One of my friends is having a issue zeroing a SGL21.  First time optics and mount on the rifle. It's a full length master mount rail with a acog.  The optic is maxed out for elevation and the impacts are still 4-5 inches low at 25m.  

I'm assuming the side rail the master mount rail locks into is angled to high?

I'm guessing the optic needs to be shimmed at the front.  I told him to try it at a longer distance to make sure this isn't a height over bore issue first but I'm not a optics guy when it comes to the glorious people's trashcan gun

There are lots of variables. It is even possible that the optic rail is off by a couple degrees. The acog itself may even have an issue.

Update on this one.  I used the same rail and optic on my PSAK74 and it zeroed fine.

Not sure which ACOG you got? I got the TA50 (7.62 Red Horseshoe) and it had elevation and windage issues. At 25 yards max elevation it was hitting about 20 inches down and 8 inches to the left. The windage would not adjust at all. I had to send it in to Trijicon and wait like 8 weeks. When it came back it worked perfectly but it was highly disappointing that it was literally broke out of the box.

I ended up realizing I didn't like the ACOG for the AK platform and sold it on the EE. Definitely loved the optic though...when it was fixed.