Yes it’s tedious, and yet it takes a lot of time, but I really enjoy load development! And this time with 300 PRC, I’ll have two nearly identical barrels to perform load development with. Both are match-grade barrels from Benchmark! One will be 1:9.5 twist (the custom 300 PRC used in this article), and the other will be 1:8.5 twist, as a part of my Remington 700 Long Range re-barrel job (stay tuned!). This time I’ll also cover something a bit different- working up a load when you don’t have complete load data. Let’s get to it!
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For this load development exercise, I used the following components:
- Hornady 300 PRC new cases (un-primed)
- Hornady 225 grain 30 caliber ELD-M bullets
- Ramshot Magnum powder
- Federal 215 Large Rifle Magnum primers
Interpolating Load Data: Enter At Your Own Risk
So what do you do if you have a powder or other component that you’d like to use, but don’t have load data for it? In that situation it’s usually best to get as much data as you can, and interpret/interpolate for the components you’re using. I could not find 300 PRC load data for Ramshot Magnum powder, but I did find enough data to safely work up a load!
First, I took a look at Hornady’s published load data for 300 PRC. I knew I was going to start with Hornady’s 225 grain ELD-M bullet since I wanted to compare performance with the Hornady Match ammunition I used during barrel break-in.
Here’s a snippet from Hornady’s load data: (for reference only, always get latest data form HERE).
I knew that H1000 was a good powder for cartridges like 300 PRC, and Retumbo as well, so I focused on those data points from Hornady’s published load data. The second task was to look at those powders on Hodgdon’s relative burn rate chart:
Aha! Ramshot Magnum falls right between H1000 and Retumbo. Very convenient! So my strategy was to put the max load charge weight right between the max charge weights for H1000 (77.5 grains) and Retumbo (78.9 grains). I assumed Ramshot Magnum would have a max charge weight around 78.2 grains or so. This is not fool-proof since I’m assuming similar densities for the three powders. Hence starting on the low side and working up to the calculated/interpolated max charge. I’ll cover that next!
10-Shot Satterlee-Style Test
As I covered in a more in-depth article:
It’s very useful to run this Satterlee-style testing when working up a load, some times as a very first step for each trial run with new components (like trying a new powder or bullet). So that’s where I started with this 300 PRC load. During this testing, I was able to monitor pressures while shooting the string in my “uncharted territory” 300 PRC load development with Ramshot powder. That’s like hitting two birds with one stone!
For this 10-shot load development test pass, I started just beyond my calculated max (do this at your own risk) and worked back over 10 charges subtracting 0.5 grains for each powder charge. I loaded up the rounds (more on that in a future article), and shot them over a Magnetospeed V3 chronograph which produced the following data:
|225 ELD-M Ramshot Magnum 10-shot ladder|
This is not the “most clean” graph I’ve produced from this 10 shot load development since the “nodes” were not pronounced near the max charge. So I had to make some educated guesses moving from 10-shot over to Optimal Charge Weight (OCW). The good news was, no pressure signs whatsoever!
Optimal Charge Weight Tests
After performing a 10-shot load development test, I’ll typically take “speed nodes” (See the main article) and shoot some groups “around the nodes”. Since I didn’t have “super-clear” nodes on my 10-shot load development graph, I decided to shoot 5-shot groups (as seen in the video) up towards the max charge weight, again incrementing by 0.5 grains for each test. Here’s the results in table form:
|Group||Charge Weight||Group Size||SD|
At 78.5 and 78.6 grains I had good results. I did not get super-consistent SD numbers for velocity and I believe that’s because of barrel heat fluctuations during testing. I’m planning to start monitoring barrel temperature for these tests with temperature strips or a non-contact thermometer so that I can take that “variable out of the equation”.
The best group came in a 0.540″ (I had another that was very close to this) which is 0.515 MOA. Now I’m really getting close to my “consistent 0.5 MOA” goal for this cartridge, very exciting!
Next steps for 300 PRC load development include:
- Varying bullet seating depth
- Testing additional bullets
- Testing additional powders
- Testing with 2nd 300 PRC rifle (following that build)
I’m having so much fun with this project, and I hope you all are enjoying it too! Don’t miss out on Ultimate Reloader updates, make sure you’re subscribed!