MOA - I worked up a load for the 300 MK using Reloader 25 in my 338 RUM Sendero. I started at 85 grains/RL25 and worked up to 91 gns. I intially settled on 90.0 grains of RL25 but found faint pressure signs on the cases after a hot day shooting. I dropped back a grain to 89.0 gns for 2605 fps. It easily shoots sub-moa in my Sendero.
I have since switched to using H1000 instead of Reloader 25. When the weather cooled, the velocities fell off to the 2500 fps range using the same 300 MK/RL25/89.0 gn load.
300 Sierra MK
88 degrees F - 89.0/RL25 avg - 2642 fps
70 degrees F - 89.0/RL25 avg - 2605 fps
30 degrees F - 89.0/RL25 avg - 2498 fps
This translates to a 2.5 fps loss per degree F for Reloader 25. I found that Reloader 22 also behaves the same way in my 300 Win and my 340 Wby.
This troubled me for it could play heck with my drop charts, so I looked at other powders to see if they behave the same way. I have tried H1000 and found it is only showing a 35 fps drop between 70F and 30F - so far. My load using H1000 is the same as Reloader 25 - 89.0 gns@2600 fps. I have a small amount of IMR-7828 that I have considered trying also, as I have read the IMR series of powders show minimal velocity/temperature spreads.
I have used H870 for the same velocity and accuracy but I have to use 8 gns more powder to match RL25/H1000's velocity. Hope this helps you a little.
