Essentially what I want to get is an upper similar to the M&P15T: I'm trying to decide on 10" or 12" handguards but that's not my confusion right now... I'll probably get an upper through CMMG just because they have tons of options that will get me what I want (mid-length gas, rails, no front sight post) but I'm not sure what to select for the "Sight Post Option" selection and the "Front Sight Base/Gas Option" selection. I plan to use Troy BUIS for front and rear just like the S&W uses... Which selection should I choose for those two? I assume for the FSB/Gas Option to leave it as an F marked FSB but I don't know what for the other.