I have to add one more thing; there is nothing wrong with simply buying another barrel for HD duty and it most certainly would be cheaper to do so. Having said that, however, Im of the mind that its always a decent idea to have at least 2 shotguns (for instance, one for sport and one for HD).
Reason being that if one goes down (for repairs) then you have another as "backup." Plus, if you plan on outfitting a shotgun with various accessories "tailored" to the HD crowd, it might be more convenient to install this stuff (and leave it) on the HD shotty vs installing/uninstalling this stuff, as needed, on just one shotgun. Of course, if you arent into the "accessory" thing, then this wouldnt be as much of an issue. Still, having a "backup" is always a good idea, IMHO.
For instance, I will have another shotgun in just a bit (making it #2) and I plan on keeping a weapon mounted light system, mag extension tube, speedfeed stock, etc., installed on one gun while leaving the other essentially stock. This is ideal for me as I could then have multiple purpose weapons without needing to constantly swap stuff around on just one.
Now, this is just me and since Im not you, well....
Regardless of which way you go, you wont be making a "bad" decision and that is a good thing!