The list seems endless as to the accessories available for a tactical shotgun; energy absorbing stocks, pistol-grip only stocks, slings, sidesaddle ammo holders, ammo pouches, scabbards, lasers, specialty shotgun shells such as rescue flares, cherry-popper shells designed to scare animals, and many other specialty products are available for popular gun models. Some of these are more useful than others. For example, a shotgun sling made with a bandolier for extra ammunition will adversely affect your aim if you are moving quickly. Whatever you purchase, be sure to test it on the shooting range.
Other than a flashlight, the most popular other accessory is a sling with sling-mounts which were made for your model of shotgun. A word of caution: Adding a sling to a pump-action shotgun can create a problem. A poorly mounted, or improperly placed sling-mount, can make it difficult to operate the pump action of the gun, so practice with the sling, is essential. But more to the point, a sling isn’t needed unless you plan to stand guard duty, or carry your shotgun outdoors. If you gun is exclusively for self defense in your home, this accessory may not be helpful.
Another popular and more useful accessory is the shot-shell sidesaddle. Most tactical shotguns will hold 6-8 shells, and hopefully this will be more ammunition than you will ever need. However, many police departments and self-defense experts mount a “sidesaddle” shell holder to the receiver of their gun. These shell-holders can be used to hold an additional 4-6 shell of your self-defense ammunition, or it can be used to hold specialty shells such as rifled slugs.
For either purpose, with a sidesaddle bolted to your gun’s receiver, these additional shells are close at hand, just in case they are needed. If you do mount a sidesaddle to your gun, be sure to test fire it after the installation. If the sidesaddle is mounted too tightly, it can adversely affect the mechanical action of the gun. Use a drop of Loctite on the threads to make sure it doesn’t come lose over time.
Sidesaddles or stock-sleeves made of elastic are generally not a good idea. If you store shells in the sleeve, the elastic tends to fatigue over time, so the shells can easily fall out of the elastic bands. And under recoil, the elastic sleeve which attaches it to the shotgun can shift, and this can cause additional problems.