Why it is there I honestly don't know, I thought it was just to make them look more like the Colt M-4, though that it probably wrong. It could possibly have something to do with the harmonics of the barrel.
what i heard quite a while ago, was a rumour that HK made the 203 barrel lug for "possible" useage with standard M203's, however i dont think anything ever came from it. (it would have required a different barel nut and handgaurd i think)
I can see it possibly happening, but with the Mil. wanting to go for the XM320's for the next grenade launchers, i cant see any possible use for M203's if the Mil. was going to actually going to spend the cash and buy 416's.
a picture is always helpful to show how it attaches to the M4. The sight that clamps onto the carry handle is a gunner's quadrant. The russians also took that idea and made one to fit on their 74, GP-30, which is much more compact, and uses the bayonet lug as a attachment point
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