Sorry I was mistaken in my previous post so I deleted it, I see you want the threads on the barrel and not the thread size on the shroud. The shroud has 27mm x 1.0 but I am not sure about the barrel, Neil.
The Vulcan shroud has a M27x1.0mm internal thread; exposed when end cap is removed. I have made screw-in MiniMods for this application that are extremely effective.
The new Vulcans come ready with a M14x1.25 mm male adapter at the end of the shroud (unscrew cap). I've made a few LDCs for them already and was told it screwed on fine. This goes for the standard vulcan and tactical. No idea why they didn't go with the standard 1/2 x 20.