HK G36 is
NOT prohibited by name. Since H&K doesn't sell G36es to civilians, G36es probably were not in the firearms catalogue that was used to generate the list of prohibs.
In fact, for mere 9000$ + shipping and tax Wolverine Supply will sell you an
authentic H&K G36P.
(As an aside, am not familiar with the P version, and that one's not restricted, so how is it different from regular G36?)
Technically SL8 is a civilian variant of G36, and those who can't afford that kind of money tend to buy H&K SL8s, and replace the stocks, sights, etc to approximate the look of a G36.
To preempt your next question, Germany has a law where civilians can not own the same guns as military, and H&K USA is a daughter company of H&K Germany. There have been cases of regional distributors in various countries selling H&K military products to civilians (Canadian distributor apparently did for a while, and then H&K found out, and switched to using R. Nicholls Distributors Inc for distribution in Canada. In fact that's probably how the G36P above made it into civilian hands), but in all cases selling to civilians is swiftly dealt with by H&K.
HK416 uppers and G36 parts (without receiver) show up on the market once in a while, at least in US, because a dealer can request from H&K a sample of HK416 for law enforcement or military customers (usually requires a letter on a letter head of end user). Upon completion of evaluation, the firearm gets destroyed. In case of HK416 the lower has the serial number, so it gets destroyed (or returned to H&K). In case of G36 commonish practice is to use a grinder and cut through receiver just behind the barrel and just before the stock block, effectively cutting out the receiver from the rest of the firearm. The end result looks like this:
http://w ww.hkparts.net/shop/pc/viewPrd.asp?idproduct=614&idcategory=13 (
HK Parts doesn't ship to Canada, so I am not sure if I even should be obfuscating the URL)
Any way, back to the topic on hand...
If I understand correctly, the difference between MR223 and HK416 is the location of the take down pin on the lower, where effectively MR223 upper will not accept any other lower then HK. Is that still the case with MR308?
(I presume that yes, since HK needs to differentiate the civilian and military versions).
So if you have MR223 and MR308, you might be able to switch lowers between the two, but not with any other AR-15s (or AR-10s) (without modding the parts), right?