Here are the steps I took to extend the barrel
1st I separated the barrel at the front groove of the mid section

2nd I made a barrel outer extension from layers of rolled .020. sheet styrene

3rd I then made an internal barrel extension for the inside the barrel at the muzzle end rather than cut the inside of the muzzle break. I had beveled the inside edge of the inner extension where it meets the end of the aluminum internal barrel so there would be no edge for the bb to catch on

4th I then inserted and glued the inner barrel extension inside the front section of the barrel. Next I glued the outter barrel section to the rear section of the barrel. Then I slide the front section onto the aluminum inner barrel and glued the front section to the outter barrel extension.

When the glue has dried I filled and sanded the joints smooth. With this method you have a solid sturdy barrel