Adding a shotgun to your sniper set up is just one more thing you have to take care of on the field. This is not real combat with real guns which can take one hell of a beating. With your primary, shotgun, and pistol, you almost have to babysit to make sure everything is in tact. If you are set on a shotgun, ditch the pistol. If you like the pistol, ditch the idea of a shotgun.
Personally, I wanted to go with a multi shot shotgun for my back up to my primary. It would be pump action, and would be less of a hassle than a gas gun pistol. Not to mention, a tad quieter as there is no loud clack like there is when you shoot a gas pistol. Also, to avoid having two big guns, the shotgun would have to be sawed off, which offers a tad more mobility. This way it is not overbearing for you when you need to run.
Overall, I say if you would like, go ahead and buy a multi shot, pump action shotgun like the UTG one. Cheap, reliable, and not extremely heavy. But, that means ditch the pistol because I know I would not like to babysit everything.
Its only my 2 cents though
Edit: Forgot to add this....
Using the pump shotgun in the winter when it is cold will be effective while your gas pistol will lack everything due to the temperature. If this suits you, buy the shotgun for the winter time, and use the gas pistol in the warmer climates.