The ACOG is a nice scope. The ability to use a 4x scope and shoot a man-size target at 500 yards is do-able, yet not practical in our real world. In the real world, if you're a trained marksman at expert level, you could do it. COD is a game and everything seems easier in a game. Hell, I took out a German SS officer at over 3000 yards with a 4x scope on some stupid game my brother gave me. Don't get me wrong. If all hell breaks loose and that's all that I had to put on my sniper rifle, you bet your momma's ass I will use it!
Trijicon makes a 6x ACOG, but at over $1000 for the 4x and $2100 for the 6x I could get a LOT more scope for a LOT less money.
Depending on what your use is, you may find that you will need to have 2 or more scopes. It's a good thing you've got a flat top on your new DPMS rifle. It will come in handy when swapping out optics and returning them to zero.
Mid-Long Range: Consider the
SWFA Super Sniper 10x43 scope. It is one of the clearest and most accurate scopes I've ever hand and have used it many times to shoot distances of 100-600 yards. It's a fixed 10x with a mil-dot reticle and you can get it with a side parallax focus or a front parallax focus. Price? $399 and $299, respectively. If you're a southpaw like me, you'll want the front parallax focus and save yourself $100.
My best 3-shot group with this scope is 1/3" @ 100 yards with my 308 bolt rifle. Basically, they went through the same hole. SWFA offers a lifetime warranty and in my 6 years of ownership, I've only used the warranty once. The 2nd day aired me a replacement and I sent mine back in their return box.
Next, invest in a quick release scope base. Either a
lever type like the American Defense ($180) or a
socket type like the Burris P.E.P.R. Mount ($70). Obviously, the lever type is faster and requires no tools. However, I always have a small tool set in my sniper bag, so keeping a 1/4" ratchet and socket is no big deal and it saves me $100.
For close and mid-range, consider a dot scope. The
Burris Fastfire is a good choice to attach on top/front ring of the Burris P.E.P.R mount, as the rings have a picatinny rail on them. It will set you back $200, though. For a cheaper alternative and maybe an increase in the Cool-Factor, check out the
Burris Trophy 1x32 dot scope. It has both a green and red reticule several power levels. A great value for around $150. The
older version had iron sights on top. They worked well, in case your battery dies and you got a zombie charging right at you.
Ok, so if you went with the SuperSniper 10x32 with side focus ($400), plus the Burris P.E.P.R mount ($70), and the Bushnell Dot scope ($150), you're only at $620. Add in the Burris Fastfire ($200) and you're barely tipping the wallet-scale at $820-$830. A FAR cry from the 4x Acog, which starts at over $1000.
Oh, just in case you want to enter the realm of the
Elite Snipers... you'll be spending around $2500 for a scope. May I suggest
Vortex Razor HD or
Nightforce NSX.