Danny, It sounds like you were using the Agula "colibri" bullets. They don't have any powder in them and only use the primer material as propellent. While they are sub-sonic. They are only rated for 6 ft # of energy and 375 Fps. for the regular and the super has 11ft# of energy and 500Fps
NOW.... If you look at CCI standard velocity witch is subsonic also it has 102 Ft # of energy and travels at 1070 Fps.
I have personally shot pigeons at point blank range with the super colibri's and they just bounced off but the standard velocity on the other hand was quite lethal at 30+ yards
The whole reason for writing this is to let others reading this better understand that there is a lot more to the energy of a bullet then just the speed
You may have something there as to what I experienced.