Been trying to get the 250TTSX to shoot out of my P64 M70 with lackluster accuracy results. After trying RL15, IMR4320 and H4895, my usual H&H powder choices the best groups yielded about 2 MOA accuracy. OK but not good enough with a rifle that shoots sub MOA groups with everything from 235 to 350gr bullets. I've been wanting to see what the new Enduron powders from IMR might do and picked up a pound of IMR4166 which according to the burn rate chart is right in the sweet spot for the old H&H.
Now my problem was getting some data to begin developing a load. Barnes didn't have any and neither did Hodgdon. While I have extrapolated data before in working up a load for a bullet or a powder that was not in any of the manuals, working up a load where there was very little data on either was a new experience. Who knew that the 375H&H was a wildcat?
I used data from the Barnes manual for the 235 and 270 gr bullets since they are about equally heavier and lighter than the 250's with 2 powders that are on equal sides of 4166 on the burn rate chart, namely H4895 and RL15. This gave me a charge weight range of approximately 70-77gr. I Loaded 1 of each and shot these in 1gr increments over the chrono at 200 yards watching for pressure, erratic velocity and accuracy.
The 72,73 and 74gr loads grouped into about 2 MOA.
At 75gr the velocity jumped significantly and was beyond my velocity goal of 2850fps so I pulled the higher loads.
I then loaded 3 of each in .5gr increments from 72.5-74.5gr and shot those over the chrono. The 74 gr load gave 1.5MOA accuracy and the 74.5gr load gave .85MOA.
More tweaking may yield better accuracy and more experience will prove if I'm on the right track but at least this is looking promising.

Like I said -- go hunt!

I've had no luck getting the 250 grain TSX to shoot in my Model 70. Like you, nothing under 2" with all the usual powders.

I have a box of 250 grain TTSX bullets so I'm going to have to try some IMR4166.

I would load more of that

quote:

74.5gr load

and shoot shoot and shoot it some more.
That will do anything you need to do.
By the way I love your shooting range, down in the trees protected from the wind.
My .375 H&H really likes 300 grain Barnes triple shocks and big doses of Reloader 15, gives me sub .75" groups when I'm shooting right.

The 74.5 grain will work, but you should shoot a couple groups of 75.0 over the chronograph again. Sometimes chronos give a slightly errant reading, and sometimes cartridges do funny things. If 74.5 is getting close to a sweet spot, you need to check and see if 75.0 is part of that sweet spot or starting to dissipate the the accuracy.

I'll work up a few more loads and shoot some more groups before settling on the final load. Anything beyond about 74.5gr is over 100% load density with Remington cases.