Firstly, there’s the way it’s mounted - the KAC M4 RAS uses the original delta ring and front handguard ring, meaning a shift in the rail system (from gripping, rests, barricades) is translated to the barrel, affecting accuracy. Systems like the DD RIS II use a barrel ring or other method at the receiver end which means the handguard does not contact the barrel further down, “free floating” the handguard and resulting in little to no accuracy shift.
Secondly, you have more rail space closer to the muzzle. This allows you to mount accessories not only in such a way that you’re afforded a better grip on the weapon, but also for weapon lights to be mounted so that they cause less shadow (barrel/muzzle blocking light beam and limiting FOV) as well as glare (light bouncing off the barrel/muzzle affecting vision and giving away position).
You don't necessarily have to get block IIs. You can get rail extensions, nice trigger and some good lot group.
