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7mm rem mag headspace

 
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  #1  
Old 01-24-2012, 11:07 PM
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7mm rem mag headspace

I am new to the whole long range hunting and precision reloading. I am having Travis Redell with RBros rifles build me a 7 rem mag to shoot the 180 bergers.

I have a bunch of questions about reloading(I know I could call him and ask but I hate bothering him, but I will ask him eventually).

It is my understanding that on the belted magnums the headspace is measured from the bolt face to the belt. That is fine but everything I have read about accurate reloading talks about headspacing off of the shoulder and bumping it back .003-.002. Otherwise with the belt the case will stretch way out because the belt. Is this something Travis may take care of in his reamer? Can you safely use the shoulder to headspace off of and not worry about the belt? Do you have to resize the bulge near the belt? Could you have a reamer built with excessive headspace and use the shoulder as the part of the case that stops forward movement?

One of the nice things is RBros will hand me a dummy round that shoots 1/2 moa when I pick up the rifle. That will make it nice and all I have to do is reload the same. At the same time I am trying to learn all that I can.
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Old 01-25-2012, 01:24 AM
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Re: 7mm rem mag headspace

Consult with Travis on this, but myself, I ignore the belt and headspace off the shoulder. If you don't already have one, get a tool like the Stony Point comparator that you can use to guage COAL by ogive length, as well as shoulder length from the datum line. Measure a fired case and adjust your dies to set back the shoulder what ever amount you feel is adequate. On a hunting rifle I go .002.
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Old 01-25-2012, 08:59 AM
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Re: 7mm rem mag headspace

It headspaces off the belt on only the first firing, then the shoulder is blown out and it uses both, I personally neck size only on belted cases until they get hard to chamber then I take the shoulder back a little. Like Brad stated .002" bump is fine for a hunting rifle, I'm just greedy and want all I can get, but if have a few cases with over 20 rounds a piece on them, Travis will assist you and you will pick up the minor details along the way. And as I always say don't let anyone give you any crap about belted cartridges being inaccurate, that is total *********!
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Old 01-25-2012, 10:21 AM
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Re: 7mm rem mag headspace

J,

The fellas are spot on here. The belt really only comes into play on the first firing. After that, we use the shoulder and headspace off it. Bumping .002-.003" is all thats needed. When I chamber a belted mag, I chamber them in such a way that the bolt drags on a "go" gauge so the case doesn't stretch very much on the first firing. We will go through it when your rifle gets ready to go.
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  #5  
Old 01-26-2012, 07:04 PM
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Re: 7mm rem mag headspace

I use a RCBS precision mic to measure fired brass and and set my dies to 3/1000's bump. Chambering has a slight resistance and case life has been excellent. I have seen factory ammo that is 25/1000's off of the chamber. That is a lot of jump. Thats also what causes the ring above the belt. Some people say belted mags are not that accurate as compared to the non-belted mags. I am here to tell you that is bunk IF you know how to set your resizing dies. Another old fashioned method is to smoke the shoulder, back off your die, and incrementally turn it down until it just nearly touches the shoulder. This has to be with brass fired in your gun. That usually ends up being about .003 bump. That's all you need. Try the precision die, it is a great help. AIM SMALL, MISS SMALL 7STW
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Old 01-27-2012, 06:13 PM
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Re: 7mm rem mag headspace

Just let your gunsmith take care of it. You would not want him to ream your chamber where you have less than normal head clearance at the shoulder on new cases. IOW even if he reams the chamber to just inside the go-gauge (minimal headspace), you will still have considerable distance for the case to expand at the shoulder. That is OK and normal.

For example I had a new barrel put on a 6.5 rem mag case and the gunsmith reamed for minimal headspace. I figured out a way to measure the headspace and the headspace was ZERO (0.00"). But when I fire a new case the case still expands .017" at the shoulder on the Remington cases. Then I headspace off the shoulder.

The reason you would not want to ream to headspace off the shoulder on a belted case is because you would then have a smaller than normal combustion chamber (total volume of chamber from case head to lands and out to body sides). That smaller than normal combustion chamber would make the loading data off. IOW it would reach higher pressure with a smaller amount of powder. Same effect as reaming a Weatherby or RUM chamber with a short throat. Then loading to max in the book or shooting factory ammo could lead to pressure issues.
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  #7  
Old 02-01-2012, 12:07 AM
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Re: 7mm rem mag headspace

The SAAMI headspace for most belted magnums is .220" minimum.
So all factory rifles are at least that big, and probably bigger.

Brass is almost always .215" or less.
Uh - oh!

What to do?

If you have a factory rifle , hope and pray the first shot works out and then headspace off the newly formed shoulder.

This is as much trouble as Ackley Improved chambers.

When I chamber my 7mmRemMag, 300WinMag, or 338WinMag rifles, I set the headpace at .215".

Then there is the question of just above the belt where the sizing die does not reach.
For that, either 1) keep brass segregated to one rifle or 2) get an Inovative Technology collet resizing die, or 3) shoot wimpy loads.
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