Take a look at Putt Putt Maru its got NMF 'sleeved' .50 cals, and there are lots of other examples to be found, I'll post them up if you like.
It would appear that that there is a number of different types of barrel, and if the barrel is 'perforated*' it was black, if its 'sleeved*' it was painted but it appears that it soon wore off leaving it in NMF like PPM. I'm no expert on these things tho, just my observations.
Considering that these are little more than blank canvases at this stage its a pretty moot point anyway as the style of armament will change depending on the paint scheme that I apply.
*For want of a better term.