I'm a newguy, please don't flame me too bad!
With info I got from this site, I bought a RRA carbine flat top, an ARMS #40 rear sight, an ARMS #19 mount and just got an ACOG TA-31 a few weeks ago. At first, eye relief was a problem for me and I had to take the ARMS #40 off in order to be able to slide the ACOG back far enough to shoot it comfortably. I have since decided to work on my cheek weld and get my nose "on the charging handle". Today I was able to put my ARMS #40 back on and moved the ACOG ahead again. That damn charging handle raps me on the nose though.
I noticed that even tough the ACOG is mounted as far back in the ARMS #19 as possible, there is at least 1/2" of the rail on the ARMS #19 behind the rearmost portion of the ACOG's mounting "foot". If I could mill or cut that off the back of the #19 I think I could move it back one notch, gain some eye relief, and still have room for my ARMS #40. Would some gunsmith be able to neatly trim it or would it be scraggly looking? I also have to wonder if the ARMS #19S might even have a shorter rail, allowing it to be mounted farther back on the flat top with the #40.
Thanks, dog guy.