Couldn't leave the old version alone. 

First thing I did is make a groove to slot in the cut-off muzzle from the old barrel into the new barrel. now no gaping hole in the front! Looks smooth! NICE.

I moved the breech a little closer to the bolt face for a better seal. I also made it flush with the breech face (for aesthetics). The Breech face (the round orange coin thing with a hole in the middle) is one millimeter thick. Next, cut a piece of 1 mm thick cooking sheet silicone to 30-33mm outer diameter, 13-20mm inner diameter hole. Super glue it to the orange thing, and you have a perfect seal.

The next thing I did was cut out a piece of packing foam (not the white crumbly stuff, the durable foam that is made with firm plastic). I cut the foam around 15 mm tall then pushed the air tube from the SledgeFire into the foam and twisted it to cut it out. Then I made sure to punch a hole in the middle for the air to come out. I found a tube that was the same inner diameter of a nerf dart and thin enough to twist and cut the foam. 

I broke out the inner part of the air tank that blocked airflow and pushed it in to take up the dead air. I then filled in the dead air space on the piston with hot glue, also making sure to plug the two little holes on it's face. I then added some lube to the tube (it is a really good seal "out the box").

I also purchased a two pack of Century Spring C-836 Compression Springs, 4". https://www.amazon.com/dp/B000BQOUIQ  It was a bear putting it in, but i figured out how to do it with no issues when I installed the spring into my modified SledgeFire that shoots the stock SledgeFire shells. 

Now, it actually shoots MegaXL darts with a whistle! Mega darts slap the targets with much disrespect, and the three dart shells are miraculous. 

Only thing I focused on was to make sure to sand the blaster shell around the breech face very carefully so it would look nice, which it does!