[1tonne]

Normally if I have a rifle firing around 500fps I would use 0.36-0.4gm bb's. The rifle won't have enough energy to push the 0.43gm bb out to long ranges though they will still shoot OK but you are better off sticking with the 0.4gm that you already use.
The reason why your 0.4gm bb is losing energy is because you have leak between the air nozzle and the bucking. Since you need more hopup to use the 0.4gm bb, this creates more downforce on the bb and so it is held that little bit longer in the bucking which allows pressure to build up. Since there is more pressure, the air bursts out between the bucking and the nozzle. If you get an AA VSR10 Hopup Chamber, you should see this issue fixed.