ARMS Rear flip up iron sight
For quite some time in my career I only used iron sights on my rifle. While it does not have to be true for everyone, I can be just as accurate with them as with any zero magnification optic, if not more accurate. Iron sights are great - light and effective, but they are undeniably slower to pick up with your eyes in any kind of rapid shooting, which is why I reluctantly reduced my irons to a backup role. So while I have to recommend always having an optic of some kind, I also have to recommend having zeroed iron sights on your weapon at the same time as a back up. And, of course, being comfortable with them. Everybody says it but it is true: Optics can and do fail.
There are quite a few options available and the price range varies significantly. At the cheaper end you can get a set of magpul’s back up iron sights for $75 (front and rear sight together) from Brownells. Troy makes a great set that runs closer to $200 for both. If you get someone else’s name on Troy’s sights, you can pay even more. ARMS inc makes my favorite rear flip up iron sights that runs about the same price as Troy.
For the value, you just can’t beat Magpul. Just don’t buy from their website, you can get them cheaper from other places. Like Amazon, who offers them cheaper even before you figure in tax. In addition to their price, the color options are nice if you want to make your rifle color-coordinated (and I often do). Remember, just because it is for killing people does not mean that you can’t make it look good.
For solid, reliable sights, Troy is generally considered king and I agree. I personally prefer the function of the ARMS, but you can’t go wrong with either. The reason I like the ARMS is that it is spring loaded and literally flips up. The Troy sights go up easy and lock into place once up, requiring a push button to get them back down, which is also functional and reliable (and offers easy near silent use if you notice your sight died at a bad time). Troy won't let you fold the sight completely flat with the small aperture, you have to select the large one - the theory being if you need your backups in a hurry, you also want to use the wider aperture, which is a reasonable assumption.
DPMS Mangonel rear sight
But all the same I am a fan of the spring loaded sights like the magpul and ARMS. The magpuls don’t always flip up as easily though. The plastic can flex and drag on your rail, or snag on one side if you are using just your thumb to pop them up.
YHM also offers a decent backup sight, but for the price I prefer the others listed already. If you like them, DPMS makes a “mangonel” style flip up rear sight. There is no link because I can’t find it on their website anymore, but they are still available from many vendors. They are pretty popular and I would say after Troy, the second most popular iron sight amongst the guys I work with.
While there are many other brands out there, I find there is a lot of overlap: You look them up and they are the exact same as one of the brands I have listed here, just with someone else’s logo on it. If you do choose to look outside the brands I mentioned, just be careful with how much play is in the sight. Troy has a little wobble if you tug them when they are up, but it is minimal and does not affect accuracy. Some can literally flop around on you and every time you shoot, the same aiming point might be 5 MOA off the last one or more.