I think it's because your using stock darts. If you want to increase ranges with those, get a piece of 1/2 pvc, it fits stock darts well. That way the dart can travel down the barrel to gain more momentum.
Retiate. I am glad to hear you modded your pump action shotgun.
Without your write ups, I never would have bought it. I probably never would've even known about it...
But I have it, and I love it. It is by far the most feared gun out of all the guns in my group of Nerfing friends. It's the only gun that people will hesitate to rush at; if I point it at someone, they'll back up or seek cover.