a second body sizing die reducing fired case body diameter a bit right to the front edge of the belt, when minimum SAAMI chamber body diameters and shoulder position are in the barrel, full-length sizing fired cases from such a chamber will enable 15 to 20 reloads per case. Rimless bottleneck cases have been properly full-length resized 80 to 100 times without any problems at all.

“In your last post you referenced people going to neck sizing and claiming improvements in accuracy. I fall in that group. I don’t even own a full length die for my 257Wby. With fired Lee Collet neck sized brass it shoots in the .2s”. With new brass it shoots ¾ to 1” with the exact same load. What do you think you could gain on this one with your method?”

First, does your .257 Wby. shoot in the 2's all the time or is that the best it’s shot?

Second, it would be nice to know what your chamber and new case dimensions are. Something’s a bad mismatch if new cases don’t do at least as good. Which means you got to do some measuring. New cases have to have their necks uniformed by running them through at least a neck sizing die with an expander ball so they’ll hold bullets straight and with uniform tension. Failure to do this often results in poor accuracy.

Third, I seldom test at short range but usually at 600 or 1000 yards. Accuracy isn’t linear from 100 yards out to 1000 yards. Muzzle velocity spread, tiny ballistic coefficient differences and the air’s never completely still; these open up the groups as range increases. I get at worst, 15-shot test groups in the 6 to 7 inch range at 1000 yards with belted cases both new and resized my way. To do that well at 1000 yards, they’re probably in the ones at 100 yards. Whenever I tried neck only sizing, tests groups were 2 to 3 times as big.
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  #17  
Old 04-20-2006, 05:52 PM
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Re: Stoneypoint Headspace gauge

[ QUOTE ]
With fired Lee Collet neck sized brass it shoots in the .2s”. With new brass it shoots ¾ to 1” with the exact same load.

[/ QUOTE ]I think I've an answer as to why those new cases didn't shoot so well. Sorry I forget to post it earlier.

New cases with their heads quite a bit out of square will not shoot too accurate. Even with a two lug bolt face that's been squared with the chamber axis. When the high point of the case slams against the bolt face, that sets up barrel movements that cause bad shots.

The good thing that happens is they're fairly squared up after the first firing. The hotter their first load was, the more squared up they'll be against a squared up bolt face. Which means they'll shoot more accurately when they're reloaded regardless of what sizing technique is used.

One can easily check case head squareness by setting a case on a hard, flat surface against a V-block, then spinning the case about its axis keeping the head squarely on the surface. If the head's out of square, the case will rotate in a circle. The mouth will appear to nutate or go through a coning motion. Of course, if the case neck's bent out of alignment with the case body, that will change things a bit.

What's interesting is to shoot a new case with its head way out of square, then reload and shoot it several times checking it for head squareness each time. They do square up a little bit at a time.
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