Jerry, shouldn't I be looking at a faster burning powder instead of a slower one to get complete burn and avoid pressure spikes?
Ivo
NO, the last thing you want to do is use a faster powder. This simply accelerates you reaching less then desireable pressures AND you will have less then ideal case capacity.
My best loads have always been with a charge of powder that fills a case as much as possible AND ignites cleanly at or UNDER 68,000psi WITH a primer that is just hot enough to get the job done.
I have definitely run loads much hotter. Although not a good idea, I have run them till I don't need to decap the cases anymore (bad idea by the way).
Accuracy has NEVER happened at these elevated pressures. At least without a whole headache of inconsistencies and day to day fluctuations. There is nothing on our shelves that will burn safe and accurate beyond magnum pressures.
I have even tried Re17 and Yes, got some real tasty speed BUT accuracy was the pits until I throttled back to what I was getting before. At which point and in my cases, load density was not what I wanted.
A simple matter of an ever so slightly smaller bore and an ever so slightly larger bullet and you have pressure WAAAAY outside what the book predicts.
Custom AND Factory actions can mask Proof pressures loads no problem at all. I have proof pressure test cases which have ZERO outward appearance that they were anything but factory loads. If they weren't marked as proof loads, you wouldn't have known.
These cases were fired in a factory to test their rifles before shipment. I bet that case was ejected with slight bolt lift effort. When you see pressure signs in a full meal deal custom, you are WAY into religious pressure territory.
My suggestion is to try Retumbo or even H50BMG/US869 - yes even slower powder. I expect that your case will hold close to 70grs of an extruded powder so you need to consider something slower.
Work up WITH your chronie with a start load that is around 2700fps. Then work up in 1/2 gr increments until you approach 3000fps. For the first few grains, velocity WILL increase in a steady and predictable manner. Accuracy will also improve.
Then the velocity will plateau, and for 1 to 1.5gr, the velocity will stay the same but the groups will continue to shrink. Become consistent in size but the orientation of the group will flatten THEN rotate to be vertical.
Then the next 1/2 to 1 gr increment will show a substantial spike in speed from 50 to 100fps and groups will open up dramatically and continue to do so as speeds rocket up.
When you are at the upper end of your load range and your speeds DECREASE, you could quickly be approaching dangerous pressure levels. Why some powders do this, I have no clue but seen it enough to know that is not good.
Usually, you will have all manner of pressures signs in a common action and it will be pretty obvious that you need to stop or buy more life insurance. You will see zip in your PGW and the cases will extract likely with 1 finger bolt lift.
I doubt that you will see 3000fps and still be bug hole accurate. My SWAG would put you around 2850 to 2950fps as your top sweet spot
You have a 24" barrel. Most competition rigs will have up to 30" pipes so we will see up to 150fps more speed per given load.
Good luck with your testing.
Jerry
PS Hodgdon lists 175gr Speer mag tip with 64gr of Retumbo at 2800fps and 61,200psi. Win Supreme 780 powder is 68gr at 60,500psi for 2850fps. They don't list a barrel length but it isn't over 26".
