I play on PS4, and this glitch appears to be present in essentially all automatic weapons and occasionally in semi-automatic weapons.
When in battle, i find that frequently, the spent bullet casings will appear in the scope of, for instance, (but not limited to) the N16, and only disappear when you release the “aim” button, and then press it again. This, as I said before, in my experience happens to all automatic weapons. However, in certain cases, I’ve found that the semi-automatic weapons like the .50 cal also does this (both standard, and experimental).