BM and Mergen are not just ‘fillers’, they are the 2nd source of dps aside from falc’s pheasant.
BM synergizes well with QS3 because of the AA nature of BM, with the help of aiming, you are also on a good spot when it comes to aoe. If you are into investing in 2 weapons, main hand for falc skills while the offhand pistol for bm, then it is ok to go.
Mergen is a bit different, while you can AA using QS3’s running shot, the damage is considerably different when it comes to AA damage compared to BM. Also, Mergen relies heavily on AoE Attack Ratio, where you will find yourself underwhelming in a sea of mobs if you have low AAR.
when it comes to benefit, imo if i will try to weigh bm and mergen, lets say bm is stronger in AA while mergen (with high AAR) is stronger in AoE (considering that build of yours)