An old file sitting in my documents of when I was trying to recreate a 1:1 model of an unused HK21 barrel. It's only the outside profile, but may be useful for making your own barrel. I tried to measure as many dimensions as possible from the breechface-forward, with the muzzle being the exception. The muzzle threads are M15x1 like the G3. The dimensions that dont really matter are the contour in the middle of the barrel, which I rounded to the nearest whole number. Contour profile dimensions were done side-to-side, starting from the furthest edge of the barrel handle slot. If you have Solidworks you might be able to see these dimensions in the first sketch. I may remake this file in the future in Solid Edge, and make a technical drawing to go with it (but without tolerances since I have no way to calculate those). Assuming perfect x.xx0mm dimensions may work the best.