Morning all!

I've been reloading for about a year now and I have just purchased the RCBS precision Mic kit for my 7mm Rem Mag.

Using the body (marked 7mm) and the headspace nut (marked #5) my case dimensions are all below 0. All case measure between 4.5 and 5 lines below 0. The line on the body points at the 45 area on the nut and the 0 on the body is not visible.I am reading this as .0045 below SAAMI minimum headspace.

I asked the RCBS tech folks the same questioin and the very brief reply was:

"means you have a short chamber, not uncommon"

By measuring before-FL sized cases and after-FL sized cases, they are the same length (to shoulder, not neck), even after trimming / deburring.

This seems like it would be a great thing for accuracy as the case are fitted ti the chamber, but will I have any issues from this situation? Overpressures? Headspace issues, etc.?

Thanks...

The only problem with tight headspace is you might find a case that won't chamber if it was made to max headspace dimension, so try all ammo in the gun before you take it hunting.
The 7mm Rem Mag Headspaces off the belt, most reloaders try to set up the resized case so it headspaces off the shoulder leaving the belt short of seating on the chamber c'bore by .001-.003" anyway.

Headspace has a tolerance the same as everything else. Usually on the order of something like +0.004/-0.006. The figures are for explaination purposes only but should be in the ball park for a quite a few bottleneck cartridges If your 0.003 below zero on your mic you are still in the exceptable range.

I seriously doubt this will cause any pressure variance at all......at least of significance.

Thanks guys...

I guess I had assumed that SAAMI specs were absolute and that I had a problem. Good to know that it's nothing significant.

Just for clarity's sake, were you referring to a "just fired" case from your rifle, or were you referring to a "just sized" case when you wrote the following:

quote:

Using the body (marked 7mm) and the headspace nut (marked #5) my case dimensions are all below 0. All case measure between 4.5 and 5 lines below 0. The line on the body points at the 45 area on the nut and the 0 on the body is not visible.I am reading this as .0045 below SAAMI minimum headspace.

If "just fired," then you might have a small chamber (or the tool might be off just a little, so measure a a factory round as a reference). If "just sized," then you probably still have your dies set as the maufacturer recommended, and that can be improved. In both cases, the Precision Mic will allow you to measure die adjustment changes for a custom headspace for your chamber.

Remember, the ammunition makers tend to make small so their product can fit into the smallest chamber, while firearm makers tend to make big so the longest factory round will fit.

Using the Precision Mic, you certainly will not have any headspace issues, since you can use the tool to set it right where you want it. How does that compare with what you're seeing? More details, if it is indeed the die that needs to be reset. You can set it to 0.006" for sureness of feeding in a dirty environment or to 0.001" for improved accuracy, or anything you want.

Jaywalker

The 7mag DOES NOT headspace off the belt. When you reload/resize, ignore the belt.
This is one of the advantages of reloading, you can craft your ammo for your rifle.

As long as your chamber accepts factory loads and brass your better off.

The 'spec's usually include some nominal dimension but actual chambers dimensions have a tolerance range and so does the brass. On top of that many things are out of tolerance.

I would rather have your chamber than a sloppy one.

Jaywalker:

I was refering to a fired case being below minimum specs, so my chamber is apparently short, but I have not had any feeding issues with factory or reloaded ammo.

quote:

The 7mag DOES NOT headspace off the belt. When you reload/resize, ignore the belt

All belted cases headspace off the belt in factory cartridges. The handloader can adjust his die to have contact between the shoulder of the case and chamber as the point of first contact instead. Most factory belted mag chambers are a bit sloppy in lenght to shoulder, usually overly long I`ve been told, and case life is improved by doing this.
There is no way to adjust the "designed" headspace contact of belt and chamber when reloading. The changeing of the point of "headspace" from belt to shoulder IS a good idea when resizeing these cases IMO. This can improve accuracy along with limiting case stretch. The fact your chamber is tight in this respect is a good thing.

mrb302, have you tried measuring a factory round with your headspace nut?

Jaywalker