300 Win. Mag case head seperation

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windy

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I have an older push feed Win. mod 70 chambered in 300 Win. Mag that has been a great hunting rifle for the past few years. I have reloaded for it and have had good success in finding a hunting load that it really likes. The other day at the range I fired one of my reloads and had a 90% case head seperation and a nice dent in the shoulder that threw me for a loop until google explained the bad neck seal from escaping gas caused the dent. I then tried another cartridge of the same batch which resulted in the same head seperation. Upon inspection of my reloaded cartridges I noticed a prominent ring just above the belt which I should have picked up on before I completed the reloading process. Tonight I headed out to the range with some factory loads in the hopes of plinking up a new supply of reloading brass. After my first shot I inspected the brass and there was about 40% head case seperation. Eeek. Does this mean my head space is out of whack? If so can anyone recomend a Gunsmith in south west Alberta who can help me out.
Thanks
Windy
PS. If this is a total rebarrel job what cartridges would work with my bolt and action length.
 
You are causing excessive case head clearance in your reload. Size the fired case as you would a rimless case, not pushing the shoulder back excessively.

Every standard belted magnum case and factory chamber have excessive clearance which causes no problem with the first firing... you need to correct your reloading procedure.
 
Hi Guntech. If all works out well I will start neck sizing only for sure. What about my factory cartridges splitting as well. That is whats freaking me out the most.
Windy
 
Checking the headspace would be a good first step.
As mentioned, reloading the rounds as if for a rimless caliber eliminates an excessive headspace situation.
But if factory rounds are separating, the process to produce ammunition to fit the rifle is more complicated than just resizing once fired cases to suit the rifle.
The rifle could be rebarrelled to most any belted magnum cartridge.
On a vintage M70 setting the barrel back one thread and setting the headspace correctly is complicated by the extractor cut and barrel contour.
 
Not to be pedantic, but even though the "cure" is the same as applied to rimless cartridges with excessive headspace due to an oversized chamber, in this belted 300 WM, this isn't caused by a "headspace" problem. The splitting is being caused by overworking the brass well ahead of the belt which is the area affected by changes in headspace.
 
Belted magnum chambers may be very generous; headspace being controlled by the belt/chamber counterbore. The bodies blow out to fireform with every shot, and excessive resizing can really work the brass. This can contribute to separations. But if new factory ammunition is separating, I would suspect a headspace issue.
 
Yes, head separations on factory loads would certainly cause me to lift my eyebrows..........I would have to say that the rifle in question is definitely in need of some work and should not be fired any more until the headspace issue is corrected.
 
c-fbmi said:
Yes, head separations on factory loads would certainly cause me to lift my eyebrows..........I would have to say that the rifle in question is definitely in need of some work and should not be fired any more until the headspace issue is corrected.
Click to expand...

^^^^^THIS!!!^^^^^^
It sounded purely like a procedural issue with the reloads (I.E. overworking the brass, sizing it back down too much, too many uses on the brass, etc) UNTIL you mentioned the factory loads separating. Is this a push feed Winny? If so, it may be somewhat easier to set it back a thread or two. If not, get it rechambered for .300weatherby.................
 
The factory cartridge were a 180gn Winchester superX. I have used these before and get good accuracy. I am concerned about the fairly sudden change from no cracking to cracking. What would have physically changed in the rifle to cause this? I will definetly be sending it off to be inspected.
Windy
 
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