What is that little plastic doohickey in the M16 handguard aka M5 RAS? I honestly had no idea myself until I stumbled on the lower image some time ago. Given how cool and popular the classic plastic heatshield on the A1/A2 is, I never paid a lot of attention to the A4 personally.
The highlighted piece is attached to the removable 6 o/clock section of picatinny, so any time a 40mm is attached then it will necessarily be removed along with said lower section. The slots in the sides of the main portion of the handguard make way for a forward mount for the M203, as shown in the lower image.
The mount shown above is the QD type, which is a lot wider than the original non-QD clamp mount that preceded it (itself commonly used with the aforementioned plastic heatshield). I don’t know specific timeliness for specific units, but I venture this particular combo wasn’t super widespread comparatively, at least it’s not as well known to the general population. The 203 was replaced by the 320 during a similar timeframe to when the US combat arms that used 203s alongside the M16A4, were changing over to M4s.
The USMC, as many will know, famously kept the M16 a lot longer than the US Army, so if you look up the 203 on the A4 it’s mostly Marine photos that appear and surprisingly a lot of them have the QD mount. The end combination of M5+QD must have been pretty heavy compared to the polymer heatshield with smaller bolt on 203 clamp mount that came before it. Plus the M5 of course gives the option to then add even more weight with a light and/or laser.
I don’t own an M5 or replica thereof, so I’ve not been able to check and see whether the older type (much smaller) 203 forward mount would actually require the cuts in the forward section of the handguard. If anyone has tried that combo though and examined it up close please leave a comment.
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