Actual distance to rifling - Hornady case vs. sized case

[deepforks]

so after getting a few measurements using my hornady case length gauge and comparator, i'm coming up with the following:

hornady case (from case head to datum) - 2.258"
resized brass w/.002" shoulder bump (from case head to datum) - 2.270"
overall length w/ hornady case and bullet to the lands (measuring to ogive) - 2.900"

regardless of where I want to seat off the lands, should I be using the 2.900" measurement w/my sized case or should I adjust due to my sized case being .012" longer than the hornady case?

I've always used the measurement from the hornady case and seated bullets according to that. i'm just wanting the true length to the rifling and I think I may not be correct doing it like this and not accounting for the difference in my sized cases and the hornady case.

[Bart B.]

If you're using a rimless bottleneck cartridge and want exact measurements for your bullet's jump to the rifling, you have to use the chamber and case shoulders as the reference. The case shoulder's hard against the chamber shoulder when the round's fired and the case head will be off the bolt face by a few thousandths. Each case will have a different length from its head to the reference point on its shoulder; it varies as much as .003 to .004 inch.

Does this make sense to you? If so, then I'll suggest a way to measure stuff to get things as exact as possible.

[Unclenick]

Deepforks,

Per Bart's question about where the headspace datum on your cartridge is, it would help a great deal if you let us know what chambering this is?

[deepforks]

this is for a 300 win mag (rem 700). projectile used is sierra 180gr game king.

I didn't specify the round I was using due to the fact I figured this was pretty general and i'm wanting to utilize whatever info I get for all of my rifles that I reload for. i'm getting pretty good groups using my distances from the hornady case, but would really like to be able to find the exact (or close to exact) length to my rifling.

bart, I understand what youre saying. it'd be nice to be able to utilize the equipment I have, but i'm open to other ideas/suggestions.

[jepp2]

Is your Hornady case actually headspacing on the shoulder or is it headspacing on the belt? If it is headspacing on the belt, the shoulder datum is irrelevant.

[deepforks]

not sure, but I am measuring from the shoulder on both cases...the hornady and mine.

[jepp2]

I just tend to find the belted magnums shoulder is far short of headspacing on the first firing. After firing the headspace datum from the shoulder will increase around 0.015". That is the difference between headspacing on the belt and headspacing on the shoulder.

You could smoke the shoulder and the belt and rotate the Hornady case and see which is contacting.

[Unclenick]

And that's why I asked about the chambering. If you buy a rimless cartridge that headspaces on the shoulder, it's common for the shoulder datum of the new commercial brass to be a couple thousandths of an inch under minimum chamber size. If the gun is new and the chamber is within the new chamber NO-GO gauge length, the case growth after firing will typically be about two to four thousandths over SAAMI chamber minimum.

But the belted cases, like the .300 Win Mag, are designed to headspace on the belt, so the shoulder can be extra short on a new case to ensure that happens and to make it easier to feed the long case. In firing the belted cartridges, brass at the shoulder just blows forward. It can do that easily and without damage since the part blowing forward is the annealed portion. Thus expanded, if dropped back into the chamber, the case then stops against the chamber shoulder before the belt reaches the end of its recess in the chamber.

Note that no harm accrues to the brass from the practice of letting the shoulder blow forward. This is hardly surprising when you consider how many wildcat cases are fireformed to blow their shoulders out and/or forward. All the Ackley Improved designs fire their parent cartridges directly without the intermediates step of using a light charge to form the cases gently, and they do just fine.

Match shooters who use the .300 Win Mag for 1000 yard shooting often prefer to have it headspace on the shoulder rather than the belt. This is to get the self-centering effect of the case and chamber shoulder tapers mating as the firing pin and rearward thrust of the primer drives the cartridge forward during the firing cycle. These folks will adjust their seating dies to set the case shoulder back one or two thousandths from as-fired length, but no more. This ensures the case shoulder makes contact with the chamber shoulder before the belt makes contact with the back of its recess step in the chamber. In the .300 WM, at that point is headspacing on the shoulder instead of the belt.

[Wyoredman]

Excellent thread. Learn something new every day. Thanks Bart And Unclenick!

