Actual distance to rifling - Hornady case vs. sized case

[Bart B.]

David, yes full length sizing fired cases does reduce the body diameter.

It's not necessary to keep the case virtually the same size as the chamber for best accuracy. I think you want to do this so the bullet in the case neck will be well centered in the bore when it's fired. Right?

But it's not the case body that centers the case neck and bullet in the bore. In reality, .308 Win. rounds fit the chamber perfectly centered up front when fired regardless of how much smaller they are than chambered dimensions. There's equal space all around the case body at the shoulder. But their back end's pressed against the chamber wall by the extractor but it's not enough off center to cause problems; and it's the same amount and direction for every round so it's not a variable that effects accuracy. Their shoulder centers in the chamber shoulder perfectly when the firing pin drives them there before the round's fired. As long as the case neck's well centered on the chamber shoulder, it too, will be well centered in the chamber neck. And therefore, so will the bullet be centered.

Even a .243 Win. round's bullet will be centered perfectly in a .308 Win. chamber when fired with a huge amount of clearance around its neck.

Most benchresters now full length size their fired cases but reducing body diameters and setting shoulders back no more than .001". Their smallest groups still are the same as when they neck only sized their cases. But their largest ones are now a lot smaller; the average group sizes got smaller. As there's no such thing as perfectly round cases and chambers, there has to be enough clearance all around the case body to not let the body interfere with the chamber wall else the case shoulder and the neck holding the bullet will be pushed a tiny bit off center.

[DAVID NANCARROW]

That's precisely what I was seeking, Bart. Thanx for the reply and explanation.

Might just go old school and smoke the case shoulder and try a partial resize enough to bump the shoulder.

I didn't like full length resizing the cases much because Id like to get as much life out of the cases as I can reasonably get without getting too crazy over it.

[stubbicatt]

If I take my fired cases, which I *try* to set the shoulders back .001 to .002", and compare them to the modified case which is used to find the origin of the rifling using the projectile choice I have at hand, I find that the modified case measures shorter from the datum point on the shoulder to the case head. I add this difference (the difference between the modified case and a fired and resized case) to the length I get when pressing a bullet into the rifling origin using the Hornady tool.

So, say the case head to shoulder dimension on the modified case is .003" shorter than the resized cases, and since I am measuring from the case head on my finished rounds, I will seat the bullets .003" longer in my ammunition than I determine is correct with the modified case. This measurement is from the case head to the ogive of the bullet in question.

The actual measurement ought to be from the datum point on the shoulder to the origin of the rifling as this dimension won't differ between fired or unfired brass, but I am unaware of any method for readily measuring this dimension.

[Unclenick]

David,

That tightening is normal for all neck sizing-only shooting. If you run mild loads it may not occur, but full loads gradually tighten the case fit in the chamber because the steel expands a bit with the brass, but snaps back to original shape much more completely than brass can. At still higher pressure you get to the point the brass can't return to shape as far as the steel can, so the steel clamps down on the brass, causing sticky bolt lift (a high pressure sign).

You can use a Redding Body die to size just the body while ignoring the neck, which you can leave to your Collet Die. This also allows you to use the die to set the shoulder back not just periodically, but 0.001" to 0.002" every time, if you choose to, which typically gives you still better accuracy for the reasons Bart described.

You can also get a bushing die or other tool to do both operations simultaneous. But if you run the body die first and the Collet Die second, you will tend to have the mandrel of the Collet Die iron out the internal donut ring that can form at the junction of the neck and shoulder, and which can cause pressure increases with a long bullet bearing surface by interfering with its fit.

[DAVID NANCARROW]

Thank you Unclenick-the issue I have is not with stiff ejection of the fired case, but camming the bolt down on my reloads. I keep the fired brass trimmed to spec and remove the burr whenever I have to cut it down. Still, I think I'm going to get that Redding body die as it seems to do what I want it to.

I've been very pleased with the collet die for the years that I've used it. The case neck run out is nil. Actually better than using the Redding full length sizing die, but that might be a subject for discussion on another thread.

[Unclenick]

David,

If you've seen this video, you'll know you're not alone with that result. I have the same experience.

The problem with closing on a case is basically the same as sticky opening—the case getting tight. I just didn't draw the connection well.

Most chambers are not absolutely dead perfectly co-axial with the bore, and most bolt faces are not absolutely dead square. As a result, cases are fire-formed slightly out of square, so when they become a snug enough fit, reinserting them with the head rotated in any different orientation than it had at the previous firing can cause its longer side to wedge against the bolt face. Also, if you are running rounds toward the upper end of the pressure the brass is happy with, you impress the brass head with the profiles of the extractor and ejector cuts, causing the brass to stand proud at those points, so you'd have to get those aligned identically to avoid them jamming the bolt face slightly.

You can check this by trying to insert cartridges into the chamber with their heads in the same orientation each time and see if they go longer without needing sizing.

[DAVID NANCARROW]

Good info Unclenick. The most accurate load I have found so far is 44 grains of Reloader 15 behind a 168 gr SMK bullet, seated to the magazine length @ 2.810 in Winchester brass. I don't get bolt extractor impressions on the case head although its listed as a max load out of my books.
I use the identical load for my hunting rounds behind a Hornady 165 grain BTSP with similar accuracy.
Ejection does show a tiny bit of resistance, but even with the softer Federal cases on the same load, there are no imprints on the case.
I didn't figure that the bolt face or the barrel was dead square and I have yet to have them trued.
I am going to try clocking the cases after a neck size and see if what you're speaking of is present. Would not surprise me if it is indeed the case.
I've thought about having the action trued, but I think my money would be better spent to purchase a new barrel from a reputable maker rather than have the work done on the stock rem.
Thanks again all of you for all the information!

[Unclenick]

David,

I'll be interested to hear what you find.

Note that Winchester cases typically are about 15 grains lighter than Federal, corresponding to about 1 grain less powder charge in the Federal to hit the same peak pressure, assuming identical external dimensions. You can measure the case water overflow capacity¹ of fired rounds to get a more accurate comparison for rounds coming out of the same chamber. Figure powder charge to change about half as much as the water weight difference to keep barrel time matching. Anyway, if your load is not imprinting on the soft Federal brass even at the higher pressure it's seeing, you're good to go in that regard.

Nick


¹ Weigh a fired, un-resized case with the spent primer still in it. Fill it with water level with the case mouth (no meniscus) then weigh again. The difference in the weights is the water overflow capacity of that case. What that capacity is when the case has expanded to the size of the chamber, determines pressure. Measure the length of the fired case, though, and subtract 0.19 grains for each 0.01" over 2.620" and add that much for each 0.01" under that length. This compensates for .308" neck length differences, normalizing them to the 2.620" SAAMI maximum case length.