I bought a M&P15 OR a few months back and I'm thinking I would like to replace the round hand guard with a quad rail. I seen lots of choices in plastic and metal. I want to get the Magpul MBUS flip-ups to go along with my Tru-Glo red dot but I've been told not to put them on the gas block. What would be my best choice that is not too expensive and can I go with a longer rail or am I stuck with the same length as what is on there now because of the gas block? Wanna kill these ads? We can help!