This may not be a bug but, by design, or the BK5 dosen't like me personaly....
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Offline I've set objects unprotected so I can test destroying buildings and other hard objects.
The MK108 and Mk103 Mine round will register hits on buldings and hard objects eventualy destroying them. The BK5 round passes through buildings and other hard objects exploding in the ground behind them if the angle of attack allows the round to pass through. I eventualy discovered I could kill the radar tower by accidentaly impacting one of the four leg girders. Trying to destroy the tower by shooting rounds into it's base building registered no hits.
All ack postions, GV and aircraft register hits by the BK5.