Here's a pic of my MK12 MOD 0 as an example of a DMR
[IMG=http://img864.imageshack.us/img864/1118/20100914033618g.jpg][/IMG]
as you can see, the hand guard is about the same length as a M16 and the rail on top extends all the way to the receiver because of the design. That's the biggest factor in choosing a RAS is being able to mesh in with the receiver in order to mount a decent scope on it. Don't worry so much about the length of the RAS because it doesn't affect the performance compared to the comfort of holding it. The length of a silencer is dependent on the length of the inner barrel if it goes beyond the outer barrel length. The inner barrel of a M16 if about 510mm in case you're unsure about it.