Its a parallax problem.  Too much space between your optic and your bore means the optic is "looking down on the target" too much and adjustments at various ranges become a pain in the ass.  

Normally with your iron sights you have about four inches of parallax and the adjustments work pretty well.  If you mount the EOtech on a flattop receiver it'll cowitness with the irons and you still have about four inches of parallax.  The adjustments on the optic are designed for this much parallax because the designers know how tall the optic is.  

Now, when you put the optic on top of the carry handle, you have the four inch height of the reticle over the EOtech's bottom, plus about a half inch of picatinny rail that you had to attach to the carry handle so the optic can be mounted.  Now you have a total of roughly eight and a half inches of space between the bore and your sight line.

So now, if you sight in the gun at, say, 25 yards, the bore is at a zero degree angle, shooting straight to 25 yards, and the optic is "looking down" at the target at a significant angle so the point of aim and the point of impact will coincide at 25 yards.  If you move the target from 25 to 100 yards without adjusting the optic, as you sight through the optic now your zero is still 25 yards, but you've brought the optic "up" so to speak because the target is further away.  Your point of impact will be ludicrously, rediculously high by a number of FEET, not inches.  In addition, your adjustments are screwed up, you'll find that the 1/4 inch per click that the optic is supposed to have doesn't apply anymore, although the difference is minimal its mathematically there.

If you sight in the gun at 100 yards and go back to shooting at 25 yards, the point of impact will be significantly low.  Its basic trig really.

The only solution is to get rid of the extra parallax by utilizing a mount that attaches to your carry handle but mounts the optic back down near the bore.  These so-called "gooseneck" mounts are expensive, but do effectively solve the problem.   Here's a nice adjustable one designed specifically for the EOtech:

www.midwayusa.com/eproductpage.exe/showproduct?saleitemid=159785



Ignore the cheap one made by leapers and marketed by Tapco.  Its garbage, I bought one as a gift for a friend and was embarrassed when the mounting screw broke on its first trip out to the range.  The mounting screw was made of pot metal and held in place by a small roll pin.  Everything popped as soon as it was tightened down.  

Quoted: But was wondering how far forward a red dot type of sight can actually be mounted before you loose accuracy...

Mine's up here, there are actually some advantages to doing it this way:




AFSOC likes to put his Trijicon Reflex way up there too. You don't lose accuracy at all, and you gain speed, especially in CQ.  This isn't AFSOC, but it is his gun.  He's deadly on steel plates out to 200 yards with this configuration.  

The tactical run-n-guns are definitely good for helping you sort out what gear works and what doesn't.  I can't count the number of equipment failures of various kinds I've seen out there when shooters are put to the test out there.  It's definitely more educational than just plinking at your local Fudd range, which I'm gonna do tomorrow anyway.

I done got me a gen-u-ine Navy SEAL SKS to test fire.  

Quoted: Quoted: I done got me a gen-u-ine Navy SEAL SKS to test fire.   They've got a Navy over there in SKS-land??  Is it 2/3rds scale like their soldiers?

Its not my kung-pow SKS, that one is remaining unmolested save for a furniture refinish.

This is a customer gun that didn't run, it had a major gas leak problem.  I welded on the gas tube yesterday and its time to test fire.

Oh, and the advantages to putting an optic in the forward scout configuration.  Hmm... might have to create a seperate thread on that, with some pics.