Any help, advice, or ideas would be greatly appreciated... So here's the details, I'm shooting a 7mm ultra mag. I shot a 0.5" group yesterday with 88 gr RL-25, one fired brass, federal 215M primer, and 140 gr nolser accubond. Went home and loaded another 5 rounds identical but with new remington brass. It resulted in big dents being left right in front of the case shoulder. There was burned powder on the outside of the case like gas entered between the chamber and the case. What I have read on this subject and thd secondary explosion effect leans towards the idea that the load is too light.... I then shot even a lighter load (87 gr) in a once fired case and everything was fine. Also tested it with the same load from the day before...... Long story short, the big dented cases were replicated 3 times, all with new brass. Anytime once fired brass was used, everything worked great. Could it be a bad batch of brass or is the once fired brass stronger and can withstand more? Any help would be great. Thanks everyone
That's exactly what I shoot, except that I run Hornady SST's (139gr). I've never had any cases that came out looking like that though. I did try some loads in the 80's, but 90-93 grains seems to work best, for RL-25. Tomorrow I plan on trying out my first RL-33 loads.
The issue that I have found with fresh rem brass, is that there is a "lip" on the inside of the neck, that doesn't want to go away, no matter how many times the brass is run through the sizer/expander die. I have a 3-way cutting head on the trimmer, but the fresh cases come up too short for me to want to trim them, until after they have been shot at least once. Basically, they need to be shot once (fire-formed, if you will), to convert that lip into more length. After that, they become pretty decent brass.
Try working the fresh cases through a sizer/expander die at least a dozen or more times, before they are shot for the first time. See if that takes care of the problem.
You may also want to try seating the bullets deeper into the cases. The 7RUM internal pressures are difficult to control, if loaded too close to the rifling lands.
I'm leaning AGAINST believing that your problems are due to running loads that are too light, because I have loaded all of the way down to 75gr without issue, for purposes of directly comparing performance to my 7RM using the exact same loads. Just for the record, the 75 gr loads fell way short on velocity, compared to the same loads in the RM. But also, you state that once-fired brass does not ever exhibit the problem.
You aren't trimming/chamfering, or de-burring, the ID of the never-fired cases before you shoot them once, are you?
I have shot this exact a lot in my other ultra mag as well as my brother for at least a couple years now. Like i said earlier, for some reason this is only happening with my new bag of brass. With 88.4 grains of RL-25, that is in the middle of the charge window according to the book. So i would think there should be enough pressure to expand the case enough to seal. I am FL sizing the new brass before their first load just to try to get some consistently in my brass. Should i not be sizing them prior? Thanks for all the help everyone.