Howdy: I hope you folks can help me. I am building an AR. I am using an LMT A3 (flattop) upper and a Colt 20" HBAR barrel. I am going to use the standard triangular front site. I was under the impression that I would need to use a sight that was marked with an "F". The place I ordered the sight from said that I would need an unmarked sight as the "F" marked ones were for the M4 style shorter barreled uppers. I also thought that "F" stood for flattop. Does someone know the answer to this question? PLEASE HELP! Also, I thought the difference between an A3 and an M4 were the feed ramps.