All-In Attack (Pathfinder Variant Rule)
From D&D Wiki
Rolling a lot of dice can be very time consuming, and combat is a long enough endeavor without having someone roll their d20 up to 4 or 5 times, then rolling all of their damage die multiple times... it's a long process, only exacerbated by larger and larger groups. If you are playing a mid- to high-level campaign with a party consisting of more than 3 or 4 Player Characters, I recommend using this variant to save time.
Instead of making cascading attack rolls with lower and lower attack bonuses, instead use your highest attack modifier for a single die roll. Roll damage as normal, but multiply the result by the number of attacks you would have had. For example, at a BAB of +5, there would be only one attack roll. However, at BAB +6, you would double your damage roll. At BAB +12, you would triple it.
This multiplier only applies if the PC uses a Full-Round Action, meaning they cannot get this multiplier if they take their Move Action. This is also not applicable to certain Crossbow attacks, unless ruled so by the Game Master, as reloading often requires a Move Action.