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7mm Rem Mag Brass Life

19 Jul 2012
@ 03:56 pm (GMT)

Erich Kuba

Hi there,

I have recently purchased a 7mm Rem Mag, and am almost done running the barrel in and am starting to think about my reloading for this rifle.

Being my first magnum I am learning a whole lot.

I've read a lot of comments on sites that I visit saying that brass life in belted magnums is limited to 2-4 cycles. I have also read that one can get 10 cycles if one sizes off the shoulder and not off the belt (partial full length sizing or using a Belted Magnum Collet Resizing die). It's all new to me and with the price of 7RM brass, I want to try and get as many cycles as I can.

I currently have 100 Hornady cases.

What is the best way of sizing these cases in order to get the most life out of them?

Have you had any experience with a Belted Magnum Collet Resizing die?

Kind regards

Erich

Replies

21 Jul 2012
@ 06:22 pm (GMT)

Quote this

Nathan Foster

Re: 7mm Rem Mag Brass Life

Hi Erich, the key to long brass life in any cartridge, is not to over work the case. But even then, annealing can bring a work hardened case back to life.

As you suggest, neck sizing minimizes how much the brass is worked.

If a rifle is made with a minimum dimension chamber, a full length die will barely work the brass as it is only sizing the case down a couple of thou. So in this instance, FL sizing is not hard on the brass.

Obviously by the same token, if the chamber is of loose dimensions, full length sizing can put the brass under a lot of stress. In such chambers, neck sizing is optimal. But as you can see from these two examples of different chambers, its not a case of 'neck sizing is always best' as it depends on the chamber.

I have one friend who has a 7mm rem Mag with a chamber that is on the tight side. His rifle likes quite a stout load for best accuracy. He finds that he gets about 3-4 neck sizings out of a case before he has to FL size for smooth feeding, otherwise its a bit difficult to chamber rounds. What you have to understand is that upon ignition, the brass expands to seal the chamber but it also 'bounces' back afterwards. In tight chambers (sometimes regardless of whether the load is high or mild pressure), after a few reloads, the brass dimensions become closer to the chamber dimensions. The brass simply doesn't bounce back as far as it did the first time it was fired, it has gradually grown in dimensions and is now stiff to cycle. In this instance its time to FL size.

One other method of minimizing stress to the brass is to simply back off the FL die a touch. This should really be done anyway as a part of the initial set up. To do this, back the die out a turn, size a fired case, then chamber it in the rifle. If the case is tight, turn the die in about an eighth of a turn and try again. Keep doing this until the brass feeds smoothly. This method helps minimize overworking the brass but there are problems with the method. Sometimes (depending on die dimensions), the case body is extruded and made longer when the body is sized and only comes right when the shoulder is bumped back- forcing the hand loader to have to turn the die right down to the shell holder.

Other times, the FL die dimensions are perfect for partial sizing of the case. Generally speaking, I find Hornady are about perfect. I have in the past found that Lyman have very tight dimensions and that if the case is not sized to the shell holder, the case body extrudes.

How do you tell if an FL die is too tight for partial sizing? Its quite simple, chamber a fired case that has not been sized at all. Then partially size a case as described (backed out a turn and then incrementally wound down). If the brass becomes tighter to chamber in the rifle than it was before it went into the die, the case is being extruded (made longer at the shoulder).

When folk neck size for a while, then find that the brass gets tight (as in my friends rifle), the general assumption is that the shoulder is the culprit, that the case has grown forwards and that it needs to be bumped back. In my own research, I have found that the shoulder usually continues to spring back quite well, possibly due to the major force direction. The body is quite often the culprit, about 1-3mm (40-120 thou) back from the shoulder. Being over zealous with a neck die can also cause swelling in this area, the shoulder is bumped back, then the case body expands and the case won't chamber in the rifle- something to keep in mind when setting up a neck die.

The brass you are using is of a very good brand, Hornady make very nice components.