Nice pics. Did you make the base image yourself?
In proper DE fashion, I will now steal that image as the core for making a Splinter Rifle to Splinter Pistol diagram.
1) Get a Splinter Rifle - while I prefer the one without a stock, most are fine.
I'd suggest staying away from the one held off to the side of the body, because removing the stock will require extensive rebuilding of the inside of the arm (as opposed to just the lower arm if you screw up).
2) Cut the barrel off at a slight downwards angle, cutting just after the fifth barrel-spike from the arm.
3) Trim off the fifth barrel-spike and the bottom line along the barrel.
4) Cut off the barrel just inside of the connecting segment -
do not discard - and then cut off the connecting segment, making the cut along the line of the fourth under-the-rifle bump. You will damage the large jewel-thing on the outside of the gun.
5) Cut off the damaged jewel-thing and glue on the (now shorter) barrel-section.
6) Trim down the bump up at the back of the rifle so that it slopes away from the arm.
If you chose a rifle with a stock, carefully clean it off of the arm.
The resulting pistol is still a bit bigger than the one from the sprue, but it's small enough to be a passable long-barreled pistol. For the proper length, you'd have to cut the barrel off another barrel-spike in, but that also would require trimming the connecting segment away from a portion of the bottom to get a decent appearance.
The early attempt at something simple tends to be stopping after step 3, but that looks too big to me - an appearance I've started calling a splinter shotgun.
