I finally had enough fire formed cases to begin load development for this case and rifle. As you can see this is really an “improved” round with a very sharp shoulder. Case volume is very similar to a 243 Win. With a custom round like this, I had very little loading data to start from. So I had to do some interpretations to get starting points. I wanted to make use of the case, so I am working with the Sierra 107gr HPMK bullets for this round of tests.
To make this development even more complicated, I needed a load that would consistently function the action on the rifle. The gas system is even longer than a traditional rifle length.
I started working with a few loads listed on the Hodgdon reloading data site. Varget seemed like a nice place to start. Good flexible powder, with good temperature stability. My testing environment was low 50s, so I did not want to develop a load that would show sign of pressure in the summer heat. I slowly worked up the powder charge seeing sizable gains in velocity. I noticed this cartridge really does have an incredible case volume. Powder charges started to approach 243 Winchester loads. Maximum velocity I achieved was 2904 fps with no signs of pressure. After that, I was burning more powder without any increase in velocity. But the action would not cycle fully.
So on to the next powder – Hodgdon Hybrid 100V. Nice jump in velocity, not so nice jump in pressure. I could not find a balance. A safe load gave me a velocity of 3077 fps, but would not cycle the action. Only when the pressure was high enough to piece primers did I get the gun to function reliably.
So I went and reviewed the powder burn rate chart to find a powder that may develop a different pressure curve than those listed on the Hodgon site. The next to be tested:
Winchester 760 looked like the next candidate, only to fail like Varget. No signs of pressure, but still not the right combination to function the action. Finally velocity – 3015 fps.
Vihtavuori N160 was next. Right away I could tell there was a difference in the cycling of the action. Working my way up I found I could cycle the action well before any signs of pressure. Maximum velocity – 3081 fps
Vihtavuori N560. Had to try this for fun. Load data very similar to the N160 for most other calibers, but results in a slightly high velocity. Well, no luck here. Only got to 3017 fps and did not cycle the action
So I now have a base line for a trip to range. Have several different powder charges of VV N160 to try. Hopefully we will get some encouraging results.
Hatcher's Notebook 