There are certain things you want to have for a competition shotgun, depending on the division you'll be shooting in.
Regardless of division, you'll want a stock that fits you and an acceptably light and smooth trigger (4 pounds or so.) You'll also want bombproof reliability.
Don't even consider a pump gun unless the division you shoot requires it. Semi-auto is the only way to go.
In Open class, you need to have 11-round capacity, speedloaders, and an optical sight (probably an Aimpoint or CMore.) Your barrel will be about 26" long, and may have a mid-barrel compensator. You'll want interchangable choke tubes.
In Limited, you need to be able to hold nine rounds, ideally nine rounds with an empty chamber. No optics, speedloaders, or compensators. Barrel should be around 20-22". Ditto the choke tubes. You'll also want a sighting system with which you can comfortably hit an IPSC target at 75 yards with slugs. Ghost rings, express sights, and hey, if you can do it with a bead, use the bead.
Heavy Metal is pretty much Limited, but you have to use a pump-action shotgun.
I shoot a Franchi 612VS in Limited - it has a 21" Polychoke barrel, custom-made +3 mag extension, trigger job, and fitted stock/recoil pad. It will be getting a set of XS low-profile sights melted into the barrel rib sometime in the next few weeks. Great gun, but the 612VS is no longer in production so I hesitate to recommend it. For a Limited or Open gun, I'd suggest a Benelli M2. Alternatively, if you want to live on the edge, the Izmash Saiga-12 is making serious inroads as an Open shotgun. For a HM gun, get a Remington 870 and cut down the barrel.
- Chris