Posted this over in the reloading section, but will also ask here........I have some of this powder, and can't see why it wouldn't work in the 243, but can find no data whatsoever. Has anyone ever used it in the 243?

I had never heard of it. I am very interested in it though.

I would Email Winchester they have a sign up sheet for new data on their reloading site. I know it is close to the 4350 burn rate but it is such a unique powder that I doubt any other powder data would match it well. You could treat it like a wildcat and watch the chrono but hard to derive a starting dose.

The load data from Winchester for the 6 Creedmoor would be a good place to start http://www.hodgdonreloading.com/data/rifle The 243 holds 2-3 more grains of powder than the 6 Creedmoor depending on bullet seating depth. With longer bullets they are almost the same due to the shorter neck on the 243.

In the Creedmoor the charge for StaBall was fairly uniformly one grain more than the charges for IMR 4350 so it should be close to H4350 as well. Once you hit the velocity of the Creedmoor or one more grain of powder than the listed loads I would stop. You may still be half to one grain under max but that is a good place to be until tested data is available.