The field manual for M16A1 and M16A2 Rifle Marksmanship states, “The larger aperture, marked 0-2, is used for moving target engagement and during limited visibility. The unmarked aperture is used for normal firing situations, zeroing”.
J, a buddy of mine uses a flip up (I believe it's a Magpul) on one of his A4 bushmaster's with the A2 FSB and 20" barrel. I was asking him how the sights line up, since he's not using the carry handle. Similar situation as the op. He is a retired Airborne Ranger and seems to know his schidt regarding AR's and other military type weapons. He says the majority of flip ups are made to work with the A2 FSB. I'm assuming he is correct and that the op should have no issues. Personally, I'd keep the flip up more towards the rear of the receiver and nose into the charging handle. What say you? Also regarding aperture size, you are absolutely correct. In low light especially, it is much better to use the larger aperture. On a standard mr31 or similar sized black target at 100, the small aperture works well..
