I have a PSA upper that I purchased specifically because it had the correct specs for an SPR build (1/8 twist 18” barrel with a rifle-length gas system), along with the bonus of being chambered in 223 Wylde. Since then, I have fallen in love with the Mk12 Mod 0, and have already started to accumulate the parts to convert the upper to this look.

Someone has advised me that the Ops Inc. muzzle brake collar so integral to the Mod 0 look may not be properly staked to a non “SPR-contour” barrel. Is there any way to tell if my barrel has this contour, or if not, are there any solutions short of replacing the barrel? I’m very happy with the accuracy and of the FN barrel and would rather keep it if possible.

"Barrel: A proprietary blend of Hammer forged chrome molly vanadium made by FN that is referred to as "Machine Gun Steel" by virtue of its use in FN's M249 and M240 weapons. The hammer forging process work hardens the steel, making it more durable. In addition, the chrome process for the bore allows for a lining almost twice as thick as a standard M16 for enhanced durability. Chambered in 223 Wylde, with a 1/8 twist, M4 barrel extension, and a rifle-length gas system. Barrel is High Pressure tested, and Magnetic Particle inspected, to ensure quality and dependability, and is finished off with a Midwest Industries SSK 15" rail, low profile gas block and A2 flash hider."

http://palmettostatearmory.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/4/2/42119.jpg

If you really want to build a Mk12 Mod 0, it's worth doing "right". There are a number of solid sources for properly SPR profiled barrels, to include Ballistic Advantage, White Oak Armory, Rainier Arms, Compass Lake Engineering/Centurion Arms/High Caliber Sales (Legit Douglas Mk12 barrels), and even BCM.

From that picture, I can't readily observe the Ops Inc/AEM profile. The 15" rail could be covering it theoretically, but I don't think that's the case.

EDIT:
Here's the BA barrel. It has the correct profile, you can see the step clearly. The PSA upper you linked doesn't appear to have the step, so no it will not build properly into a Mk12. $210 is well worth it for the right kind of barrel if $425+ is out of your reach for a Douglas.