Covenant.
Sorry mate - You're analysis is faulty.
I assume an organised group of terrorists attempting a replay of 11/09/01.
If the flight crew are armed (as opposed to Sky Marshalls or anyone else in the Cabin), then the terrorists only gain possession of the weapons after they have secured the flight deck.
My preferred response to their attempt to do this would be to order the cabin crew to defend the flight deck door, (from the outside) to prevent the terrorists from reaching a point where they could batter it down. This means hand to hand fighting, in which the remainder of passengers are available to assist in exactly the manner you describe. Probably, total chaos ensues.
If the terrorists manage to subdue everyone, and break the door down, we then quite likely have a situation where quite possibly they still have strength of numbers. If so, by your own analaysis, I need a 'Force multiplier' to prevail.
Fundamentally, if cabin crew plus passengers have already been defeated in hand to hand, what chance does flight crew have?
CPB