Optimal speed for Polymags

I've noticed that once you break the 800 fps wall with them, accuracy goes down hill after the 20 yard mark. I can get 1/2" groups out of my P-Rod at 637 fps at 30 yards, 1/2" groups from the Trail NP & NP2 at 20 yards going around 727 fps, but when I put them in a Gamo Bone Collector at 834 fps or the Daystate Regal at 868 fps, they are all over the place.

I think the same thing is happening in .177 but I've only tested them out of a Gamo Big Cat so I can't really say but in .25, my Marauder handles them fine out to 40 yards over 800 fps but the Air Arms S510 and Brocock Compatto don't.

I've questioned if it has more to do with how the lead performs in the barrel itself than with the velocity of the pellet.