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. It isn't necessary, but the stock springs can only do so much. Mega XL wouldn't clear the barrel, Mega darts were pretty good, and the 3 shot silly shells were… standard factory distance. 

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!