In TripleA (LL setting) SBR is handled like this, if you have 1 bmr, then AA gun rolls @1, if AA gun misses then the bmr rolls 1 die, this means in the long run will average around 3,5, but it could be 1 ipc damage, or 6 ipc damage.
If you have 5 bmrs doing SBR, AA gun rolls 1D@5, so 5 or less is a good chance to shoot down a bmr, and lets assume the AA gun rolled 5 or less, then the 4 remaining bmrs roll 4 dice, which could mean 46=24 ipc, or 41=1 ipc.
IF you have 6 bmrs doing SBR, or attacking any territory with AA gun then one bmr is lost automatically.
With 7 bmrs, one bmr is lost and AA gun shoots @1, 11 bmrs, 1 bmr is lost to AA gun, AA gun rolls @5 or less, so if you got lucky to escape AA gun with 11 bmrs, you loose only 1 bmr, rolls for 10 bmrs, and can do 60 ipc damage…