Some of the pics are lacking but I am feeling semi lazy and I do not feel like going back to open up the gun to do internal pics. The descriptions and photos provided should be enough to complete the modification.
Open your gun. You will need to crack a couple grey rings of plastic at the front and back. Take out the plunger assembly. Here is a view of the front. This plunger system works like that of the scout and longshot front gun.

Now cut away at those bungees try to keep as much as possible. Then take your pipe cutters and cut along the line. Do your best to get all the way through. Use a dremel if you have too. I got semi frustrated at opening this up so I cracked it open with my bare hands.

Then pull the two pieces apart and it shall reveal the plunger head. It's massive.

Then take off the O-ring and apply some fitted E-tape around the groove where the O-ring would go and then put the O-ring back into position. Then lube it up with some O-ring lubricant.

Now lets take care of the air restrictor in the back.

Now take out your screw driver and poke out this flat grey plastic nub.

Take your dremel and cut out the cross shape still in the barrel.
Wrap some E-tape around both ends of the barrel of choice and stick it in the stock barrel.
I went with about a foot of petg. I also added a wire to make sure the dart doesn't get stuck in the plunger tube.
Then be sure to hot glue or plumbers goop around the bottom to make sure it is air tight.

Then re-attach the stock bungees to their old spots by dremeling a hole and knotting them. You can see that and the dwarfing experience the Mega pistol has on the Longshots front guns plunger assembly.

Then take some 8 or 9 inch bungees and attach them to your pistol by drilling small holes for the bungees to go into.


The final product.

Range was only 89 feet. It would have gotten more but the weather is brutal for shrinking Stefan circumference and the cold air is more dense which means it is harder for ballistics to travel through it.
Summertime this thing should be able to punch out over 100' for sure.
It survived it's first winter war at WCNO.
One tiny internal band snapped during the range test. It did not affect it much. The bungees are pulling all the weight around here.
A couple things I really like about this gun are: How light it is. How far it shoots for a pistol, The size of the plunger tube, The sound it makes when firing. The sheer terror it beings to peoples faces when they are looking down the barrel at it.
Video is here.
Lord of the Rings crossbow modification demonstration.
This has been a 20 minute write up and video edit session. So I hope you didn't expect much.