No expert on night vision gear, but here how Crane optics guy explained it, more or less.
Early Mk 13 rifles with MIRS/MARs stated with 1.125” rings, circa 2004-2005. Lowest centerline to bore was desired. (Crane ordered it first 600 MIR rails circa 2004, along with AN/PVS-22 units).
Nightvision optics progressed and ring height was adjusted to get proper center lines between day and night optic. He joked something about the old PVS-17 being “last century” and couldn’t remember the height of its centerline.
At some point the 1.375” rings were adopted to get proper centerline with NV gear, which might have been the PVS-26? He implied this was during PSR contract time period, which had a separate optics procurement. I assume this is Mod 5 era.
I got the impression that a “flat rail” system was desirable to standardize ring heights. (KAC Mk 11/M110 and FN Mk 20 for example). I think the iMUN rail achieved that goal on the Mk 13 Mod 5, with front rail height alignment, correct?
At some point SOCOM instructed vendors to centerline their NV optics at 1.535”, so all flat rail systems could use the same NV gear with perfect centerline alignment, thus allowing a standardized ring height (based on day optic tube diameter of 30 or I guess 34mm?). Maybe that’s why he said 1.5” was “close enough” although spec is 1.535”.
He didn’t say what year that occurred, only that the PVS-30 was designed to that SOCOM spec.
Based on your pic, your PVS-30 aligns perfectly with the flat Mod 7 rail, but sits a little too high (~ 1/4”) with the MARs rails seen above with the 1.125” rings, hence I presume his remarks about settling on the 1.375” rings with the NF 5.5-22x56mm scopes. Not sure about Mk 11, but alignment looks very slightly off, with PVS-30, maybe about 1/8” taller than scope centerline...I think, but hard to see.
(FWIW, it seems SOCOM has ordered all of the following clip-on NV optics over the past decade and a half: PVS-22, 26, 27, 29 and 30, but obviously I don’t know what NV optic(s) operators preferred on their Mk 13 platforms over time).
KAC night vision devices are designed to be positioned in front of conventional day scopes’ on the user’s rifle providing the same night time zero as their unaided day scopes.
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