The main benefit of a Longshot shotgun grip is a more efficient method of priming the blaster. Users can still support the front of the blaster while priming it, thereby maintaining steady aiming and better rate-of-fire.
In addition, the shotgun grip's dual arms both act in tandem to push back the bolt sled, which helps to balance out the load on both sides of the bolt sled during the priming action, thereby reducing uneven load stress (one of the common causes of bolt sled breakage, especially when using stronger aftermarket springs).
There are many ways to convert Longshots into "shotgun-gripped" versions. You can find many examples of similar mods at the various Nerf forums. Most of the mods require a fair bit of blaster modification and custom work though.
I use a slightly different (and simpler) method, with the objective to shotgun grip my Longshots without drilling or cutting the blaster casing, so that they can be reversible back to stock condition anytime and the shotgun grip can be transferred to another Longshot easily.
Okay, let's start...
Step 1: Remove the original bipod legs from the Longshot.
Step 2: Measure and draw out the arms and grip dimensions.
Here is my sample template (download and print it out in A4 size borderless mode).
Superimpose the schematics on the polycarbonate sheet. I use 3mm polycarbonate for the grip but 6mm polycarbonate for the arms (6mm thickness is highly recommended for the arms, especially for Longshots with stronger springs).
Cut out the sections using a jig saw or scroll saw. Please be careful when using power tools. Use a hobby knife or file to round off the corners slightly and smoothen the rough edges.
Step 3: Using a heat gun, i shaped the grip section into a U shape with twin inverted guide rails. Again, please be careful when using heating tools.
Step 4: Position the grip and arms.
Line up the screw points and use a motorized rotary tool (ie. Dremel) to drill the necessary holes.
Step 5: Position and solvent glue 2 small pieces of polycarbonate within the inverted guide rails, tinker with the dimensions until you get them to the right size and position.
This helps keep the grip in a level position when in operation.
Step 6: Assemble all the parts together.
I used 4 x 3mm diameter screws, washers and nuts to attach both arms to the grip, and a 3" length 4.5mm diameter bolt and nut, along with nylon spacers to thread through the original priming handle points and secure the arms to the bolt sled.
This helps keep the grip in a level position when in operation.
Step 6: Assemble all the parts together.
I used 4 x 3mm diameter screws, washers and nuts to attach both arms to the grip, and a 3" length 4.5mm diameter bolt and nut, along with nylon spacers to thread through the original priming handle points and secure the arms to the bolt sled.
Here is another example of my polycarbonate shotgun grip with some additional shaping and trimming for customized ergonomics to suit my personal grip style.
As you can see, no drilling or cutting of the original Longshot casing is required.
The polycarbonate shotgun grip's inverted guide rails basically ride along the Longshot's existing groove at the bottom part of the front section.
This is just a quick guide to show a simple way it can be done... there are many other methods to do it too, see what suits your preference! :)