I fully understand Sine waves and barrel harmonics. But you stilldida good job explaining that. I am just of the opinion that the bullet has not left the barrel, before the barrel starts to raise(because of the rifle recoiling). I used the pistol reference, because when the barrel was strapped down, it could not raise due to firing the round. IMO it is the same as the rifle jumping off the hood of the truck in my earlier reference. You said you agreed with me about the POI being different when you shoot with bipods off the hood of a truck. The flimisiness of the hood could only effect POI if the bullet was still in the barrel when the hood affected the rifle? How could the hood affect the POI unless the bullet was still in the barrel? I agree that there may be a velocity when the bullet is out of the barrel before the barrel starts to rise from recoil. And I do believe you when you say the lead sled does not affect your POI. Were you shooting a heavy rifle? Or a rifle that was ported? Both of these factors would affect how high the barrel would rise before the bullet left the bore. I realize that we are splitting hairs, but it sure is fun. I have been thinking about this all day. Tom.