Dear all

I have owned a 7 Rem Mag for the past 20 years but have never known of what you might describe as an idiosyncrasy of the design of this cartridge.

That is to say that new factory brass is, 20 thou shorter on the shoulder than the SAAMI spec of a 7 Rem Mag chamber.

A smith friend of mine informed me of this a few weeks ago and I have some new Winchester and R&P brass so when I got home I meansured it with my head space comparator and sure enough he was right! Anyone else know of this fact? I was amazed.

Given that it has the belt, I guess it’s not a big deal.

quote:

Given that it has the belt, I guess it’s not a big deal.Matt

It isn't until you full length re-size and push that shoulder back several thousandths. Then it blows back forward and after a couple cycles of this you get a head separation.
I think it's a damned shame that as long as the belted cases have been around the ammo manufacturers cannot offer more consistent ammo.
Use the case comparator to set your die to only bump the shoulder .002-,003".

I think the belt bares no relevance and I’m also talking about brand new unfired brass as opposed to once or more times fired brass which is, after the first firing, formed to your chamber spec. So new unfired brass is 20 thou shorter when measured with a comparator on the shoulder than once or more fired brass. Hope I’ve explained it more clearly in this post.

the round actually head spaces on the belt.
so everything else just floats out in space until the neck is laying on the bottom of the chamber on the other end.
this is how it matters.

what the other guys are telling you is to fire-form your cases and use the shoulder to set your new head space.

the manufacturers not only make them short but they also make them skinny.
the problem most loaders have is either stretching the cases out and cracking them right above the belt. [after 1-2 loads]

or getting cases that are too large in that same area and they can't size them down enough to chamber in their rifles.
you need a special tool to fix the problem.

the belt has no bearing on either problem.

Guys,

This is easy. Don't full length resize. Shoot new cases and only partially resize so the sizing of the neck stops just short of the shoulder. I've been using this method for years with belted or non-belted rounds for hunting or otherwise. My primer pockets wear out well before any case separations etc. As others have said the belt is irrelevant after the first shot.

Mark

Ammunition manufacturers tend to make all cases of all calibers at SAAMI minimum dimensions since they want them to be able to chamber in all rifle chambers. The manufacturers tend to fudge even smaller with belted magnums since the case can headspace on the belt (although most belted magnums are better off headspacing on the shoulder).

Treat any bottlenecked belted case the same as you would a rimless case: After firing resize it only enough to allow it to rechamber without undue resistance when placed back in the gun. Typically, only neck sizing is needed for several firings -- unless you are playing with the upper boundaries of pressure and stretching the case (and action) beyond its normal springback tolerance. If so, either reduce your pressures or be satisfied with only two or three firings of a case.

Like Mark and Stonecreek said

quote:

Originally posted by Stonecreek:
The manufacturers tend to fudge even smaller with belted magnums since the case can headspace on the belt (although most belted magnums are better off headspacing on the shoulder).

Treat any bottlenecked belted case the same as you would a rimless case: After firing resize it only enough to allow it to rechamber without undue resistance when placed back in the gun. Typically, only neck sizing is needed for several firings -- unless you are playing with the upper boundaries of pressure and stretching the case (and action) beyond its normal springback tolerance. If so, either reduce your pressures or be satisfied with only two or three firings of a case.

Good stuff. Would add that greater number of firings simulates fewer firings at high pressure. So eventually a partial sizing will be needed. I'm lazy, just partial size everything.

Second, some belted round rifles stretch excessively on first firing. You'll know because there will be an incipient, detectable, separation above the belt. If so, run new brass over a .308" expander button and back. Then run it through your 7mm Mag sizer backed off some. Screw the die down a little at a time till you get some resistance to chambering. You've made a false shoulder, and that will help mitigate stretch on first firing. You only need do this to new brass. Obviously, no way to rescue factory load empties fired from such a chamber.

Alternatively, you can stick bullets into the lands on first firing of new cases. But be sure you have enough neck tension. And an appropriately adjusted powder charge.