Rock - In most cases that I know of, less lethal markers are a little more hardcore. They usually have much higher FPS, with not a whole lot of room for adjustment, among other things.
As for the T9 thing, well its Spec Ops. I have a hard time considering just about anything they make as "MILSIM."
And Viper, idk how active you are on MSEP, or if you're even over there at all, but I think MILSIG is going exactly in the direction that you want. Maybe its not an exact replica of an M4, but there are many stocks and handguards to get it just about as close as you can hope. Unfortunately, their site has been down for some time and is only slowly getting fixed, but
www.tacticalreplicas. carries some of MILSIG's stuff.
Schematics of their base markers:
http://www...iewtopic.php?f=93&t=3290Different threads with pics:
http://www...iewtopic.php?f=12&t=2935http://www...iewtopic.php?f=12&t=3102