From my Sizemaster, YMMV. I use this powder exclusively in the Idiot Stick, found several kegs a while back.
12 15.4 gr WW 1250
12A 16.8 gr Fed 1250 (also works with Eurotrash hulls)
13 17.4 gr R-P 1250
13A 18.1 gr R-P 1300
I have settled on 1250 fps with a full load of Magnum Eights for all my Wienie Gun Clay Target chores. Breaks targets very well, good case life. I use the Claybuster 410 HS wad and Rio primer in all loads.
The 1300 fps load is great using Copper-Plated 5's or 7 1/2's for game.
Very interesting question. I use 300MP in my 410 reloads. The recommended recipes by Alliant cause me some concern. Using WinHS components one recipe is 16.4grains = 10,700psi and 1300fps. Using Rem 209P and Rem components in a STS hull the recipe is 18grains = 6800psi and 1300fps. However, when using Rem 209STS and Rem components in a STS hull there is no published recipe. Strange!!!
Remington's web-site says 209P and 209STS are equivalent. See the cut and paste below. Last year I called Alliant and asked if there was an error. Told - no errors. In my Spolar I use a 272 bushing hand-filed to 280 and it drops 16.4grains as measured with my electronic scale.
Cut and paste from Reminton:
209 Premier® STS® Primers
Extensive testing with a variety of commonly-used reloading powders has shown that new 209 Premier STS primers, in comparison with other commonly-used primers, create reductions of up to 40 percent in the high-to-low spread of velocities in reloads. This resulting improvement in velocity consistency is an important benefit to those reloading target loads for practice or competition. New 209 Premier STS primers can be substituted for Remington's previous 209P primers without changes in published reloading formulas. Check with powder manufacturers for recommendations.