More on Case resizing

[survivalshop]

  He's some more interesting reading on case sizing .

CASE-SIZING ADDITION: APPLES, ORANGES, AND BANANAS

MARCH 4, 2016 MIDSOUTH SHOOTERS 8 COMMENTS

The following is a specially-adapted excerpt from the forthcoming book, “Top-Grade Ammo,” by author Glen Zediker, owner of Zediker Publishing. Click here to order from Midsouth.

by Glen Zediker

First, I want to thank everyone who’s reading “Reloaders Corner,” and especially those who take the time to post comments and questions. That’s the fun of the web for me: it’s a closer connection to my readers and fellow shooters. Judging from a few responses to the last couple of articles on case sizing, I’d like to offer a little more detail to ensure there’s been no misunderstandings or misinterpretations.

Here’s a nice 1000-yard target from David Tubb, 11-time NRA National High Power Rifle Champion, 6-time NRA High Power Long Range Rifle Champion, two-time Wimbledon Cup winner, and current Long Range World Champion. This target was fired from prone using full-length-sized cases, with carefully constructed rounds. The rifle is a TUBB 2000 bolt-action chambered in the 6XC cartridge. Tubb sizes the case bases an additional 0.0005 smaller than SAAMI specs, and sets the case shoulders back 0.002 inches.

I don’t have a lot of space here to cover all the smaller but often important peripherals associated with any larger topic. In the few hundred words I have available, it has to be a more specific treatment of a more specific thing, and that’s why I’ve been doing some of this material as series installments. There are a number of folks who claim neck-only sizing is necessary for best accuracy, and, in effect, that full-length sizing is a compromise, favoring function over accuracy. I want to use this space here this time to clarify a few potential confusions. And I’m not even really disagreeing with anyone.

To review, neck-only sizing is when only the case neck, all or some portion of it, is sized to adequately retain a bullet for another firing. The case body is not touched by the die interior, and the case shoulder may or may not be set back; that depends on the die design and operator preference. The idea is to better preserve the fired case dimensions; that is, make the case more closely mirror the rifle chamber’s dimensions. One advantage of neck-only sizing comes to those who expect dozens of loadings from a case. This tactic does, indeed, minimize case stretching on subsequent firings.

Mostly, do not get the impression that full-length sizing — essentially following the steps and methods I suggested in the past two articles — is short-circuiting on-target accuracy. It’s not. Not if tooling is what it should be and the operator makes the investments in money and time to gauge influential dimensions.

One old accuracy trick is to reduce ejector pressure. That’s easily done in most rigs: just shorten the ejector spring, if you know what you’re doing. The author does that, or has it done, on all his rifles. The ideal amount is to have a stress-free contact of the spring against the ejector at installed height, such that the spring isn’t compressed until the ejector moves in as a round is chambered. That’s usually the minimum pressure needed to make it functional and doing its job 100%.

There are some who maintain that they only get good groups from neck-only sizing, and, moreover, that they get gatherings rather than groupings when they full-length resize, or when they use factory ammo. There can be some reasons for that, and they may have something to do with rifle-chamber dimensions.

A lot of factory-produced bolt-actions have fairly generous chambers; they are a little larger diameter and usually favor toward the longer end in headspace (but with all numbers within SAAMI tolerance). A rifle produced for across-the-counter sale needs to accept virtually any commercially available ammunition. If someone measures as many representatives of factory ammo as I have, it’s pretty clear that there are dimensional differences, significant differences. Additionally, it’s common to find some slightly oval chambers in factory guns. That has a lot to do with the freshness of the tooling when that barrel was reamed.

So, let’s construct a circumstance where we have a chamber that’s a tad amount big and a cartridge case that’s been manufactured on the smaller end of SAAMI blueprints. And this rifle has an ejector. As soon as the bolt closes, the ejector is bearing against the case, and it’s bearing well off-center.

