Friend of mine picked up a Colt's M16A4 upper that has a M4 upper receiver, and the barrel has M4 feed ramps (lucky puke, he always finds stuff like that). The upper is a later one with the numbers on top, and the flash hider has the newer crush washer. I would GUESS it dates from the late 90's in that it has a C A marked upper, rather then a Diamaco upper. Very slick set up, can't get him to sell it though. It also has a Colt's carrier but a FNMI bolt. I have also seen commercial MT6700s with M4 uppers and M4 feed ramps in the barrel. Looked two over at the last show, both were in the 36,000 serial number range, funny thing was the older one (CA marked, and no numbers) had a M4 upper and the newer one did not.
Am well aware that the Colt's M16A4 came (past tense?) without a M4 upper, just so happens a friend of mine sent me some pics of a Govt issue mid 90's Colt's M16A4. Guess I need to post them up some time, great detail shots and it has a standard upper. Also sent me a picture of an FNMI M16A2 lower, because I never saw one while I was in. Kinda of disappointing, real plain Jane markings.
Any how, I am going to update my M16A4gery with an M4 upper, don't know where I am going to get the barrel, guess I will have to cut the feed ramps in the extension myself? Or maybe wait till they are easier to find?
I did some measuring of non F marked and F marked front sight bases myself (being the curious type), and could not find a measurable height difference. There was more variation in height from sight base to sight base, then from the F's and the non F's. Maybe I had to small of a test group?
I just love modern small arms variation trivia. USGI M16 variation is boring compared to those Russians who change about every dang part on their AK-74s every couple of years, darn head scratcher that is. Am I a rambling again?