This could prove to be very good advice depending on what boolit you're launching. When I was loading right at, or just over supersonic velocities, I could get nice 1.5" groups at 50yds. Move it out to 100yds and groups would open up to 4"+. I was loading the Lee 170 fn gc, without the gas checks. My only guess is that boolit goes unstable when it transitions to subsonic in flight. I can load the same boolit to 1400 fps.+ and hit 2-3" groups at 100yds. The only difference is that the boolit is still supersonic at 100yds when loaded at, say about, 1350 + fps. The same boolit loaded subsonic at the muzzle, will also print sub- 3" groups. Go figger. My goal after this bit of frustration is to load at velocities that will avoid the transition from super to subsonic while in transit to the mark. Different boolits will behave differently. Simply adding a gas check my Lee 170 may have cured my problem, but I haven't tried a back to back experiment on that theory.