I've been meaning to experiment with slower powder in the .243 and have a little bit of RL-26 and 100 grain Sierra FBSP to play with. Started out a couple of grains below book max and based on these ES and SD numbers think this shows a lot of promise despite leaving some velocity on the table.
46.2 grains
3029
3020
3033
3021
3027
ES 13
SD 5.4
46.4 grains
3036
3028
3028
3035
3026
ES 10
SD 4.5
This is out of a 21" barrel. Did not try to shoot groups with the Magnetospeed dangling off the end.