Since the gun was such a disaster, I had to start from scratch - and that meant singling it. I managed to find its plunger catch, and got to work.
Here it is now:

I ended up with what is essentially the opposite of Ice's singled LS mod - instead of a coupler sliding over a barrel, it's a pipe going into a coupler that's attached to the barrel.
Internals:

It works just like a singled longshot, it's just made differently. To counter the safety issues created by singled Longshots (like finger loss), I did a few things:
First, I cut notches in the side for the bolt. Should the catch fail, the bolt's not going anywhere:

Second, I put a ridiculous spring on the catch:

If that shit slips, I will be very fucking surprised.
This entire gun was constructed just to give this little guy a purpose. I saw CS's integrations, and thought about guns I could slap the SMG onto. Then I thought about the poor mangled black and red Longshot I had in my closet, and decided to bring it back from the dead.
I added Velcro, and attached the Spidey Gun to the side of the LS:


Once I got that all working, I did all kinds of little mods and cosmetics.
Stock Reinforcement (1 1/4" PVC):

A new bolt handle, with a lock nut that fits on the bolt glued inside, so you just screw on the handle.


Ammo Holders - PETG inside Firefly ammo holders:


Barrel and Light from a recon - removable:

I added the recon barrel because I didn't like that long, bland piece of PVC sticking out of my gun. The barrel looks fucking awful in this picture, but in real life, you can't see any yellow at all. I guess the camera flash brought it out.
I also cut off the LS's carry handle, since it was in the way.
Some more pictures:
Bolt Opened:

Bolt Closed:

Its barrel is 17/32 Brass and SCH80 PVC, and it has a silver LS and streched NF spring in it. It gets around 80', and the SMG gets around 40'.
I like this gun alot. It's ROF leaves something to be desired, but you always have the Spidey gun in case someone rushes you. It's trigger is always there as a sort of panic button.
Q's/C's/F's?