In general....any general purpose shotgun is fine for 5 stand...12ga or 20ga with a 28" barrel is fine....and every 5 stand is different in terms of target presentation / but for most 5 stand operations, a Modified choke is a good compromise.
For the 5 stand at my local club ....they have about 25 machines ....and the various options for which combination of targets is thrown is referred to as "Cards"....and I think they have about 15 different "Cards" they can elect to have pulled for a round....from pretty easy, to "black" as in nightmare difficult.../ the tougher cards will often have more "true pair" combinations...but a typical card...the first target is a single ( and you can fire 2 shots if you need to on it / then a report pair ( 2nd target is pulled on the report of your gun by puller ) / then a true pair (both targets in the air at once). Targets will vary - like sporting clays....some on ground as rabbits, some rising up from ground, some coming from behind you from a tower, some dropping in from a distance....or from right or left...depending on how many machines they have.
In general, you can never load more than 2 shells..( in a pump one in chamber and one in mag )...you shoot within a cage, that limits your gun swing and where you can see the targets, make sure action is open and no shells in gun when you move between stations ( there are 5 Stations / " 5 Stand"....)....on the most entertaining cards...the shots are fair to a mid level competitor ...but they require some "thought" especially on the True Pairs on which target to engage first...so you still have a good shot on 2nd target in the air....
In a 12ga....1 oz of shot is plenty ...and most shooters will shoot 1 oz of
8's at around 1200 fps ....but 7 1/2's are fine since that is what you have ( most clubs will not let you shoot anything bigger than 7 1/2's....so no 6's or 4's, etc...)...
If you wanted to shoot a 20ga.....I'd still go with 8's ...and 7/8 oz loads at 1200 fps...
Focus on having a good time.../ proper safety procedures...
Stay balanced, good fundamentals ( follow thru on the shot - pulling trigger is the start of the shot not the end of the shot ), good gun mount, rotate with lower body ( don't push the gun with arms), focus on leading edge of the target, think - analyze your lead, consider a clock face ( numbers 1 - 12 ) ....so evaluate your lead like "shoot that bird at 7 O'clock with about 3 feet of lead or whatever"....and of course every target is different.../ ....same kind of fundamentals in upland bird hunting, or Trap or Skeet...
