Shooting a 338 Win with 75 gr RL-22 behind Sierra 250gr gamekings and FED-215 primers. Velocity is right at 2800 fps. SD is good. COL is 3.35". Pressures "appear" a lot lower than 74 grains of RL-22 and the 225 gr bullets.
The symptom appears in the muzzle blast. When groups are good (less than inch at 200) there is a "certain" muzzle blast. When groups go to pot muzzle blast sends the chrony reflectors like scattered quail. No kidden! Muzzle blast can be seen in the tree branches/leaves about 30 feet away.
Saturday after I got the dies set up 3 3-shot groups averaged <0.6". Velocity spread was tight. Then things went to pot. Muzzle blast blew the plastic reflectors every shot for the next 3 groups. Velocity spread was still good. Groups opened to 4 or 5 inches.
I may be the slowest guy on the mountain . . . . but . . . . I'm on the mountain!
Great powder..shoot it in a 6mm/284, great groups with just about everything else I use it in..only drawback..it's dirty, really makes a mess with all the residue it leaves behind, at least in my bores...just switch over to "H-4350" and end most of the problems..I've found the the "H" versions shoots better in all my magnum rifles..