I'm currently running an Apex1 Tip on the end of my Stock G1 Barrel with NO mods required, and I've fitted an Apex 2 onto the end of a J&J 16" Barrel. There no REAL reason to have to purchase an adaptor or a specific "Apex Barrel" to use the device. If you're accurate with some Electrical Wiring Tape and a Sharp Blade, you can mount an Apex 1 or 2 onto almost ANY barrel, as long as the Outer Diamter of the Barrel you use isn't bigger than 23 mm's.
Here is a Pic of the Open Apex 2 Tip, and the J&J Barrel. You can see the position of the tape on the barrel, as well as the various rubber "lips" inside the Apex tip which you need to put tape onto the barrel to sit in between these lips.
The three red circled lips need to have tape on the barrel to fit snugly against these lips. The Blue curcle is where you need to wrap some tape to fill the gap between the barrel and the plastic housing of the Apex 2, so that it doesn't wobble around on the end of your barrel.
Here you can see the barrel in the Apex 2 tip, and showing where the lips are in comparison to the tape you need to put on the barrel.
Here's a closeup.
Nothing to it.. it's not a complicated thing to do, and as long as the barrel you use doesn't have a big end fitting or muzzle tip, it should fit inside an Apex Tip without issues.
My Standard G1 Barrel is inside an Apex 1 Tip with no tape needed, and it works perfectly. As long as you've got the screws tight on the Apex 1 tip, or you've put a few rounds of tape on the Barrel in the Apex 2 tip so it doesn't wobble around, it'll be 100%.
Here's a Pic of the Apex 1 tip and the standard G1 Barrel.
Hope this helps a few guys out there asking if it's possible to fit Apex tips onto barrels..