Hey!
More progress today. After a cold morning at hilltop- where it was 10 degrees colder than the rest of NSW,
I managed to test the bullet exit sensor with the 6.5- it survived!
I then got off 10 rounds ranging from 2300 fps (39 grains AR2209 with a 147gn ELDM) up to 2625 FPS (42 grains). I measured the bullet exit time and it ranged from 3.8 to 4.2 msec. I had the sensor way out in front of the barrel at the end of the magneto-speed so these exit times are not accurate. I am after a change in time (dt) and the difference in time is accurate so it works for my purposes......
The data tells me that a muzzle velocity change of 750 fps will vary the bullet exit time by 1 msec.
So 750 is "X" in the equation in my last post. Solving that gives me a 2.625 millirad change per millisecond.
What that means is that my barrel needs to be whipping up at 2.625 mrad per msec to completely compensate for point of impact changes due to changes in muzzle velocity.
Next step is to measure the barrel whip on the 6.5, with bullet exit time, plot 2.625 mrad per msec as a tangent line on the graph, and see how close I am to it - like this:
- tangent.png (2.92 KiB) Viewed 2821 times
I am guessing way off as my great groups got bigger recently.
This is great timing, as the MDT chassis arrived yesterday (
No issues with it at all Tassie!)(it did feel like Xmas Die Judicii!). I set it up and based on today's results my accuracy has worsened by more than half. Of 5 groups with my (previous) best load, I managed only 1 group under 1 MOA today. Previously I would average 0.3 MOA with a pb of 0.1MOA. The groups are all strung vertically so changing the chassis means I have changed the barrel whip harmonics. The action screws are certainly tighter at 60 inch lbs and the shape of the material supporting the barrel has changed so a harmonic change is no surprise. It is actually a timely opportunity as I can use the measurement rig to dial it in. I never thought I would be happy with being 200% less accurate but I am...
It is also great that I am done testing the rig- it's like a test pilot handing over an airplane to a pilot who will fly it for it's real purpose. Now I am gonna put that thing to work!