What would cause these cases to split??

With a rimmed or belted case, if the headspace is good, an oversize chamber or an undersize round will not stretch as those have. The case will simply fire form to match the chamber.

I will say again, the only reason for that kind of case separation is excess headspace. The fact that it shows incipient separation with new cases pretty well rules out overworking the brass as the primary cause (though it can make the situation worse).

I agree that the amount of firing done and the use of normal loads should not result in a headspace problem so soon. That is one reason I recommend sending it back to Ruger; if it looks like a manufacturing defect, they may well repair or replace it free of charge.

But I think you can be sure of one thing - it is only going to get worse, and perhaps very quickly.

Jim
 
It's hard for me to believe that you have built up excessive head space in a gun like yours just yet. Let's rule a few things out first. Take one of your recently fired brass, decap it and put a good dial caliper on it and see if it has stretched beyond allowable limits for your gun.
When you seat the bullet and load the rifle dont let the bullet touch the lands, it builds too much pressure, if it is a moot point ignore it.
I find it hard to believe that headspace is lost from a Ruger M77 from 300 rounds, I am not ruling out headspace. Get a set of go and no-go gauges or go to a gunsmith and get it checked out like you said but I would like to lay odds that you are developing too much chamber pressure for either the weak brass that you have,improper resizing or seating of the bullet.
 
imq707s: I agree that the new brass seems to reflect debris in the chamber caused by the case separations and not necessarily thinning of the case due to excess headspace. The only way to be sure is to cut a case in half longitudinally and check for thinning in the area ahead of the web.

It's a good idea to mike the case head, but it won't tell you anything if you mike the area ahead of the belt, because even if there's thinning at that point, pressure pushes the exterior out to meet the chamber walls.

The places to measure are at the head at marked locations and/or at the belt, since those areas aren't restricted by the chamber. A good test is whether the primers seat too easily or not. If they do, the head has expanded from high pressure.

Misfires aren't common on belted cases because if the case is too short, the belt will stop it from sliding ahead in the chamber. On non-belted cases misfires and pierced primers are common.

You may not have mentioned trimming cases, but I hope you're keeping them within allowable limits. Cases that are too long can increase pressures.

I had a Savage 110 back in 1961 that had a loose chamber, though still in tolerance. We loaded ammo on my buddy's press set for his Mod. 70 Win and it had a tight chamber. I experienced numberous misfires, primers popping out, pierced primers; all the signs of excessive headspace or overloaded ammo. The rifle was returned to Savage and they checked it and found it to be within tolerance, but replaced the bolt. I still had problems until I got my own reloading press and set the dies according to instructions.

Picher
 
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I would have your gunsmith check the headspace and while the gun is in the shop I would have him make a cast of the chamber using cerrosafe.
The reason I suggest the chamber cast is I believe you may have some gas cutting from the splt cases; the cerrosafe cast will show this if it's so.
Regards
Bob Hunter
www.huntercustoms.com
 
Well I looked down into the chamber of the rifle and I can definitly see some markings in the chamber. I'm going to try to use a brass brush (12ga) and some copper solvent to clean it out and see what that does.
 
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