The biggest question when firing a shotgun without a stock and pistol grip only is accuracy and recoil. It will take some practice because you cannot hold a pistol grip only shotgun and fire one handed as you would a handgun. Your wrists simply will not be able to take the recoil and you cannot maintain target discipline, meaning follow up shots are more difficult. There are advantages though to a pistol grip only shotgun in tactical situations.

Hogue Tamer Pistol Grip and Forend for Remington870
Hogue Tamer Pistol Grip Forend

Hogue Tamer Pistol Grip Forend

You can get Hogue Tamer Pistol Grip Forend here: http://tinyurl.com/bnz96hy

Some shooters have modified their weapons for varmint control. Farmers and others that may encounter snakes or rats have been known to saw off the barrel and modify the stock and load with buckshot or birdshot to take care of snakes or rats at close range. In some cases, it is possible to fire one handed.

Keep in mind current state and federal laws make it illegal in most cases to own a shotgun with a barrel length less than 18 inches.

Pachmayr Vindicator Grip for Remington 870
Pachmayr Vindicator Grip for Remington 870

Pachmayr Vindicator Grip for Remington 870

You can get Pachmayr Vindicator Grip for Remington 870 here: http://tinyurl.com/cour547

Close quarter combat is just that close quarters and you may not have the room or time to bring a weapon to your shoulder. Some military and law enforcement agencies will use a pistol grip only shotgun as a door-breaching shotgun because it is easier getting close to the door and you do not have to shoulder the weapon in confined spaces.
Typically, the firing stance is where the grip is close to your chest and you are looking over the barrel. Keep in mind the weapon is for close range only however, some hunters use a pistol grip only shotgun when hunting turkey or other animals that are stationary when the shot is taken.

Knoxx Breachers Grip for Remington 870
Blackhawk! Knoxx Breachersgrip

Blackhawk! Knoxx Breachersgrip

You can get Knoxx Breachers Grip Blackhawk! here: http://tinyurl.com/c5ayza3

You can bring the weapon in tighter just as if you had a stock, then tuck your elbow and forearm in close to the chest for sighting along the barrel. You can also hold your elbow straight back from the body and let the elbow swing to absorb the recoil, this is a point and shoot method because the muzzle is at chest level and you cannot sight along the barrel. Some when shooting this way may tend to dip their head toward the weapon. Keep your head straight and eyes on the target. Practice shooting this way and over time, you can become accurate at close ranges.

ATI Talon Pistol Grip for Remington 870
ATI Talon Pistol Grip for Remington 870

ATI Talon Pistol Grip for Remington 870

You can get ATI Talon Pistol Grip for Remington 870 here: http://tinyurl.com/cl8s92b

Use the combat stance when moving in on a target making sure your weak foot is forward and you are slightly bent at the waist so you can swivel from side to side and absorb the recoil. You may have a greater advantage when firing at moving targets at close range when you have a pistol grip only.

Crouch slightly with the weapon at chest level and make sure the barrel is not dipping toward the ground keep it horizontal to the ground. Remember you cannot obtain a sight picture because you will be firing from chest level. To engage a target in any direction you will swivel the weapon and your body by moving your feet. You have to be a position so that when you fire the recoil does not put you off balance. You weak hand is always on the weapons forend.

Pachmayr Vindicator Grip and Forend for Remington 870
Pachmayr Vindicator Grip and Forend

Pachmayr Vindicator Grip and Forend

You can get Pachmayr Vindicator Grip and Forend here: http://tinyurl.com/ar94eah

Some may find it a bit awkward at first working the pump action because you will have to hold the weapon by the pistol grip when cycling the weapon. This will require you to push forward with your shooting hand while the weak hand works the action.