The Trijicon NSN has iron sights mounted on the top of the rifle scope which can come in handy in closer situations and are a less expensive alternative to mounted a red dot on your ACOG.    That is worth the money if you are in the line of work that calls for it, but for general blasting and plinky, I think that $1800 optics is a bit much.   The Eotech 512 is probable my recommended optic for an M4.  I think people can get carried away with optics on the M4 because even though the rifle is pretty accurate, it really is not very powerful or effective past 200yds.    With a growing selection of combat optics you can put on the AR15 type rifles, one other suggestion I have for shooters is to always keep your iron sights on the rifle.   I’ve seen a few people show up at classes with just an Eotech mounted and they complained the front sight blocked their FOV.   Well,  taking a 200yd shot with an Eotech that has a dead battery is like parking a mini van from the furthest back seat.

There are numerous reasons that your rock solid rifle optics will go down.   Some optics use batteries which will always go down at some point.   Your tough Trijicon ACOG will hold zero under most combat conditions, but a 30 caliber bullet can still dismount it.     Reticles can come loose, rings can fail, glass can break and numerous other things occur in a fight that don’t normally happen with occassional hunting trips.   Most rifle optics won’t handle real big drops, but having the ability to have a backup just incase the glass goes down.  Co-witnessing your sights is a simple answer for those using rifle scopes and quick detach mounts in my opinion are worth considering.   It has never happened to me in a hostile situation, but having a MOA rifle with broken glass that is solidly mounted on my rifle has got to be a nightmare sceneario.