I played the Gunnery School in 2 campaigns about 20 games with them in each,
Rules as written you must "Resolve each shot individually", Misfires are far less common than you think,
1/6 = 16.66%, you can ignore the misfire on a 3+ 16.6/3 = 5.5%, of the misfire table only results 1-3 are going to stop the working of the gun so 5.5% - 50% = 2.75%, which at this point you use your rabbits foot not before hand 2.75%-50% = 1.375% plus 2 shots that don't have a rabbits foot 1.375 + 2.75 + 2.75 = 6.875% of misfiring a 3 shot from a repeating handgun or pistol.
Early Campaign Gunnery School is very powerful if they can turn a profit, having a long rifle blow up or losing a handgun henchmen will seriously hurt this warband. Hired Swords are vital for this warband as this warband becomes quickly outclassed in melee so Ogre Bodyguards and Freelancers are good.
Also consider getting your heroes the Skill Ride Warhorse to force people into the open your enemy will avoid your firing lines.
I played beastmen for significantly longer being my 2nd most played warband. I did terrible terrrible things with my beastmen, using "the power in the stones" supplement to appraise the wyrdstone shards i would create super-powered Beastmen which would inevitably turn into spawns and i filled my henchmen slots with heroes who became spawns (and other terrible things, but that's a story for another day).