There are numerous reports that the small BATs have marginal ignition. Unless Bruce has addressed that issue -- and he may well have -- I would go with a B.
Actually, I would go with a 1.125 tenon M, but then, I'd want to allow for a bigger case than a Dasher. A 1.125 tenon will let you use the 404-size diameter case with no qualms -- as when you decide a .338 with a 300 grain Berger is an advantage... ;-)