I'm going to go out on a limb here (I'm sure others can confirm or correct me) but from this and other pics of him and his team mates I'd say that's an M4A1 with SOPMOD block 1 parts fitted NOT a MK18.
Either way, as you say all the parts are available from the interwebz and the rifle in he uses in the film (which I'm guessing is what the OP was actually asking about) is a MK12 SPR.