I got the idea from 375 ultra mag thread.
I decided to do a 375 Rum with a 40* shoulder, moved ahead .053" and taper reduced to .006"
@yorke-1 had done a similar cartridge several years ago and called it the 375 Box Monkey Express.
Initially I tried to seat the bullet hard into the lands to jam the case head against the bolt face but that wasn't working. I believe the firing pin strike was pushing the case onto the bullet causing excessive head space and I was almost getting case head separations. I ended up having to neck them up to .416 and then size back down to .375 and make a false shoulder (wish I'd have taken pictures of those cases). I ended up with some fairly crooked necks so I had to trim off a bit more than I would have liked.
-Formed case on the left, Factory brass right.
I have the intention to run this rifle out at Ko2M Canada which has a 10k Joules limit, I was hoping to get the 390gr A-tips to 2900 fps. which I can get it there but the node seems to be around 2870 unless the barrel speeds up which I'm not expecting now. I have about 140 rounds through it (including 100 FF)
I did initial powder test 104-110 gr and never saw pressure to stop (I don't have this data handy)
Second powder ladder was 107-112gr N570, with 390 A-tips. BTO 3.177" (barrel 32" long)
107- 2830 fps
108- 2840 fps
109- 2875 fps
110- 2893 fps (light ejector mark)
111- 2898 fps (light ejector mark)
112- 2940 fps (light ejector mark, Bolt click at top)
Next I went to testing load stability. I did 5 Shot groups and on paper they all shot the same about 1.25" which I am not happy about so I hope I can bring that in with seating depth.
108 - 2858 fps, ES 9, SD 3
109 - 2881 fps, ES 8, SD 2
110- 2887 fps, ES 39, SD 16
111- 2906 fps, ES 43, SD 20
At 111 I started to notice a very slight click at the top of the bolt throw, that and the massive ES makes me want to stay away from here. I wish I would have loaded 107gr to see if the window goes that low. I am planning to run powder charge at 108.5 gr and do a seating depth test. I also have a couple new tricks to hopefully lower ES/SD a bit that I didn't do to these cases.
I also need to add some weight to the rig. It weighs about 20lbs. I have a steel arca rail coming that is about 3lbs and then I will likely add a bipod extension and a belly weight and see where it ends up.
I will try and post some groups when I get to seating depth testing.
I also have some solids to try at some point but I wanted to shoot these a-tips for the splash.
In the end I don't know if the effort of custom reamer and dies was worth it for the few FPS over what the standard chambering is doing but it's fun to learn.
You guys can say "I told you so" but I've still got my fingers crossed it picks up 30-40 fps
I decided to do a 375 Rum with a 40* shoulder, moved ahead .053" and taper reduced to .006"
@yorke-1 had done a similar cartridge several years ago and called it the 375 Box Monkey Express.
Initially I tried to seat the bullet hard into the lands to jam the case head against the bolt face but that wasn't working. I believe the firing pin strike was pushing the case onto the bullet causing excessive head space and I was almost getting case head separations. I ended up having to neck them up to .416 and then size back down to .375 and make a false shoulder (wish I'd have taken pictures of those cases). I ended up with some fairly crooked necks so I had to trim off a bit more than I would have liked.
-Formed case on the left, Factory brass right.
I have the intention to run this rifle out at Ko2M Canada which has a 10k Joules limit, I was hoping to get the 390gr A-tips to 2900 fps. which I can get it there but the node seems to be around 2870 unless the barrel speeds up which I'm not expecting now. I have about 140 rounds through it (including 100 FF)
I did initial powder test 104-110 gr and never saw pressure to stop (I don't have this data handy)
Second powder ladder was 107-112gr N570, with 390 A-tips. BTO 3.177" (barrel 32" long)
107- 2830 fps
108- 2840 fps
109- 2875 fps
110- 2893 fps (light ejector mark)
111- 2898 fps (light ejector mark)
112- 2940 fps (light ejector mark, Bolt click at top)
Next I went to testing load stability. I did 5 Shot groups and on paper they all shot the same about 1.25" which I am not happy about so I hope I can bring that in with seating depth.
108 - 2858 fps, ES 9, SD 3
109 - 2881 fps, ES 8, SD 2
110- 2887 fps, ES 39, SD 16
111- 2906 fps, ES 43, SD 20
At 111 I started to notice a very slight click at the top of the bolt throw, that and the massive ES makes me want to stay away from here. I wish I would have loaded 107gr to see if the window goes that low. I am planning to run powder charge at 108.5 gr and do a seating depth test. I also have a couple new tricks to hopefully lower ES/SD a bit that I didn't do to these cases.
I also need to add some weight to the rig. It weighs about 20lbs. I have a steel arca rail coming that is about 3lbs and then I will likely add a bipod extension and a belly weight and see where it ends up.
I will try and post some groups when I get to seating depth testing.
I also have some solids to try at some point but I wanted to shoot these a-tips for the splash.
In the end I don't know if the effort of custom reamer and dies was worth it for the few FPS over what the standard chambering is doing but it's fun to learn.
You guys can say "I told you so" but I've still got my fingers crossed it picks up 30-40 fps
!!!!!
