Okay, I'm getting better at hitting something with the bump fire stock on.
I went through 20 rounds and one of them hit the target (lol) I'll shoot 1-2 hundred rounds and maybe 10 will hit the target. Basically the first one out of the barrel when you first pull the trigger. It's still a lot of fun. It's a great stress relief for my wife.
How heavy is your rifle? Try just cradling the forestock rather than gripping it. If the rifle is heavy enough, it will settle back down, but if you are gripping it, you will make it "rise" because you're holding it at the top of the recoil point. At least, that's true of a Thompson fired on full auto. I used to know a 90-pound woman who could put most of the rounds in the black because she just let the Thompson settle back where it started. Hold it tight and it will "rise" higher with each shot. Try this and see if it works with the bump stock. The worst I've ever seen was the M-2 carbine because it was so light. I have a 9mm Ingram that is just okay to shoot full auto.