Try this, it's a visual thing, watch the barrel come out and go back into the slide and you can see a adaptor would likely not work. Try it on your full size with the full size barrel.
We are actually talking 3/4 of a inch total so it is about 20 mm. At least on the P99 full without the smooth extra part sticking ouside the slide the guide rod will slap the can. 3/8 inch threaded, total including the threads and smooth is 3/4 inch. If the P99c needs the same length of smoothness I don't know, Call Joe at SWR and he will know.
Take your P99 (fullsize), put in a empty mag and lock back the slide, see how much more the guide rod sticks out more then the barrel, you can see why it can't be threaded all the way but must have more barrel than just thread hanging out of the gun, without this it would even hit the front of the frame. Now release the slide and the barrel is about flush, I think a adaptor even if it could retreat into the slide would tear up the slide as there would be a slight crack/gap between the barrel and adaptor that would catch in the muzzle hole as it went back into the slide. And you can not just do this with the P99 or P99c to take mesurements as live firing makes that rod shoot out even more. I can tell you what works on a P99 but on the C I would call Joe of yoy plan on doing it down the road, someone had a factory P99 barrel on Ebay but it may not be there anymore.
Rockets Out....