I dont find any published data for the 500 Linebaugh using IMR 800x but a friend has been using it in his 500 GNR. I started working up loads with a 410 grain wfn. Results were 10.0 grains went 725 FPS, 12.0 grains went 850 FPS and I am trying the 12.0 grain charge again but it goes 980 FPS...