Well, I finally took a bit to sit down and share the preliminary load development testing I have been doing on my new 375 Allen Magnum. First a bit of back ground on the new round. As many of you well know, my 338 Allen Magnum has been around for a while now and has been performing extremely well. There have been some hic-ups but that is expected when you take the performance envelope and push it to the max and then some.
All in all, it has performed extremely well. The main issue has been a quality parent case to use for these wildcats based on the 408 CheyTac round. Jamison International has been working on this and they have finalized their design. I had my shipment of ordered brass show up so I decided it was time to test things on the 375 Allen Magnum. I also had a supply of the new 350 gr SMK for testing.
Above is a pic showing three 375 Allen Magnum rounds on the left compared to three 338 Allen Magnum rounds on the right. As you can see, the shoulder location is the same for both meaning it has been moved forward roughly 65 thou compared to the parent case. This just lengthens the case body section to increase case capacity while still leaving plenty of case neck for each. By the way, the neck length on the 375 Allen Magnum is roughly 0.305" which should be considered min but works just fine from my prelim test results. These rifles are designed for big single shot rifles, so the commonly used standard of one caliber of neck length is really not needed. Its not needed in most applications for that matter.
Anyway, you can see the 350 gr SMK seated in the 375 AM cases. Very similiar in design to the 300 gr SMK in 338 but with a shorter baring surface and longer boat tail. My chambering reamer is set up for a heavier bullet. Something in the 380 gr range and heavier, as such, there is not alot of full diameter bullet in that case neck. Plenty but not alot.
First thing that has to be done when working with a brand new wildcat is to get cases for the rifle for testing. I had the parent case which were bare headed Jamison 408 CT cases. By this I mean I ordered them with no headstamp on them at all so they could be used for any of my wildcats or 408 CTs for that matter.
You can see in this pic the bare head compared to a 338 Allen Magnum round formed off a TTI 408 CT parent case.
I had my set of 375 Allen Magnum custom dies that Hornady makes for me. With the 338 AM, there are several steps needed to take the 408 CT neck down to 338 cal for fireforming. I was very curious to see how many steps were needed to neck down to 375. Really what I wanted to know is if this could be done with the use of only my 375 AM FL die.
I took a case, deburred the outside of the neck very well, lubed it well with imperial case forming wax and then dipped it in graphite just for added slickness.....
To my surpise, the case sized rather easily in one pass. This was a big deal as it makes forming cases Dramatically easier then for the 338 Allen Magnum. Care had to be taken to debur the case mouths well because if there were not even, the neck would size down uneven with a noticable stepped shoulder on one side and hardly any on the other. This can lead to some case squareness issues until high pressure fired one time.
I necked down 20 cases and then loaded them up with my corn meal fireforming load. For the 338 AM, I use 22.0 gr GreenDot with the rest of the case filled with plain yellow corn meal and topped with a TP plug pressed into the case mouth. Works very well. With the larger neck diameter on the 375-408 CT cases, I decided to up this to 24.0 gr. Worked perfectly when fired in my case forming fixture. Cases come out with a slight radius on the shoulder, I would say 95% formed, just what I want in my formed cases. Ready to be FL sized, trimmed square and to length and loaded with full pressure loads.
This is where I ran into a hic-up. I had designed the reamer off of the old Jamison cases. These cases have very thick case necks. THe new case design, has a much thinner neck which is good but with custom dies, they are set up for pretty exact neck sizing dimensions. With the thinner case necks, they were not being sized down enough to hold a bullet properly. After 15 minutes I had a bushing made out of 4140 steel to size the necks to a 0.400" diameter which would offer a 2 thou neck tension fit.
I neck sized all the cases in my Redding Comp 338 AM bushing neck sizing die which worked perfectly.
Here you can see the 408 CT parent case on the left, 375-408 CT case ready to be fireformed in the middle and a formed and loaded 375 Allen Magnum on the right. You can see the shoulder position change on the formed case evident by the stepped shoulder on the case ready to be fireformed.
