Quality product, but... - Meprolight MMX4 4x24mm Micro Magnifier w/ Quick Flip, QD Picatinny

I have a S&W M&P-15 with an Meprolight M21 sight. When the M21 is installed fully forward on the top rail, if you try to install the MMX4, the rail isn't long enough for both. The only way to get it to work is to get an extended riser, that is long enough to allow both the M21 & MMX4 be installed. I haven't checked if this only with the M&P-15 or other AR-15's, but it's something you need to consider if you have an M21. Other sights may not need it, so check before getting the MMX4.