+1 for Mossberg 590A1
My personal favorite is my 14 inch barreled 12 gauge Mossberg 590A1 shotgun with ghost ring sights, a Hogue Youth stock, a Surefire 621FGA Forend Weaponlight, Blackhawk front sling adapter, Specter Gear 3-point tactical sling, and 3GunGear.com 7-Shell Side Saddle, which has a nylon side saddle with elasticized shell holders backed with velcro that affixes to a base on the shotgun receiver. This side saddle carrier allows single-hand removal and attachment so that additional side saddle carriers can be carried on tactical vests or plate carriers and your ammo source can be easily replenished. This shotgun has an overall length (OAL) of 35 inches, which is a nice compact size for manuevering inside a house or building.
I'm a big fan of the Mossberg 590A1 for tactical use and/or HD, if you choose a shotgun for HD. My PD carries 14" Mossberg 590A1's for duty use in our cruisers. We used to carry the Remington 870's for duty, but we switched to the Mossbergs more than 10 years ago and I'm not sure why we did so.
However, I prefer the Mossberg to the Remington...to me the Mossberg seems more sturdy and solid and I like the placement of the safety better with a rifle stock. If you're dead set on using a pistol grip, then the Remington 870 may be better due to its safety location, but I'm not a big fan of pistol grips on a shotgun.
I do prefer pump shotguns over semis, largely due to reliablity in feeding ammo over the semi in most instantces. I know from personal use that my Mossberg eats everything I've ever fed her! She's a nasty beast!
I'd also like to add that I strongly believe that a light source should be MANDATORY for any tactical or HD weapon, so as to positively identify your potential target! Lest it be a family member, such as a teen son/daughter sneaking back into the house...that could be extremely tragic.