Home Forums Hunting & Shooting Custom Rifles & Wildcats Why have I not heard of this before (7 Mag case dimension query)

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 spacing collet tool and sure enough he was right! Anyone else know of this phenomenon? I was amazed.

I’ve seen the same for several cartridges of factory brass, not just limited to 7 Rem Mag.

This is due to the fact that the cartridge headspaces off the belt. Thus the reason after the first firing you should setup your die to bump the shoulder about .002. Should definitely help with case head separation also.

Belted cases in general start out with what is in my opinion an excessive amount of "headspace clearance" in the shoulder area.

BTW, how have you been sizing the brass for your 7mag rifles?

Another good reason for using a custom reamer, tweaking the somewhat sloppy SAMMI specs to suit your needs. Proper headspace, minimal chamber neck clearance, less freebore, like a Weatherby, and throat angle immediately come to mind.

I have been shooting many 7 Rem mags since 1978, Rem 700's, browning A bolts, BAR's.

Got started with Rem brass, worked up many loads that seem to be very close from rifle to rifle other than adjusting seating depth.

I found out that I got better accuracy with full length sizing....got a neck forster sizer for sale if you want one.

I set the FL sizer for the particular rifle, where the bolt just starts to close easy on brass that is hard to chamber. This method has worked without too much head banging.

As a benchrest shooter, I started measuring the shoulder set back, darn near lost my mind. So, I abandoned the whole project in setting back the shoulder xyz amount, just get the bolt to the point to where it closes easily. My 700's shoot 2.5-3.5" groups at 600 yards, pillar bedded in walnut or laminated stocks with Gentry muzzle break.

Throw brass away when primer pockets get loose.

I have bought a PTG match reamer and have a Brux barrel on hand, 28" will get you 3200 fps with extreme accuracy using Retumbo/160g bullets.

Dies are often problematic with the 7 RM. In order to get the web dimension to where it is easy to close the bolt, the shoulder has been pushed back .006+.
Answer, watch web area in brass, when you see a line start to form, throw the brass away. I usually get 5 firings on Rem brass, Winchester would be more but all my loads have been developed with Rem brass.Fed is soft and you loose primer pockets in hot loads that the 7 RM really loves.

Good point on the line near the base. I made a home made tester some years ago just for this purpose after a discussion on this forum about separations. It is a simple paper clip straightened out and with a small 90 degree tip bent in the end and sharpened. Stick it down in the case and feel the inside of the case with it and you will definitely feel the separation starting even before you see it on the outside of the case. For a simple tool, it works surprisingly well and may save you a separation in your chamber, causing hot gases to crater a spot in the chamber- which I have seen happen....

Bob

It is, IIRC from my reading, common practice among belted-mag users to fireform new brass after creating a false shoulder on the neck so that it fits that chamber as well as possible. A little searching on Al Gore's (peace be upon him) amazing internet should turn up a description.

Just look into the case with a flashlight; you'll see a shadow from incipient separation long before you can feel anything with a paperclip. You'll also see that the stretched area often doesn't go all the way around the case, so it's easy to miss with a paperclip what you can see plainly by looking inside.

I've done it both ways and I like the bent wire better.

Why? I honestly can't see any advantage to doing that, and obviously have done it both ways myself. It's so easy to just look in the case and see if there's a shadow ring or not. I've had to check hundreds of pieces of 308 brass for this, and it was way easier and faster to just look inside.

This sort of makes me wonder if I should forget my project with the 7mm I have and just go back to the 270 and forget about dicking around with this brass issue

What do you want for the foster neck sizer 7mm ?

Interesting, a false shoulder on a belted case? I guess for the OCD types maybe. This may cause the stretch to be in the shoulder area instead of the web but I'll pass on this.

mjbgalt: The 7 mag is no more problematic than any other cartridge. The belted cases are notorious for being short, the 300WM is even worse. But any case can be short and just needs to be fire formed.

The 7RM is one of my favorites but the difference between it and the 270s is shrinking. Especially a fast twist 270 that can handle 170 VLD spitzers. The 7 mage gives more energy and less wind drift further out than the 270s. But most hunters don't shoot at ranges long enough for this to matter which is somewhere around 500+/- yards.

I'm only reporting what I read. Too much trouble for me too, and I don't need such a beast anyway. If I ever do, it'll be something sans the belt.

The last piece I saw mentioning the practice was by John Haviland, IIRC.

Nothing wrong with using the shoulder to headspace a belted case. There are many magnum cases that don't need a belt - more of a sales gimmick, magnums had to have a belt. I think the 300H&H mag was the first case to have a belt but needed it due the weak shoulder.

I think they should've put a belt on the 8mm Mauser - it's the only case that I reload where I have to watch for incipient head separations. I did once have a head separate on a 375H&H though, musta been a dodgy case to start with.

I forgot to add - cases only stretch above the web. And if you have a belted case or rimmed case rifle with excessive headspace the head spacing off the shoulder is the way to go.

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