For my KillSAW, I had to shorten the barrel by 2.31 inches. I had drawn all the parts how I wanted them to look, and the CAD file said cut this much off, so...
To get the cut this close, I made my initial marks on the barrel with a scribe, and made both cuts on the inside of the scribe lines. This left the cut ends exactly the length they needed to be.
Since you are using the inner barrel for a gluing armature, it is ok if your cuts are a bit crooked or don't quite match up. The JB will make up the difference. You might need a second application after sanding, as I did. If I wasn't showing this to anyone, I wouldn't bother with the cosmetics like this.
The inner barrel had to get shorter also, but I wanted to keep some accuracy. 1.2" was removed with a tubing cutter. The end was painted black, it reaches almost to the end of the flash hider in the assembled gun.
Then I had to put it all back together reliably. To do this, an inner barrel was used as a temporary brace.
I put a single layer of clear box tape around it, this made the diameter a smooth fit into the outer barrel parts.
The epoxy (JB-Weld) can't stick to the tape. So with the parts all in place, I put a good dollop of Epoxy on the freshly cut/cleaned (the
epoxy won't stick if you used a cutting oil when making the cuts, as I did. Some acetone or nail polish remover will do just fine to clean them) ends, I immediately wrapped the barrel exterior in several layers of masking tape, taking care to keep it tight and unwrinkled as I wrapped it.
Now the barrel has a "splint", so I grabbed the inner barrel with pliers (I was using an old junker) and pulled it out. I probably could have left it and pulled it out after drying, but I wasn't sure that any small imperfection would be holding it fast. Out it came, the barrel parts didn't budge, the tape was doing a great job. I set the assembled parts aside for a few hours while I did some other work. It's very important not to handle or manipulate the parts during this time, the resulting bonds depend on it.
The glued-up shorty barrel:
Sanded smooth, and a coat of satin black:
