I am going to try and clear some things that I think is causing some confusion.
BBA might be okay if you are using a first strike friendly riser along with your optics of choice. Those adjustable risers give you an additional 2" or so, not including your optic's mounts. At that point you should be able to clear a mask even against a bba tank.
Or you could get a fancy $200 crossbow scope with adjustable fps settings and forgo the riser, but then you would definitely need a bottom line.