For more clarity: rifles have extractors and ejectors. The extractor is the “claw” that rides in the case rim groove. It’s there to pull the case from the chamber. The ejector is a small, cylindrical piece that’s spring-loaded; its job is to lean or tilt the case toward the ejection-side of the action as the case is withdrawn from the chamber. It’s not commonly possible to encounter a bolt-action that doesn’t have an ejector (custom Benchrest actions and some Long Range Rifle specialty actions don’t).

Here’s an ejector and here’s what an ejector does. Pressure levered against the case will warp the case. It’s a small amount — all these things are small amounts. Case-body sizing helps straighten out the “banana,” that is, a curve in the case body, making it a smaller banana.

Back to the reason I said anything about ejectors in the first place. It makes a banana out of a case. This is unavoidable. The pressure steadily being put against the case base by the ejector warps the case under fire. It’s going to happen on each and every case fired. The bigger the dimensional differences, the greater the warp. Of course, as the uses and reuses add up, the nature of expansion changes. A case can warp one way, and then another way, and then another. Brass has a “memory,” by the way, and we’re always fighting that. Cases tend to follow the same expansion pattern regardless of orientation in the chamber.

There are some, and I’m among them, who think case-body sizing is a good thing to help allay the effects of warped cases, when they return to a correctly-dimensioned chamber. If a rifle chamber is on the larger side, then I honestly think that neck-only sizing may be doing a better job working around it, or working with it, and that’s the primary source of accuracy improvement. I also know that little bit there will get pounced upon.

The “70” on the dial indicator isn’t a measurement of anything; it just happened to be the position of the indicator. What matters is the area the indicator sweeps, measuring points about the circumference of the case. More needle movement means more warping. This pair of photos shows the amount of warp on a new case measured using a V-block-style concentricity fixture with a dial indicator. Even new, they’re not perfect.

I’m shooting custom-chambered custom-made barrels in my rifles. I do not request “tight” chambers (in any dimension), but they sure aren’t oversized. I like to have headspace set to closely accommodate the case brand I plan to use; doing that minimizes case stretch from the get-go.

Using a concentricity fixture that’s designed to allow isolation of points of measurement along the cartridge, it’s easy to see the warp. Spin a new case, spin a fired case. Some experiment with marking the “high” (or low) point and reinserting the round in the same orientation each time. That’s tedious but possible using a single-shot approach to firing. The point is, that after full-length sizing, I see less runout. I have also seen additional body sizing improve the accuracy of rounds destined for use in rifles with smallish chambers, and that’s one of the first steps many competitive Benchrest shooters take when they’re losing the gilt-edge on groups. For that, they use a die that doesn’t touch anything but the case body.

This pair of photos shows the amount of warp on a fired case measured using a V-block-style concentricity fixture with a dial indicator. As before, the “70” on the dial indicator is just the position of the indicator. What matters is the area the indicator sweeps, which shows they are a little less perfect than the new ones above.

If a case becomes a banana, it should be a smaller banana rather than a bigger banana. More technically, it’s a question of if the chambered round can sit in the center of the rifle chamber.

When David Tubb designed the sizing die for his 6XC cartridge, he added an additional 0.0005 inches downsizing right at the case head area. He maintains that is a key to good accuracy at long distance. He’s also setting the case shoulders back 0.002 and running a little more “neck tension” than you might imagine (difference between sized case neck inside diameter and bullet diameter). Tubb maintains that the consistency of case expansion has been an overlooked element in accuracy. Believe me, he’s tried everything, including neck-only sizing, to improve scores at 1000 yards.

And this pair of photos shows the amount of case-diameter variation after they’ve been full-length sized — the variation amount is nearly back to where it started in the new cases.

If you’re running a factory bolt-action rifle, by all means try neck-only sizing. If you want to compare results to full-length sizing, just make sure you’re doing that second operation correctly.

[wetncold]

I like David Tubb. When he speaks you will do good to listen.

[Dane Armory]

SS + 1 .... Looks like it's your turn to teach the class !