I wounder why S6 AP 3 when a Heavy Boler is S5 AP4+ ...
Well, Forgeworld rules were originally (and, to my knowledge, still are) designed by tweaking the outputs of the Vehicle Design Rules (VDR). Under VDR, the "mega" upgrade improves the AP and the S of a weapon by 1.
The 15 shots, on the other hand, is a departure from the VDR rules, which would give, at most, 3D3 shots.
Inquisitor Psychologis Ruminahui