From Michael Haugen, inventor of the MIRS and MARS rails (very abbreviated history)
Two or three things I might note. One, Crane makes the sniper rifles for SOCOM, which includes Navy NSW/SEALs and Ranger Regiment, etc. Two, Nick Irving was in the Ranger Regiment. Three, its at least possible the MIRS rail in the above pic on an M24 was one of the 600 units that Crane acquired in 2004ish. (I don't think they had that many Mk 300s at the time, maybe 300 or a little more? - not sure).
In fact, I have seen pictures of M24s with the MIRS/MRS rail being used by SOCOM guys and possibly Rangers like Nick Irving, I think in Iraq. (see below pic with 2 M24s equipped with McCann rails, along with a Mk 13 Mod 0 in the middle)
Hence I don't think McCann rails were unit purchases, but rather a Crane purchase for SOCOM sniper rifles, mainly the new Mk 13 Mod 0/1s, but possibly M24s as well. (I have no other knowledge that I just wrote, so its just my opinion).
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