I have one of those ARMS #38 EX MOD-L rails on a carbine.  The Aimpoint sits nicely with a GGG standard 30mm ring.  I want to be able to switch from time to time to my ACOG TA01NSN.

From a pratical standpoint, are there any downsides to having the ACOG sitting a little bit higher than it was designed for?

I assume this is also true for the SIR?

You can not use a correct cheek weld.  This means you cant have as consistent of head placement as you could if the scope were mounted at the correct height.  This degradation in accuracy will be more or less depending on the individual.  The BDC lines will be off by only a small amount due to the height.  That is not as big a deal.

The ACOG was designed to fit into the carry handle and adopted/issued that way world wide, to include US SPEC OPS. To say mounting it onto a #38 is making it highe than it was designed for is not correct, as the carry handle is quite a bit higer than if mounted on the #38. In the carry handle the irons weere seen thru a tunnel under the ACOG.
The ACOG and other optics with magnification are traditionaly mounted higer than a red dot/reflex type optic, as they take the place of the irons. The magnified optics are for enhanced distance shooting and are better served by being higer than the irons.
Good shootin, Jack

While it is certainly true that it will raise the ACOG up slightly than what it would be if mounted to the flat top, I think the rest of the above statements are highly subjective.

Mounting the ACOG to a rail does automatically mean it will be higher. Being higher absolutely, positively does not automatically equal more problems with consistent head placement or degredation of accuracy.

I guess all the snipers, highpower competitors and everyone else who competes in the world of high accuracy rifle shooting is just blowing smoke when they preach good cheek weld. So should they mount their optics high and should they all just raise their heads up then?  Why is it that people complain about the M14 and its lack of cheek weld?  They can just hold their heads up too right?  I dont think anything I said was subjective at all. [;)]

Maybe if you were talking about building a sniper rifle where you are settling in behind the rifle from prone, or a bench... a lot like snipers, highpower shooters, and scoped M14 shooters do. But I because... at least for the moment, we are talking about putting an ACOG on a SIR, and SIRs only fit on carbines, I presume we are talking about a running and gunning CQB weapon. In which case the gun will see a lot of fluid movement and fast paced shooting. I dont know how much of this you do, but I can tell you one thing... things like the correct shooting stance and position are the last thing on your mind. In this scenario, Im unsure how a conventional cheek weld is going to help to any descernable degree in any way shape or form.

On the other hand, I can give you a list of reasons how an ACOG on a SIR will help you on a CQB gun. I just dont see why a half inch of movement in your head should keep someone from using one of the best handgaurd systems available, with one of the best optics available, using one of the best mounts available. I cant say it will work for everyone, but saying lifting your head up a little will degrade your accuracy is quite subjective in my view.

Put it this way, there are probably many more things on most rifles that will inhibit a shooter from acquring the most accuracy out of their gun than a *slightly* elevated cheek weld. Like a stock trigger for example.

My face must be a ton longer than everybody else's since my Compact ACOG mounted on an ARMS#5 is perfect for my face and my cheek weld.  Not only does my nostril still settle right on the charging handle when I mount the rifle, but the ACOG falls right in line for me.  Quite honestly the A2 sights feel like I am mashing my cheek over the buffer tube when using them.  Individualized....this is why you will see many serious shooters long range/BR shooters with custom-fitted stocks or an adjustable cheekpiece.