Since you got the mid length, I'll assume that you want to keep the rig pretty compact.
I don't know if you've ever shot with an ACOG. I'd stay away unless you have a compelling reason to go that route. I'll say most (because I haven't used them all) of the 4X ACOGs have lousy eye relief. Something like 1.5". They are very uncomfortable to use. There are 1.5X, 3X and 3.5X models with more forgiving eye relief though. The thing is that the ACOG really isn't a good scope. It's a good piece of battle gear. That's what they were/are designed to be. They have a very unique illumination system that works, but the optical quality is only on the better side of mediocre. They are very tough. I doubt that you could kill one if you tried, but it's only built for shooting at man sized targets. I had a TA33 3X ACOG and sold it the week after I got it because it wasn't nearly as clear and didn't have nearly the resolution of the little Sightron Big Sky 1.25-5X scope that I had. The funny thing was that I paid over $800 for the ACOG and only $300 for the Sightron.
As far as red dots go, I do like them and the concept. I don't use one any more because my eyes aren't what they used to be, but I do run a 1.5X ACOG on my general purpose rifle. It operates just like a red dot in use. They are good for close in, but I wouldn't call it a 300 yard optic. Maybe for war, but not for sporting use and not on a .308. There's just no way in hell that I could hit a clay pigeon at 300 yards using a red dot. As for using a magnifier, I don't know. They are heavy, the optical quality sucks, they have super short eye relief and are a PITA. The only guys that I know that like them are guys that are trained to use them. If you do go with a red dot, go with an Aimpoint. Tough as nails, the batteries last forever, and you'll never have to worry about it. Put in on the rifle, turn it on and there it stays. I had an Aimpoint that was about 15 years old and it was still working right up until it was stolen a couple of years of ago.
Again, I'm assuming that you want to stay on the compact side. If for some reason, you decide that dropping around $1500 on an ACOG will be worth it, I highly recommend that you at least check out the Nightforce 1-4X. At $1200, the optical quality will far surpass that of the ACOG and it really isn't that much bigger or heavier. If you want to stay in your price range of $300 to $600, I'd first recommend the Sightron SII Big Sky 1.25-5X. It's a compact little scope with great glass and very repeatable adjustments. You'd have to spend a whole lot more money to get an appreciably better scope. If you can stretch you budget a little, there is the Trijicon Accupoint 1-4X. It's probably the most popular of the Accupoint line. I'm not crazy about the optical quality, but the triangle reticle is really neat and easy to use.
Of course, once you get the scope, you'll need to mount it. That's going to be another couple hundred. Don't cheap out on the mount. Especially on an AR type of rig. It's really advantageous to go with a one piece mount for your scope's sake. To stay in your price range, I'd go with the Sightron and then take your pick of Larue, American Defense or Bobro for the mount. Check out manventureoutpost.com Good deals there.
