Make your barrel like this: " I Made a longer (up to the muzzel) air soft barrel with K&S 9/32 brass tubing, slid into 5/16x.014 tubing to get a better out side diameter. I wish I had measured it when it was apart but it would be easy to figure out for any one doing this mod. I drilled the two hop up holes, the gun shoots better than stock.) note: when you cut the inner barrel you must use a new hobby knife and go around the id of the barrel on both ends, so the bb will not be hung up on the burnish from the pipe cutter or dremel."

This barrel will work perfect and will fit the breech on the air soft unit nice and tight.