The AR-15 is a rifle comprised of two halves; the upper and lower mated together, each responsible for their own essential functions.
While the lower is the half of the rifle that’s considered the “firearm”, and therefore gets a lot of attention, the components that most largely determine the performance of the rifle reside with the upper.
The upper contains the bolt carrier group, and is the half of the AR-15 the barrel attaches to, so making the right selection of upper parts to match your desired performance is critical.
A significant advantage of the upper – unlike a lower receiver – is it's not considered a firearm by the Bureau of Alcohol, Tobacco, and Firearms.
This means you can purchase the upper and have it shipped right to your door, no FFL transfer and no paperwork. A shooter can assemble a variety of upper configurations and use all of them with the same lower. Meaning you only have "one" serialized firearm.
Perhaps you’d like a few options in alternate calibers, with different gas systems, different barrel lengths, or different optics.
Maybe you’d like a 5.56 NATO upper with iron sights for plinking and target shooting, and a suppressed 300 Blackout upper with a 1-6X variable zoom optic for hunting. Having two dedicated rifles is certainly an option, and if you want to get the absolute best performance from each specialized set-up, that is probably the way to go. But if you’re on a budget, or you just prefer to keep your trigger, stock, and all other factors consistent and have the flexibility of different calibers and set-ups, all you need is different assembled upper.
Many manufacturers offer a number of options when it comes to AR uppers, so what are the most important things to consider when choosing your next one? What makes a good upper receiver?