Retumbo worked the best in mine with 130 gr Swift Scirocco's at 3200 fps from a 24" barrel. With the 140 gr Accubond and Partition I worked up to Hodgdon's max of 63.5 gr for speeds in the 3050-3070 fps range in the same gun. You should expect a bit more speed with a 26" barrel, others load the 264 WM much hotter but mine did well at those speeds and I know they were safe. When I first got my gun built I tried both Winchester and Remington factory loads, the Remington wasn't bad but the Winchester was awful. Earlier this year I ran a bunch of the remaining Winchester loads over the RCBS Casemaster and some of them were just terrible, the ones that were seated straight though grouped nicely.......
