There is a rail cover there. It's the standard SOPMOD rail cover that's typically used on M-16A4s. Since it was originally sized for the M-4 it doesn't cover the full length of the rail, leaving part of it exposed. This is the most common configuration for the M-16A4 in US service.
If only the devs would pull their finger out and give us the fucking sdk already..... such a simple change to reintroduce the railcover, such a simple mutator.....
I think we will see the death of Fortnite and another two generations of pokemon games before we ever hear about the mythical source development kit...