I'm not sure about the hand guard thing. It should just slip off and you can put together the RIS/RAS and just tighten the screws.
As for the silencer, check to see which way the ends are threaded. They're usually 14mm however as to the direction of the threading it can be either CW or CCW (that is: + or - threading if you look online it will usually say something like -14mm or -14mm threading). There is also the option of getting a CCW to CW or vice versa adaptor so if the silencer you want is only offered in CCW and you have CW threading just get the adaptor and you're good to go.
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