Most ACOGs have a carry-handle base that is actually part of the scope tube. When you see an ACOG on a flat-top rifle/carbine, it has been modified by the addition of a rail-type mounting system, and that attaches to the carry-handle mount built into the tube (there are a few exceptions to this "rule", mostly for special mounting systems intended for subguns and the like).
Originally Posted by jason-hart
The carry-handle bases that bruce333 referred to can vary quite a bit in quality and finish, but they are usually plenty strong. Some will block effective use of the iron sights (even if they claim to be "see through") due to the sight tunnel being too high. Find the lowest-height one you can, and use low rings on top of it to keep the sight from getting too high above the barrel/stock, which can strain the neck, make getting a good cheek weld challenging, or mess with your zero/trajectory distances. Also make sure the rings allow enough clearance for the magnification/power/zoom ring on the scope to rotate, if your scope is a variable. I had a nice Leupold 1-4x on one of these mounts in low rings, and you could barely get a piece of paper between the power ring and the base with the rings I grabbed out of my optics junk drawer. I later found a slightly higher set, and switched to those instead, just for piece of mind (and a little better finish match, I'll admit).
Generic carry-handle mount and low rings (both aluminum construction):
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)