I just got back from the range and was sorting my big bore brass to tumble andd prep.
I found one case with a split about a 3rd of the way around the case just above the web.
I then closely examined the rest and found seven out of the 15 with tell tale signs of pending case head separation.
The other 8 were from another manufacturer and have no signs.
Cases have all been fired 4 times.
Loads are just under factory spec and there are no pressure signs. All have been fired in my Ruger #1 custom.
I have not encountered case head separation in a big low pressure case before.
What I did do is check the other packet of unfired new brass which I had bought at the same time.
I found that there is a slight dimple right around inside the cases just above the web which is a potential weak spot.
Has anyone had this hassle with straight wall cases?

Seems like case head separation is a "pressure sign" My guess is that the head space should be measured first. I seem to remember this being a problem with the Trapdoor Springfield, which was never loaded above 21K psi.

Probably your brass had fatigued..Double rifle brass is thin and with near max loads it will do just that at about the 5th loading as a rule..

Other factors will cause that also an overloading is the most common...Excess headspace with a long chamber will cause it also as they don't fireform well, and that fireforming will cause shortened case life..then there is the problem of case head thickness that can in itself ocassionally cause case seperation...

I base the above on my dealing with double rifles, not a Ruger No. 1, but that should not change things much other than you might have a tendency to overload it for a Ruger as the gun will take it, but I assure you the available brass will not....What brass were you using btw?

case speration, while it can be high pressure, in a double is probably more of a headspace COMBINATION rather than a true problem..

here's why...
speration, due to pressure, is about 60kpsi+ loads, which basically can't be used in a double.. we are talking 40k max max max in a double.

so, what you may have is a middle headspace chamber, and the brass that is failing is minimum spec, and the brass that is doing okay is larger.

when working with manson on the flanged reamer, he suggested (and i accepted) a +.005 diameter over nominal cart spec. in a bolt gun, that would have been .002 or so..

so, brass that is on the small end of spec would have to move ALOT to fill the chamber, and then move back down to be resized. if it was small to start you have an accelerated start on case fatigue on shot 1 from the brass. think of it as being some number of reloads harder on the brass than the other maker.


just my thoughts
jeffe

I have experienced a few case head separations and as Ray Atkinson said,I believe most were from loading too many times----but more times than you stated. That is the reason I dont anneal necks,I want that split neck as first sign. To me,making the neck last longer is like putting a penny behind a fues.

With the nitro express calibers, plan on 4-6 reloads.

If case had a little step in it just below
separation, then it is one made with a hard head and a real fast transition from web
radius to the
sides, where it is soft.Usually Bruce's cases.
Separation on these is normal like that, not a sign of high pressure. If you have a case style with a large gradual transitioin through the radius to the sides, like Bell it would reload many more times.Section one of the
thinned cases and you can see the design.
I had Bruces 450NE cases I built 458HE from
last fron 5 to 8 rounds and Bell lasted dozens of rounds, but Bruces case heads were much harder and I could get 10 % more energy from them while they last.Ed.