Quoted:
The RAS II makes it difficult to mount a conventional scope...FF RAS is the best for that.
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I think "difficult" is very "scope" dependent. Some scopes of short overall length will have a front scope ring mounting issue with the RAS II "bridge" (at least until someone comes up with a ring of appropriate heigth). But on some large variable power scopes, where the power ring literally moves the scope closer to your eye, I find mounting the front ring on the RAS II a very good option to having the scope mounted too far back, i.e., hanging back over the charging handle. Especially if you follow the old gunsmith's rule of spreading the scope rings as far apart as possible on the scope body for its best stability.
Remember, most all these commercial scopes are designed for bolt action rifles and have about a 3 1/2" eye relief, and then just so much scope body area appropriate for scope ring attachment.
Of course for a 20" barrel capable of sub-moa, the FF RAS would be the best choice, all other things being equal. But with the RAS II's lower rail and its integral barrel nut lip clamp and 2 indexing dowel pins that help secure the upper RAS II as well, once assembled, RAS II is extremely stable for a free-floating rail forend system that installs in less than 30 seconds.
ColdBlue got to do yard work now...