[deepforks]

so I found a local machine shop that is going to order the correct tap to thread my brass so I can use it with the hornady case length tool. he's only charging a few bucks for all of them and not going to charge for the tap, since he's keeping it. hornady charges $15 a case, if you send them in. I figure this should get me pretty dang close to what i'm after. I was going to load up some new brass for each and go fire them. do you think it would be better to use a piece of 300wm brass that has been shot a few times, to ensure its fully expanded in the chamber (at the belt and shoulder)? I think i'll be fine to use once fired for the 243/270/22-250.

[Bart B.]

Deepforks, when 30 caliber magnums were popular for long range matches shot slung up in prone, the top scoring folks learned that brand spankin' new cases shot just as accurate as any properly full length sized ones. But they had to resize their fired cases with two different dies.

The first one was a standard full length sizing die (with its neck lapped out to a couple thousandths smaller than a loaded round's neck diamter) set in the press to set the fired case shoulder back a few thousandths. The second die was a body only die made by cutting a standard die off just above the belt clearance groove and just below the shoulder. It was squared off at the bottom then set in the press to resize the full length sized case all the way to the belt. That swaged down that tiny ridge a few thousandths in front of the belt caused by the case head being hard against the bolt face, the belt's front edge back from the headspace point in the back of the chamber and the case expanding a bit from peak pressure.

Unless that ridge was removed, it caused chambering interference issues and accuracy suffered. In other words, the fired case was returned to near new case dimensions. www.larrywillis.com now has a collet die to do the same thing; it is the best thing going in my opinion for getting best accuracy from reloaded belted cases. Seems Larry Willis has taken down his website; maybe he's quit making those great collet dies for belted cases.

[zplinker]

How about a slightly different approach to setting your sizing die to correctly size your brass to your rifle: back the full length die off the shell holder a couple turns (often referred to as partial sizing). Size and load a couple rounds, fire them, and repeat. About the 3rd time, the partially sized brass won't chamber. Now, turn that full length die down in small amounts until you can just close on that case, and you have it. Obviously your brass will have to be the same brand, similar number of loadings, or annealed, etc. At that point you can measure with your head and shoulders gauge, record that number, and have a useful number to shoot for next time.

[Bart B.]

Zplinker, backing out a sizing die two turns moves it up about .143 inch. That's enough to often force the fired case shoulder forward several thousandths as the case body sizes down. Some bottleneck cases will not chamber after such die repositioning even after being fires only once; their shoulders got moved forward too much. This is more predominant with cases with minimum taper on their bodies. And with sizing dies having minimum diameters sizing cases from maximum chambers, it gets worse. .300 Win Mag's one of them. My .300 Win Mag as well as two other 30 caliber magnums' cases were that way.

I've maintained for decades that the best measure of a belted round chamber's exact shoulder headspace is to first is to full length size several fired cases different amounts so a half dozen have lengths incrementally by .001" as measured by your case shoulder headspace gauge. Then find one the bolt will easily close on and the next longer one will not let the bolt easily close on it. The number representing your chamber's shoulder headspace is half way between these two. For belted cases, full length size the fired ones so their shoulders are about .002" to .003" back from that number.

[zplinker]

" backing out a sizing die two turns moves it up about .143 inch. That's enough to often force the fired case shoulder forward several thousandths as the case body sizes down. Some bottleneck cases will not chamber after such die repositioning even after being fires only once; their shoulders got moved forward too much."

My point exactly. Then when you slowly take the die back down, you will find the perfect fit.

[Bart B.]

Zplinker, I didn't see where you instructed to turn the die down before shooting the first partially sized case. As I've never been able to chamber a belted case after setting the die up two turns, that's why I said what I did.

That aside, your method may work if the die height's lowered only .001" at a time and the application of lube on the case is the same each time as well as the press springs the same. Otherwise, there'll be a few thousandths error.

[zplinker]

Jeez, Bart B., why don't we just nit-pick this until no one will ever attempt to do things in a simple way that works?

I leave this forum to you to handle the technical end of things. It seems you know it all.

[Bart B.]

Zplinker, if you don't want details on sizing cases to some exact measurement such as using one to represent chamber headspace exactly, then don't ask for information that explains what's needed for them to be within .001" of being so.

[Unclenick]

Bart,

The Larry Willis link is working now. Must have been a temporary situation.


