7mm rem mag question

[nmbarta]

Lots of good info here, and some good explanations on why so many people run into problems with belted cases.
headspace in relationship to gunsmithing is different than headspace is in relationship to reloading and have to be looked at in different light.
When a smith cuts a chamber he sets the headspace to a specific dimension that falls somewhere inside of the tolerances allowed. On a belted case, the belt is used for this, on an unbelted case the shoulder is used for this.

The reloader sets the actual headspace (distance between the head of the case and bolt face) of his loaded ammo by sizing the case to meet his needs. (usually .002-.004) On a belted case the reloader is going to change where his ammo headspaces from. (from belt to shoulder) I realize this has already been said here, this is just another way of saying the same thing. The reason I bring it up this way is so that the OP can understand exactly what's happening, which leads me to my next point.

It is very important that your cases are fully fire formed before you set your die. Because belted cases are headspaced by the smith off the belt, they are allowed to have more slop in the chamber. (I've seen belted cases grow .016) This will usually not happen in one firing and may take as many as 3 depending on how hot your running them.
I always sacrifice one case to figure this out, fire the case then measure using comparator, neck size, load, and fire again. If the case grew on the second shot it was not fully fire formed. Repeat. If the case did not grow, it's fully fire formed and this case can be used to determine where you set your sizing die.

 

[epoletna]

NMBarta explains the process of cutting and measuring a magnum chamber very well. And he differentiates well between a measurement for chambering (which must meet SAAMI specs) and for reloading, although I don't follow his explanation when he says "The reloader sets the actual headspace (_distance between the head of the case and bolt face_)". I'm having a hard time seeing what he is referring to there.

His description of the amount of "slop" the magnum chamber may have because the case headspaces off the belt is important: only in belted cases is is possible the chamber can be more than about .005" longer than spec. In a chamber that headspaces off the datum on the shoulder, having excessive space between the bolt face and the shoulder could well lead to inconsistent ignition or fail to fire. But because the case on a magnum is "grabbed" by the belt, that creates the amount of distance the case can move when the firing pin hits the primer.

But this raises another issue that NMBarta addresses when he tells you to fire the case a sufficient number of times that the case has "grown" as far as it will grow before you set your resizing die. In a case without a belt, the datum point on the shoulder establishes all the "slop" the case can have. With a belt, there can be a good deal more.

This can possibly allow your case to grow over successive firings, and if it does that, you could well see separation somewhere between the belt and the shoulder.

His recommendation to measure an unfired case _to the shoulder_ and compare that measurement to the same place on a fired case is a good one -- that way you will know if your cases are being "stretched" excessively when they are fired.

You're getting a lot of good suggestions here -- another reason to follow Long Range Hunting!

 

[nmbarta]

What I was trying to explain is the reloader gets to determine the actual distance between the head of the case (where the letters and numbers are and the bolt face. The smith determines that distance based on saami specs and has some wiggle room on where he ends up. Brass manufacturers have tolerances as well, so if you have a chamber cut on the deep end, and have brass that's on the short side, your "actual" headspace is going to be different than if you have a chamber cut on the deep end and brass that's on the long side. This is one of the many advantages of reloading.

If you fire a case over and over with just neck sizing, the case will become hard to chamber because there is zero headspace. When you close the bolt, the bolt face is touching the head of the case and pushing the shoulder tight into the chamber. At this point the case is basically a mold of your chamber. Now you can measure that case and proceed with pretty good accuracy how far your going to "bump" the shoulder back to allow for easy chambering, while keeping a very minimal amount of "slop" If you move the shoulder back .002, you have allowed for .002 between your bolt face and the case head. The space you create isn't between the shoulder of the case and the shoulder of the chamber, it's between the bolt face and the head......that's why it's commonly referred to as "headspace" When the firing pin hits the primer the case will move forward into the chamber, if the ejector isn't already holding it all the way forward, then the case will expand when the powder ignites and the case will grip the sides of the chamber, but you have room between the head of the case and the bolt face, so the case wall (typically right in front of the web) will be the only place the case can stretch (or grow). This is why the case almost always separates just in front of the web where the case wall just begins to thicken and taper into the web.

When you size the case, your going to try to push that stretched area back together, this work hardens the brass and leads to failure. If you FL size all the way, you'll move the brass way more that what is needed to ensure proper chambering and over work your brass leading to shorter case life. "bumping" the shoulder minimizes the amount of stress/ work hardening on the case. Obviously this portion of the case can not be annealed and will continue to weaken, but if you don't move the brass too much, you'll likely end up with primer pockets that won't hold primers anymore and you've basically gotten everything you can out of that case.

Long winded, but that's what I meant by "the reloader gets to determine the actual headspace"

 

[WahooYahoo]

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[HighKnob]

I bought one of the Tupperware 700 Adl a few years back. First round through were federal factory. First reload with full length size separated. Never had that before. Bumped shoulder back ,002 and never another issue.