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Well it's nothing speacial but here is my first mod. The bottom half of a Defender T3, one of the crapy guns of our age. Got about 10-12 feet when I got it, now it gets 50ft. with sawed off mega stefans. Ok so open the gun up. Nothing fancy, open those big cyber strike screws, and remove everything of the internals except for: The shaft that the rod with the spring goes through, we'll just refer to it as the shaft. 2. The cocking mechanism. 3. The trigger. 4. The spring with the cocking lever.
I took a 6 in. barrel of 1/2 in. PVC and, after making the hole on the end of the shaft bigger with a drmemel, and glued/taped it on. (I made it big enough that I can get my thumb through the hole) Now here is the internals that are left along with the barrel glued on.
Stretch the spring if you want, put it back together, layer on the bands, and your done! I use sawed off mega stefans to get 50+ feet. 
10 shots, level, average 49.7 feet. Short, 45.1 feet. Long, 55.11 feet.
10 shots at a 2x2 foot target from 20 feet away, 6 out of 10 hit, 1 hit dead center.
Mod by Steven Roughton.