I don't think the OP thought about his weight limit when he stated it. He said it was to limit the 105mm howitzer responses. Regardless, it isn't the "ultimate" question since it is asked about once a week (at least) on this forum. My choice would probably be my XDm 9mm (no clue what it weighs, but not much compared to my SBH or Walker). I feel certain I could get off multiple shots with it in stated scenario. Maybe not all 20, but more than one.
'Merica: Back to back World War Champs