Just finished buiilding my first work-up loads using the Barnes 115g TSX and H4831SC for the .257 Roberts. And have to admit I'm somewhat disappointed.
Barnes recommends using standard "X" bullet load data and working up to 1-2 grains heavier for the TSX due to reduced friction. Great - the max load for H4831 is 50.0g, so I thought I would build loads up to 52.0g, take them to the range and chrono them, and stop shooting when I hit the "X" velocity of 2956fps or pressure signs, whichever came first. But it was not to be.
I had to stop at 51.5g because the powder charge was becoming too compressed with the long TSX bullet and the COL was starting to grow. I'll have to be careful when I shoot these to make sure that they haven't grown even further while sitting in the safe waiting to be shot.
It looks like H4350 would have been a better choice - good load density but with room for another grain or two, and a loss of only about 50fps.
Oh, well, now I have a pound of H4831SC to play with in my 7mm Mag, been planning to switch from the IMR4831 I've been using for 20 years anyway.
Here's a question: How come I can find good data for Varget for the .22-250, .260 Remington, 7mm-08, 7x57 and .308, but not for the .257 Roberts? About the only data I can find is from Hodgdon, but its not +P data. Bummer. Well, I have 10 of the Barnes TSX bullets left, think I'll see what Varget does with them.