Tony, I have 2 of the MR'S, both are SS, my older one has a .500 barrel, and the newer MR LE has the .501 barrel along with the NEW Optima that has the Nitride coating. I have used the supplied Red sabots from Hornady, but I like the feel and how the Harvester easy load black smooth sabots work, I use mainly the Harvester smooth sabots unless I will be shooting the 458"s then I use the MMP.
The reason I was using the RED CR sabots 260 PT Golds was, the last time out with them, the first 1 shot great,then the other were all over, In other guns I have had no problem with them, so I knew I could do better, and after I switch sabots it seem to help. The 300's I used both sabots and they both shot great. The Barnes I like the Black smooth sabots.
What I was trying to show, if you like a bullet and you are having a problem grouping with it, try a couple of different sabots.
Dave