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7mm Remington mag sizing problems

6K views 15 replies 7 participants last post by  LDBennett 
#1 ·
I'm new to reloading, and recently started reloading 7mm mag. After doing 20 rounds I noticed a couple rounds were mushroomed right at the shoulder.im using a hornady full length sizing dye. Is it ok to shoot them. I checked for size on the case length befor sizing, and a few of them were .005 over book lengths. Is that too much?
 

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#3 ·
Like to see what they look like. Not sure what a mushroomed shoulder looks like! What I found when I shot the 7mm mag is that it's best to partial resize belted case's. Problem with a lot of them is they headspace on the belt but have sloppy chamber's. Reg FL sizing with a lot of them leads to case head separations after relatively few rounds.
 
#8 ·
Back your seating did out a bit. It's screwed down too far. Take a properly sized case and put it in the press. Raise the ram and screw the die down until you feel it contact the case. Lock the die down and then adjust the seating stem until you get the proper length you want and lock it down.
As howlnmad said, they won't chamber with the shoulder being bulged out as they are.
 
#16 ·
Typically when the shoulder is "mushroomed" or "ballooned" it is a crimping problem created by over crimping. The way normal crimping works is the case throat edge is pushed up by the ram against a ledge inside the seating/crimp die. That folds the edge of the case over into the bullet. But adjust for too much crimp and the shoulder of the case balloons (mushrooms).

The solution is obvious: don't use so much crimp. OR.... Use the LEE Rilfe Factory Crimp Die (FCD) which uses a collet to push the crimp into the bullet horizontally instead of vertically. Over crimping with the FCD can distort the bullet but will not ballon the shoulder of the case.

Case length should be measured after sizing. There are two dimension that are important: The maximum case length and the trim-to case length. The first should never be exceeded and the second is normally about 0.010 inches shorter than the maximum allowed length. Any case in that range measured after sizing is good to reload.

But Don Fisher (#3 comment) is absolutely correct. If you full length size every time the life of the brass will be reduced. Magnum rifle brass has a short life anyway and extending it save having to replace it from case head separation caused by over working the brass. If you partially size the case then failure will normally be an eroded (by the high pressure gases) hole in the case neck. Case head separation is not desired because it is hard to get the front of the case out of the chamber and the gas gets to touch the chamber walls and maybe erode them.

I too have a 7mm Mag bought before I recognized the reloading problems with any of the rifle mag calibers and the extremely short case life. The added gain in performance is not worth the short brass life, in my opinion. The problem is the head spacing on the belt rather than the shoulder. Magnum level cartridges without a belt is the way to go if you just have to have the fire power of a magnum.

LDBennett
 
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