Backup sights are a controversial issue these days. There are an increasing number of shooters who feel that the current crop of military-grade reflex and prism sights (Aimpoints, Trijicon ACOGs, etc.) are tough enough and reliable enough that backup sights are no longer useful – that is to say, any event that’s traumatic enough to destroy an ACOG is probably not going to leave behind an intact rifle.
To my knowledge, most militaries still embrace backup sights as an essential. About the only first-rate military I’ve ever seen field optics without backup iron sights is the IDF – you can find plenty of pictures online of optics deployed on flat-top M16s without a rear backup sight..
Therefore, I assume FAB Defense developed the FBS / RBS backup sights for export. I have a 9mm AR-15 that needed some backup sights, and the FBS / RBS seemed like an interesting option. How did I like it? More after the break.
The FBS and RBS are folding polymer backup “iron” sights (BUIS). They seem sturdy, but I didn’t drop-test them to find out. Remember that you should not mount a polymer front sight to a gas block – it probably won’t melt, but the heat will affect your zero. They fold down pretty far, and I would not worry much about trying to mount a scope with them. You can mount them to the rails without having to slide them on, which makes them a bit easier to install than some other choices.
The RBS is pretty similar to the Magpul rear MBUS, except it doesn’t have a spring-loaded flip-up capability. I have mixed feelings about that feature – it certainly helps deploy the sites more quickly and keeps them locked down, but you also run the risk of them popping up because your rifle hit a piece of your gear, which is a bit annoying. Given that the real-deal military BUIS I’ve used don’t have that feature, I don’t think it’s essential.
The FBS, on the other hand, actually reminds me more of the Magpul MBUS Pro front sight, as it allows tool-less adjustment of the front sight. Given how annoying AR-15 front sight tools are to use (4-pin? 5-pin? Deep enough?), this is a very welcome feature. On the other hand, it also means you can’t use a standard front post, which rules out fancy fiber optic or tritium after-market posts. You can buy the FBS and RBS in tritium versions, though, so if that’s important to you, you have the option.
In use, well, they’re BUIS. I didn’t notice any real difference between them and the Magpul MBUS I’m used to. Given that the FAB Defense BUIS are somewhat cheaper than the Magpul BUIS, and the FAB Defense FBS has a tool-less adjustable front post, I think they present a pretty good value for the money. At the end of the day, backup sights are just that – backup. You’re better off spending the money you save from buying polymer backup sights on a better primary optic that’s not going to go down as easily.