Deepforks,

That Hornady Gauge case tap is a 5/16"–36 tpi. They are available from MSC. The tap drill size for 65% thread is a letter L drill. You can hold cases in a Wilson case holder during the drilling and tapping operation if you have one.


Zplinker,

Bart acknowledged that he'd missed the adjustment part of your post. Neither of you said anything technically incorrect in the end, so let's take the win-win and let readers pick and choose whatever degree of detail interests them.


It should be explained to those not following that a full length die first squeezes the body of a case down before its shoulder finds the shoulder in the die. That lengthens it. This is exaggerated in my drawing, below. I'll also link to the die calibration templates I've put up before. Print one, cut it out and stick it down by wetting it with a little oil. Put a registration mark on the die body with a Sharpie and use the scale to adjust the die any specific amount you want to.



Die Adjustment Template

[F. Guffey]

“How about a slightly different approach to setting your sizing die to correctly size your brass to your rifle:”

Zplinker, good luck with that one, The tap and die for the modified case is not and has never been necessary, I simply drill the flash hole and primer pocket out to a diameter that will accommodate a cleaning rod. Rational? I want all the bullet hold I can get, and I want to control the length of the case from the shoulder back to the head of the case, mistakenly called head space of the case and going by the drawings of SAAMI, the case does not have head space, it has a length from the datum to the head of the case. If you understand that you understand why “How about a slightly different approach to setting your sizing die to correctly size your brass to your rifle:” will not get consideration.

By drilling the primer pocket/flash hole I am able to seat a bullet, chamber the case and push the bullet out of the case until it hits the lands. After pushing the bullet to the lands I can then adjust the seating die up to zero the seating plug to the bullet. After zeroing the seating stem/plug I use a height/depth/dial caliper to adjust the bullet off the lands. Advantage, all of this is accomplished with a drill and cleaning rod. I use the drilled out case as a transfer to transfer the chamber dimensions to the seating die. I know, there are those the believe I do not have the the Sinclair/Hornady tool, I have the tool, I have modified cases, problem, I am the fan od bullet hold, I want all the bullet hold I can get, because I am the fan of transfers and standards.

The modified case has a fixed length and is a little light on bullet hold.

Then there is the fundamentals of the incline plain, the threads on the die and in the press are incline plains. Instead of making guestimates in fractional converted to thousandths, I use the feeler gage, even if I used guestimates, I would still verify because I am the fan of verifying, when verifying wild guesses it should dawn on the guesser it makes more sense to verify and skip the guess, it saves time.

F. Guffey

[F. Guffey]

2.195 off case length from the datum to the case head.

http://www.saami.org/PubResources/CC...r%20Magnum.pdf

F. Guffey

[DAVID NANCARROW]

Not trying to toss gas on this particular fire, but what does a Lee collet neck sizing die do for something like this, if anything?

[Bart B.]

David, all any neck only sizing die does is reduce the case neck diameters; both inside and outside.

The Lee collet die's no different except it squeezes in the case neck onto a mandrel while conventional dies squeeze down the neck by pushing its smaller diameter down the case neck from mouth to shoulder.

[DAVID NANCARROW]

So the neck still needs to be bumped back by a small amount in addition, I take it.

[Bart B.]

David, it's the fired case shoulder that needs bumping back a thousandth or so. Most folks use a gauge such as the RCBS Precision Mic or Hornady LNL one to measure the fired case before full length sizing it then afterwords to see how much the case shoulder gets set back.

[DAVID NANCARROW]

I have a Stony point gauge, now marketed by Hornady with the modified case for my rifle. I don't know how much better or even if I should take a fired case from my rifle and drilled/tap it to use in place of the factory part.

I had been using the Lee collet die and calling it good, but after a few firings on a new case, you can tell that the case needs to have the shoulder just slightly set back as I can feel some resistance to chambering a reloaded round.

Id prefer to keep the shell as close to fired chamber dimensions as possible with the exception of the case neck for obvious reasons, but wont running the case into a full length resizer just enough to push the shoulder back also resize at least part of the case body? Or am I worrying about something I don't need to be worrying about?

The platform Im shooting off of is an older Remington VS in 308 so I don't have a belt to contend with, and when Im in practice, it will shoot groups of about 1/4 inch with the ammo it likes at 100 yards. I probably cant get much better than that because the rifle is completely stock-but Id rather not wear the bolt lugs out prematurely.