stagpanther
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Forgot to mention--given the notorious barrel-burner nature of the STW I only shoot one charge group at a time spacing out the shots at at least 3 minutes--so it's going to take a while to find the best loads.
7mm STW--any thoughts/experience with?
- Thread starter stagpanther
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stagpanther
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I've abandoned the 7mm STW brass and decided I was better off making my own out of 375 H&H--unfortunately that means a significant part of the barrel life is going to be used up fire-forming.
Using a low-end charge in a cavernous case is a tricky thing--this can lead to the potential for hang-fires and trailing secondary pressure spikes for "multipart" ignitions. I'm only doing a few shots at a time to keep the barrel wear down--today I widely spaced 8 shots to fireform the cases--all using 77 grs of H1000 and 150 gr GMX's. Even though I was justr shooting to fire-form the brass, I wanted to make it as much fun as possible, I brought the labradar out to see what I was actually getting and put the target out at 355 yds. I also had to boresite the scope having worked on the gun and removing the scope and base--wasted 5 shots just getting on paper at that distance--but still fun. I finally found the target on the 5 th shot, here are the last three which had an SD under 10 fps and an average vel of 3196 fps--not bad for a low end charge and fire-forming brass if I may say so myself.
Using a low-end charge in a cavernous case is a tricky thing--this can lead to the potential for hang-fires and trailing secondary pressure spikes for "multipart" ignitions. I'm only doing a few shots at a time to keep the barrel wear down--today I widely spaced 8 shots to fireform the cases--all using 77 grs of H1000 and 150 gr GMX's. Even though I was justr shooting to fire-form the brass, I wanted to make it as much fun as possible, I brought the labradar out to see what I was actually getting and put the target out at 355 yds. I also had to boresite the scope having worked on the gun and removing the scope and base--wasted 5 shots just getting on paper at that distance--but still fun. I finally found the target on the 5 th shot, here are the last three which had an SD under 10 fps and an average vel of 3196 fps--not bad for a low end charge and fire-forming brass if I may say so myself.
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stagpanther
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PPU 375 H&H brass sized to 7mm STW prior to fire-forming the case.
The parent H&H brass has more taper to the case which even after being "blown out" by fire forming still allows the case to snap back enough to release from the chamber--the factory 7mm STW brass I bought would not do that adequately in the match chamber on the barrel I bought. !50 GMX, 77 gr H1000 COL 3.65 (low end load for fire-forming, though not a bad performer).
The parent H&H brass has more taper to the case which even after being "blown out" by fire forming still allows the case to snap back enough to release from the chamber--the factory 7mm STW brass I bought would not do that adequately in the match chamber on the barrel I bought. !50 GMX, 77 gr H1000 COL 3.65 (low end load for fire-forming, though not a bad performer).
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stagpanther
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The barrel has been so hot even after just 8 shots timed 3 to 5 minutes apart I cut back to just 5 shots at a time. Since I have only one box of the 150 gmx's I switched to Nosler ballistic tips to fire-form with. Except for the bullet I loaded everything identically to the gmx's specs for fire-forming the brass. The group looks quite promising--unfortunately I don't think the bullet would perform well on game at 7mm STW velocities.
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stagpanther
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Yes, I thought about that, but I'm not confident enough of a good hit at 1,000+ yds (yet, anyways).
stagpanther
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I'm almost done making the brass out of PPU's 375 H&H brass--just a few more to fireform so that I have enough cases to work with for load development. Even though there are wide variations in the pre-fireformed cases and I'm using light powder charge weights, the accuracy has been spectacular at my target ranges of 175 to 200 yards. I've already decided I'm going to order another barrel just like the one I purchased after I finish load development which I figure will use up most of the life of the barrel I have.
Jim Watson
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We see that a lot, a barrel used up looking for that last silly millimeter on the target. But good luck on that "just like" part.
In the early days of the 7mm STW, the local dealer got all enthused and had at least one 7mm RM rechambered and one rifle rebarreled with a very heavy tube, pretty much just the blank with the lathe marks polished off. It was very frustrating, he never could get as good accuracy with fully formed brass as he did from his fireform shots.
stagpanther
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Really? Why is that?
Jim Watson
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I don't know, neither did he. If he knew, he would have changed it.
stagpanther
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Funny--I was just writing a response to this. I've been burning with curiosity as to why that might be, and finally decided I had to go find out for myself. Today. In the pouring rain. And high winds.
I took 4 fire-formed cases and did the same loads as I used to fire-form, so not optimal velocity wise. Here's the interesting part: because of recent construction activity I had to move much closer to the target, and the scope was set for the previous day's shooting at 178 yds--so without adjusting the elevation I was expecting the impacts to be hitting high, but instead they were hitting lower. I suspect the case capacity widened a bit, as well as the neck perhaps loosening a bit, but it doesn't look like the accuracy been adversely affected.
I took 4 fire-formed cases and did the same loads as I used to fire-form, so not optimal velocity wise. Here's the interesting part: because of recent construction activity I had to move much closer to the target, and the scope was set for the previous day's shooting at 178 yds--so without adjusting the elevation I was expecting the impacts to be hitting high, but instead they were hitting lower. I suspect the case capacity widened a bit, as well as the neck perhaps loosening a bit, but it doesn't look like the accuracy been adversely affected.
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stagpanther
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After examining the chamber and bore with my Hawkeye scope (yes, it's an affliction) I noticed it has a couple of very unusual features which I've not seen before in another rifle chambers. They appear to be in the SAAMI specs for both the 7mm STW and the cartridge it was necked down from--the 8mm rem magnum.
There appears to be a large tolerance between the case length for the cartridge and the length of the chamber. Because the case headspaces off of the belt, this can potentially result in a wide space (especially when following recommended case trim lengths in manuals) between where the case mouth ends and the case mouth cut in the chamber steps down to the beginning of the freebore. That step-down is quite steep (70 degrees) and quite deep. My drawing skills aren't nearly as good as unclenick's--but here's my attempt to convey this:
What I'm seeing so far is the gas-cutting and other "barrel burning" is happening mostly at the case mouth cut in the chamber and in the space behind it to a point where the actual cartridge case mouth ends.
I finally got around to making a modified case (I had lost my 5/16 36 tap and had to order a new one) and was quite surprised how far out I could seat a bullet before the ogive engaged the lands considering how short the throat freebore appear to be--resulting in a COL well beyond 3.8" in the case of the 150 ballistic tip.
There appears to be a large tolerance between the case length for the cartridge and the length of the chamber. Because the case headspaces off of the belt, this can potentially result in a wide space (especially when following recommended case trim lengths in manuals) between where the case mouth ends and the case mouth cut in the chamber steps down to the beginning of the freebore. That step-down is quite steep (70 degrees) and quite deep. My drawing skills aren't nearly as good as unclenick's--but here's my attempt to convey this:
What I'm seeing so far is the gas-cutting and other "barrel burning" is happening mostly at the case mouth cut in the chamber and in the space behind it to a point where the actual cartridge case mouth ends.
I finally got around to making a modified case (I had lost my 5/16 36 tap and had to order a new one) and was quite surprised how far out I could seat a bullet before the ogive engaged the lands considering how short the throat freebore appear to be--resulting in a COL well beyond 3.8" in the case of the 150 ballistic tip.
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