Im always willing to pass on a bit of info to help out a fella THR member. The reason all those shotguns look shorter is because they ARE shorter. Your average hunting shotgun ahs a barrel that ranges from about 26 inches up to 30 inches (most of the time, there are 24 and 32 inch barrels out there)
These longer barrels are to control the pattering of the shot. Now, could you imagine trying to sweep you house with a 28" barrel, I did it once, was NOT EVVECTIVE AT ALL. That’s why you chop the barrel down to 18-20 inches and it gives you a more nimble gun to maneuver.
I wish I had the link to the test, but testing shows that there is a very minimal velociety difference between an 18" and a 26" barrel.
Some would say that having one shotgun with two barrels is the way to go, however I would have to disagree with them on the sole point, who is going to reconfigure the shotgun from HD to hunting EVERY TIME they change uses.
I would prefer getting 2 of the same model gun, one for hunting and one for HD.
As far as being custom or buying like that, its both ways. You can buy a HD/Police gun already set up or you can go to a pawn shop, get a used gun, and build to you taste.