Such as the JG-S-system.Forgive me if I'm wrong, but I've been told that RIS-ing an M203 onto the M4 (S-system) would make it incredibly over-balanced and front heavy.
Pretty much yeah , I have an M203 on my CA M4 along with a MOE handguard and it is much more front heavy, not a problem when the stock is fully extended but when collapsed it is a bit unwieldy.
I meant in comparison to an M16 w/-M203 or M4 w/-M203 (standard). This after a conversation I had with an M4 enthusiast who has tried various M4 / M16 and M203 combos.