As for custom building a 12 ga bear gun, here's some points to consider:
- choose a repeating action,
- minimum 2 shot capacity for those who prefer doubles,
- choose stainless steel or nickel plated finishes if possible, but parkerizing and bake on finishes are also very good,
- use a full length stock, rather than pistol grip only,
- whatever the LOP you need is, cut the stock an inch shorter to allow for bulky clothing,
- install a good quality recoil pad,
- keep the barrel shorter than 26", but not so short that you could inadvertently get your support hand out in front of the muzzle,
- choose an open choke and if your gun is equipped with screw in chokes, choose one that extends past the muzzle to protect it from dings and burrs,
- use a ghost ring and post sight or at the very least a bead mounted on a pedestal to bring it to the height of your receiver to prevent you from shooting high,
- the front sight or front pedestal and bead should be soldered in place,
- if you need a barrel mounted swivel, have it soldered in place so it can't move,
- install a flashlight mount, and acquire a good quality light suitable for the application,
- understanding that in some circumstances you will only have access to the ammo that is in or on the gun, consider a side saddle, a speed feed stock, or a sling with shell loops, with the knowledge that all of these things add weight and bulk to your gun making it slower to use effectively.
- shoot the gun under a variety of conditions with the same ammo you intend to use in the field to ensure it is as close to 100% reliable as possible. If there appears to be deficiencies that detract from the gun's reliability, address those deficiencies before going afield with that gun or ammo.