OK, my turn.
Iron sights work - I consistently hit 300 yard pop-ups with them when I was in the service. They truly suck in low light though, even with the large aperture on the rear sight engaged. They also require you to focus on the front sight which throws everything downrange out of focus. You can still shoot well doing this, but it works best with high-contrast targets and young eyes.
Optics allow you to focus on one focal plane -- there's no issue of the target being blurry, the front sight being sharp, and the rear sight kinda-sorta in your awareness if you think about it. Rather, you get to look at your target and see where the point of aim is at the same time. This really is an improvement over irons.
There are some problems though. If you throw your rifle off a waterfall, or if you trip while walking up a steep rocky incline and accidentally throw your rifle so that it bounces down 50-100 feet of trail, will the optic hold up? If it's parked in the closet and you suddenly need it Right Frigging Now will the battery still be running? These are issues that worry folks.
Luckily, we've been putting optics on weapons for a while now, even in combat, and there are a lot of vendors who make optics that are the next best thing to bulletproof. Search on youtube for torture test of optics and you'll be surprised. I was looking for one where a guy grabs the muzzle end of his rifle and swings it so that his US Optics scope hits a wooden fence post a few times, but found this first. It's not all that abusive, but it's more than I subject $2,500 optics to:
US Optics, Trijicon, Aimpoint, Schmidt & Bender, Nightforce, ELCAN, and others make optics that can be counted on to be durable and waterproof. In my opinion they all are a better option than iron sights, but I'll point out that my carbines have backup iron sights as well.
The question you should be asking is this: what kind of optic are you looking for?
Option 1: non-magnifying optics
These are generally seen as iron sight replacements and are fairly quick to get on target. You have options here:
Aimpoint. This one is an older model but they all work comparably. What you're seeing is a durable housing for a battery, electronics, and some optics. Look through the tube, see a red dot at your point of aim. Kind of hard to screw up.
They are battery dependent, but battery life is long (years) and many setups include a spare battery as part of the basic kit. They work well, and are issued to troops and seem to work pretty well in combat.
Trijicon Reflex: This is kind of like the aimpoint, but different. You look through the optic and see an amber triangle that represents your point of aim. No batteries though -- brightness is controlled by fiber optics, so the brighter it is outside the brighter your reticle becomes. In low (or zero) light there's tritium so you still have an aiming point.
Less to go wrong, but sometimes the reticle can be hard to find. I've never had that problem, but it's been reported.
Eotech: I think these are about the easiest thing to shoot with for close-up work, though I've never bought one. Battery dependent, options for reticles (like this one)
There are others, but these are the big players. All offer easy target acquisition at close range and in questionable lighting situations, and there are trade-offs with each one.
The problem with them all is this: they're not nearly as good as some other optics for ranged work. I think my Aimpoint has a 12 MOA reticle (meaning it covers up a foot-wide piece of the target at 100 yards), but it looks like current models are typically 4 MOA. This means that pretty much any carbine you mount these to will outshoot the optic -- the reticle is big enough to cover up the target and make real precision work hard.
Now, does that matter? 4 MOA is still more than good enough for hits on a man-sized target at 300 yards, but it's not perfect. It also hurts if that guy at 300 yards is shooting back at you, blends into the environment, and is taking cover. In those cases you want more magnification.
Option 2: Fixed Magnification.
The Trijicon ACOG is pretty much the standard here, at least to my mind. 4 power scope, doesn't require batteries because fiber optics pipes in ambient light, durable, and seemed to do pretty well over the the middle east.
Some have reticles that are designed to look through for close quarters combat, and it seems to work. I've never tried it.
You can modify an Aimpoint or EOTech with a magnifier (Justin did a review of one
here). I saw precursors to this at the 75th Ranger Battalion ~ 1992 so this isn't that new of an idea, and it works. Sort of.
There are ELCAN optics in the 3.5x range as well that mostly do well (I'm biased -- I love ELCAN optics) but have some weaknesses at the mount when soldiers beat them up. Really though, the Trijicon ACOG is the archetype in my mind.
Option 3: Variables
There's another class of optics that 3-gunners have embraced pretty strongly. These are scopes that go from 1x power up to 4-6-8x power, depending on the model. This allows you to run with no magnification and a lit reticle at short range like the aimpoints and EOTechs, but you can crank the magnification up when you're shooting targets at 200m and beyond, up to the ballistics of the rifle.
Here's the one I'm currently reviewing, from SWFA:
It's not perfect, but it may be the value leader in this range.
It's competing with optics from US Optics, Schmidt & Bender, ELCAN, Trijicon, Leupold, Zeiss, and others. All seem like reasonable choices. I really like the ELCAN Spectre DR -- it uses a lever to swap from 1x to 4x and the optics are impeccable, of course it's quite pricy. Others who shoot more than I do seem to favor the S&B Short Dot or the Zeiss Z6i, both of which probably cost more than your rifle did.
This class of optic seems like the best solution for carbines to me. They work well at any engagement range, don't
require batteries (they're traditional scopes and have etched reticles) but offer illumination if needed, and allow you to choose the magnification based on what you're doing. The only one with a lever is the ELCAN, the others have a ring you twist like with any other variable scope.
Finally, mounts
If you look at the photos above you'll see durable, well-built optics that are mounted on quick-release mounts. If you need to move to irons with these sights you can, and quickly. Well, it'll be between 5 and 15 seconds to perform the swap, but I'm more than willing to assume that possibility in order to enjoy the advantages that modern optics offer for carbines.
