Well, I'm pretty sure it goes on the top so save that one.
Anyway, I have an M&P-15 with the handle removed and a Primary Arms M4 sight on the rail. My question is where is the best placement - all the way forward, back, or in the middle?
Personal preference. Me, personally, I like to place them towards the middle, basically around the front of the receiver/delta ring area. If you have a free floated rail platform inline with the upper receiver rail, I place them directly above where the barrel nut is. For me, I find a little bit farther forward is quicker to acquire, plus you have plenty of room for mounting a magnifier behind it, and on top of that I find it balances out the weight distribution on many setups (not necessarily all).
On my flat-top, the Primary Arms M-4 had to be mounted fully front to allow for the lens caps to open with the back up rear sight that I had installed on it. If you're going with no back up rear, then mount it wherever it pleases you, full forward, middle or back. But if you want a BUIS for co-witnessing (which makes a huge difference) then you're going to be a little on the forward side depending on what BUIS you go with. Example, with my YHM A-2 rear sight block, the Primary Arms M-4 had to be fully forward, but with a Matech BUIS sitting rather out of the way, the M-4 can be mounted at about 3/4 forward while still allowing full use of the BUIS.
My PA M4 is mounted just in front of the MagPul BUIS; think it is mounted in T8 slot. Really personal preference as has been stated. This works well for me on the M&P 15.
Thanks for the info y'all! I don't have a rear sight yet - the placement issue would have been more apparent if I did. I'll wind up moving it forward since a rear sight is on my Christmas list...