What if they overpower one? Then they have a gun.
Already have 'em; have had 'em for years. Piece of cake to get through.........believe me.
Then we need stronger doors, and they need to be locked for the majority of the time.
In the meantime we'll have to settle with putting policemen (or "Sky Marshalls") on the planes.
Why not use extremely weak calibers, with full auto fire? An SMG firing .22 safety slugs at a very slow rate of fire (for an SMG, maybe 400 shots/min) would make control very easy, limit the chance of blowing out a window, and still give the firepower necessary to drop anyone who wasn't wearing armor on their face. If the rate of fire was too high, several bullets at once could slam into a window, breaking it.