Well? I have to mount it on a A2, I'm guessing over the handguards to be used as a co-witness. Do I use the factory mount to achieve the right hieght? You can tell I'm use to an A3. Anybody liked this set-up on theres?

I have my Aimpoint on a QRP atached to an ARMS #39A2 on my Bushy carbine and love it.  I tried the Aimpoint gooseneck mount and did not care for it.  It worked fine, just liked the look of the ARMS better.  When you leave out the spacer it will cowitness in the lower third of the Aimpoint.  For most short and intermediate work the Aimpoint over  the handguards is great.  

If I understand your question properly...  the M3 is the newest model with the most options and latest technology.  I have the M, and it works just fine.  Get what ever one you can afford.

Welcome to the A2 club!

Sarge

I've got an A1, A2, and A3. This particular one is a 11.5 w/perm. 4.5 f.h.! I am really into the co-witness factor but I like a large objective.

Objective size and field of view are non-issues with red dots, especially when you mount them forward over the handguards.  Proper shooting technique is to keep both eyes open and view the dot only with one eye.  Since both eyes are open the field of view, and objective for that matter, are the same as when you are looking at it since there are no restrictions on your view.

Sarge