ORIGINAL: cayugad
It might be the APP. I would suggest trying some Triple Se7en 2f 100 grains and a projectile or some Pyrodex with the same charge. Some rifles just do not like APP.
I think cayugad is correct. My sidekick doesn't like APP either. Using 80 grains and 385 I get velocities ranging from 975 fps to 1500 fps. And of course, the just spray in the vertical with such a broad range of velocity and recoil.
I don't know why APP refuses to burn consistentlyin my Sidekick. I suggest pyro P which I have had luck with using light projectiles or maybe 777 which I want to try.
Phil