I can see where RedRiot is going here. Basically the look of the SA17 rifle kit with a more practical use of the silencer in the form of an Apex2 tip, correct?
If you are willing to put in the money to purchase a barrel (that would ultimately have to be chopped down anyways= machine time= some monies) and the DW adaptor, I'd say forgo the adapter and have the barrel machined with the three cuts to directly accept the Apex.
Considering the barrel will be so short (aprox 4" total, threaded area and all), I would assume, as long as the barrel's id is bigger than the SA17's "insert" barrel, that the "quality" of the barrel is a non issue.
-Purchase this barrel (kinda assuming it's a 1" since it is a bull type, and it appears the major of threaded portion for the tips [assuming again it is the standard 7/8" fare] is smaller in diameter than the barrel's od ; I'd hope that this would work
), the cheapest I found searching eBay using the "lowest price+shipping". 6" 3SKULL Scout Tactical Paintball Barrel Autococker | eBay
This may be a cheaper option, after shipping; and is advertised as a 1" od. Viewloader 1 Inch Diameter Recon Rifled Barrel
-Have it sent directly from the eBay seller to any machinist here to be chopped down and have the grooves cut into the barrel. tjd10684 on here LOVES to do projects like this; but be sure to tell him that it must take priority over any other current projects.
All kidding aside, he does solid work, as I'm sure most all here would do.
I would assure you this route will be the cheapest and quickest. Other than searching for a used barrel, which will take time to find and research the od, the price of the above option should be spot on. As far as machine work price, I can't quote someone else's work; but I'd say it would be equal to or less than the price to cut the barrel and buying a $30 plus shipping Apex adapter.
The only other option I can possibly think of would be to have a cocker barrel cut down and have the barrel's od threaded for 7/8-20 threads immediately after the cocker's threads. This would allow you to 1) use damn near any barrel of any taper or shape, as long as the first 1" of the barrel was at least 7/8" in od; and 2) allow you to change the apex out for any number of barrel tips later if that strikes your fancy.
I'd suspect that it would be more expensive to machine; but the cost difference from the option above may be offset by finding a dirt cheap barrel or a freebie a friend is throwing out.