The AR-15 has become one of the most popular firearms among American gun owners and for good reason. Due to its lightweight frame, high level of accuracy and versatility, the weapons system is a testament how innovation can transform an older design. For years now, firearms accessory manufacturers have been turning out various parts for the AR-15. While many still prefer to buy pre-built firearms, many others enjoy shopping around for the coolest and best parts on the market and create something special and tailored for them. One of the fundamental elements of the build starts with an upper receiver.