I like the idea of this mod. A longer stock seems necessary for it to be comfortably shouldered, and for a better
cheek weld. The pump fore end seems designed primarily to speed cycling the rounds in the action.
I have two concerns.
The first one is simple; the sound of the pump action makes me cringe, but Coop already covered that.
The larger concern is the fact that the pump device raises the surface of the top of the receiver by two inches or more,
from what I can see. This will likely cause some sighting issues for those who wish to add custom sights to the top
of the receiver. While not a deal breaker, in terms of sighting along the trajectory, it would also force the shooter to raise their
eye level slightly, which could be adapted with a modified cheek piece, but still.
I would have hoped they would cut off the top charging handle, and instead created an attachment
which would wrap/bend it onto the side of the gun, thus, allowing for a side charging handle rather than a top one.
This would allow the emplacement of "iron sights" to the top of the receiver without raising the sight, "height over bore."
Don't take this the wrong way, but you're trying to apply real steel practices and ballistics to Nerf and related blasters in such a way that isn't practical or worthwhile (unless like you like doing things for the sake of doing things, which is cool). The tolerances, workmanship, materials, and other factors are so poor with Nerf blasters that trying to maximize accuracy doesn't make any sense. Yes, the Nerf Rival line represents a new generation in accuracy and precision, but the yellow foam projectiles and Nerf blasters that use them can't take advantage of refined sights or ergonomics (at least with respect to vastly improving accuracy).