I'm doing a gestalt game where my munchkin friends actually get to go nuts. One of them wants to do a summoner with a bunch of extra arms, and each arm holds a double barrel shotgun. The summoner specifically says you are limited to the number of attacks given by your eidolon, but is should at least double from the double barrel. He's a lvl 10 summoner synthisist/gunslinger. How many attacks are we looking at and what would his bab actually be?
Bab would be just like a 10th level gunslinger
So only 2 attacks at 10/5?
Are they at a cumulative -2 as multiattack or -5 as normal?
4 arms with double barrel shotguns... So 8 attacks?
The -5 to hit/-2 with multi attack only applies to natural attacks.
Shotguns are not natural attacks.
Penalties are going to stack up fast.
To start you are talking about -6 to hit with one weapon, -10 to hit with every other weapon. That penalty gets reduced to -4 to hit for all attacks if you have two weapon fighting or multi-weapon fighting.
Shotguns fired one handed suffer a further -4 penalty to hit (and I imagine can't be easily reloaded).
Firing a double barrelled shotgun suffers yet further -2 to hit if using scatter shot, -4 to hit if using bullets.
You get a number of attacks equal to the number of weapons you have plus 1 at -5 to hit with one weapon (and you will need a way to reload that weapon as a free action if you used the last bullet with your main attack).
Double shotguns don't grant you extra attacks, you can fire both barrels as a single attack for extra damage.
4 arms with double barrel shotguns
4 attacks at -12 to hit and possibly 1 attack at -17 to hit with bullets or 4 attacks at -10 to hit and 1 attack at -15 to hit with pellets.