So now I had my 20 cases ready to load and start load development but where should I start. This is an issue when your starting load development for a brand new chambering, especially one that is roughly 10% larger in case capacity then even the 375 Chey Tac. I found some references on line that said the 375 CT was being loaded with 139 grains of Retumbo under a 300 gr TSX bullet for 3300 fps.
I had figured Retumbo would be the best choice for this powder or at least a very good one. Again, just an educated guess. With my larger case capacity, I know that with the same bullet, the same max load should produce less pressure, but I was using a bullet that was 50 grains heavier.
I decided to start at a very safe point and loaded the first 3 rounds up at 125.0 gr. under the 350 gr SMK and lit with a Fed-215 primer. I was very happy to find that there were no hang fires at all even with the very mild pressures. Velocity averages 2866 fps with an extreme spread of 55 fps. Understandable with the very low pressure. I was also seated well off the lands just for that extra cushion on chamber pressure.
Below are the results for the initial load development, including powder charge, muzzle velocity, extreme spread and accuracy at 100 yards for three shots. OAL was set at 4.315" measured to the meplat. I was surpised how consistant the 350 gr SMKs were in OAL. Generally not the case with SMKs.
125.0 gr...........2866 fps............55 fps..............0.788" ctc
130.0 gr...........2979 fps............41 fps..............0.641"
135.0 gr...........3086 fps............28 fps..............0.433"
140.0 gr...........3181 fps............24 fps..............0.489"
143.0 gr...........3219 fps............17 fps..............0.414"
146.0 gr...........3301 fps............11 fps..............0.398"
At 146.0 gr, there was a slight tap needed at the top of the bolt stroke to free the case from the chamber, not much at all. There was also the first signs of a slight ejector ring on the case head and very mild cratering on the firing pin divit on the primer. Really not much at all, had to use your fingernail to really feel it well. Still, I figured 146.0 gr was a max working load with these componants and decided to load the last two formed cases with 144.0 gr which I would call my top working load for the time being until I can test it in warmer summer temps anyway to see if there is a pressure increase.
Here are the results for those two shots at 144.0 gr
144.0 gr............3242 fps.............4 fps.............0.110" ctc
Now these were only three and two shot groups so extreme spread really do not mean alot at this point. Accuracy was good and met my 1/2 moa standards with any load above 135.0 grains. The very tight 144.0 grain load was probably a fluke with the one hole group but I have no doubt it will shoot 1/2 moa out to a very long distance.
Now onto the case performance. I had told myself I wanted 3200 fps with the 350 gr SMK at least as my 338 AM will push the 350 gr ULD RBBT to 3200 fps in this same 32" length barrel. I knew the Jamison cases would not take the pressure that the TTI cases would but I was hoping to get 3200 fps anyway.
For all the test loads, the only load that noticably loosened the primer pocket was the very top 146.0 gr load which was good for 3301 fps average. All others showed very quality primer pocket feel after the first high pressure loading. Just to check the top load, I took a single case and loaded it over and over with the 146.0 gr load. I got 4 firings on this case before the primer pocket got looser then I care to deal with. I am sure I could have gotten at least one more firing out of the case before throwing.
The 144.0 gr load gave me a comfortable 5 firings before the primer pockets became a concern. At 3200 fps levels, 6-7 firings would be easily attainable. SO far, I am happy with this Jamison brass.
I will soon test it in my 338 AM and get an apples to apples comparision with the TTI cases and its top loads. That will tell us alot but for now, they work fine for the 375 Allen Magnum and I am more then happy with all the inital testing so far. Only issue so far is the FL dies but that will be corrected on the next batch.
Take care, if you have any questions, feel free to contact me on this new chambering. Personally, once the new bullets get to the market, I believe this will be just as good if not better then its smaller brother, the 338 AM. That is saying ALOT!!!