This instability of a projectile just before it goes subsonic is a well known phenomenon, and displays itself in varying degrees regardless of whether you are shooting a rimfire or a centerfire. Occasionally, there will be a load that seems to defy this destabilization effect, but it is rather rare. I shoot a lot of rimfire, and almost all hi velocity 22 rimfire ammo shows this effect to a fair degree. The 17HMR also shows it, but out considerably farther, due to the sleeker bullet design and much higher initial velocity. When we shoot 1000 yards, we are always concerned, [with smaller capacity cases, thus lower initial velocity] about keeping the bullet considerably supersonic until it has reached its destination, because of the detrimental effect of this destabilization phenomenon. FWIW, this is why "Quality" match grade 22LR ammo is always subsonic from the get-go. My present lot of Eley Match EPS is 1059 fps avg muzzle velocity. Regards, Eagleye.