I just picked up one of the Matrix handgards and I love it got mine through gunbroker, from the same gentleman you mentioned. If I can figure out how to post pictures I'll put one up.
I've already assembled a 5.56 with a second upper in .458 socom. So an AR.308 was the next step.
Sorry for the double post hit the back button by accident.
So does anyone have any experience with the JP silent captured spring and the magpul UBR receiver extension tube? Will that combo work or should I just use the CAR 10 And Armalite modified spring?