The Service Rifle game Slow Fire Prone stage is shot at 600 yards.
The stage is single feed, but the most popular loads are too long for the magazine anyway. That is because the 80 SMK is one of, if not the most popular rounds. The other stages use magazine length loads because there are rapid fire strings.
Of those 600 yard line loads, the typical 80 grain loads shoot inside the 77 OTM and 77 TMK. The 77 TMK comes very close, but still doesn’t match the 80 SMK.
A typical SR has a 20 inch National Match Profile barrel, a Wylde chamber, and a 7 twist.
I’m not sure what the bbl twist options are on the Ruger Predator, but if your twist won’t stabilize an SMK80 than the 77 TMK is a good option.
My typical SMK 80 Service Rifle load is a LC case, 24.0 gr RL-15, and Rem 7 1/2 primer. Seating depth is roughly 7 mils of jump in a WOA bbl. The velocity is going to run between 2650 and 2725 FPS depending on the bbl and the batch.