BAR may not be far off. For what it's worth I find targets where my POA is on a square or edge versus a circle are harder to center on at distance.

To be clear, when you shot the string of six did you shoot three, reload, shoot three? Shoot one at a time, checking your scope each time? Shoot six in a row without moving up off the gun? Did you have a spotter calling each shot? This can help narrow down what is happening.