After my recent post covering research and decisions on setting up a co-witness sighting system I ran into a problem. When I tried to achieve a co-witness view through my Trijicon RMR (on an American Defense Mfg riser) it looked like this:
Where is the front sight? Why is the amber triangle so low in the RMR? This sight picture is way to low to be a lower 1/3, much less an absolute co-witness.
I talked with the armorer at my LGS (local gun store) and we covered a bunch of diagnostic questions. Specifically:
- Is the upper a continuous rail from upper receiver to end of handguard? <yes>
- Was the upper factory assembled or built at home <factory assembled>
- Are you holding the gun properly?
- Your nose should be touching the charging handle when you acquire a sight picture. That is the method I was using.
- I used Troy Battlesights- fixed front sight and flip-up rear. Both are standard height. Troy does make micro sights now that are not a standard height- don't use these for a co-witness because they are very short.
We ruled out all those common causes of a problem like this. What's left? The actual co-witness riser mount appears to be too high.
To be certain the armorer grabbed an Aimpoint T1 and put it on the rail next to the RMR. The mount heights were indeed different. Next he removed the RMR and looked through the aimpoint. Sure enough there was the front sight – centered in the reticle as expected for a co-witness.
I got in touch with the folks at ADM (American Defense Manufacturing) to explain the situation. They were very easy to work with and after hearing a recap of the troubleshooting they made my day. ADM has offered to build me a new mount to my specification. I measured up the mount and the sight plane and asked them to take about .250 off the existing design.
I hope to have the new mount back in hand in about 2 weeks. American Defense really went the distance to make this right– I am now a customer for life and suggest you give them a hard look if you're looking for any mounts, rails, etc. Although if you're looking for an RMR co-witness riser you might want to specify that you want the low one…