I was shooting my Winchester mod 70 243 today. Brand new Winchester brass loaded with 37.5 of imr4895. I had two cases blow out 1/4" above rim. Others were swelled . I have never experienced this before in this rifle. Bad brass??
What bullet weight are you loading?
Case head separation in new brass would indicate over pressure to me. My experience with 4895 in the. 243 is that it works ok with 70 to 80 grain bullets but over 80 accuracy falls off pretty bad.
I have Winchester .243 brass I've loaded 7 or 8 times with no sign of separation.
I'd try some Varget. It works very well in the .243.
It might be worthwhile to have the headspace of your rifle checked by a gunsmith . That would eliminate that possibility . The powder charge you show is lighter than the starting charge for that bullet weight shown in Speer #14. Sometimes too light a charge can cause strange things to happen .
IMR 3031 and 4064 were always my go to powders before Varget in the . 243. I've killed more chucks than I can count with 4064 and a 70 grain Nosler Ballistic Tip in my Mod. 70 sporter.
Stick with it . You have a great rifle /cartridge combination .
I would suspect the brass first. I used to swear by Winchester brass but recently acquired some that was so far out of spec it could not be loaded. Mostly the case rim was not fully formed and it would not go in the shell holder. Case necks were so thin I had trouble getting sufficient tension on the bullets.
If you have shot other brass with that same load without failure, in that rifle, then it's the brass. The old "all else being the same rule". If it is the first time you've used that load, then try another load with the same brass. Rcope822 is right on about light loads. Sometimes they can go over pressure.
If it’s brand new Winchester brass, did you fill length resize the brass before it was loaded ? Ive been using Winchester 243 brass for many years, never had that problem, and I’ve loaded from 70 Gr up to 100 Grainers.