M4 Upper Receivers
What is an M4 Upper Receiver?
The M4 emerged in 1984 and was officially adopted by the US military in 1991. Since then, it's earned a revered place in the heart of every gun-enthusiast as one of the most formidable combat rifles of all time. The M4 had a few distinctive features, like it's barrel-cut for an M203 grenade launcher, as well as it's vastly improved feed-ramps. Chances are that if someone is referring to an "M4 Upper Receiver" nowadays, they're probably talking specifically about it's feed-ramps. As technology has advanced in the AR-15 platform, people are no longer running the dated M4 barrels and hand guards.
M4 Upper Receiver Feed Ramps
The M4’s feed ramps were developed to rectify full-auto feeding failures once the weapon starts to get dirty. Previously, feed ramps only existed inside the barrel extension, and the M4 Upper sought to modify that for improved reliability. By widening, and deepening, the feed ramps of the barrel extension, and extending them down the upper receiver to where the lip of a magazine sits, it removed the issue of rounds hanging up when feeding into a dirty gun. Largely any AR-15 Upper Receiver on the market to day is going to have the M4 feed-ramps, as a vast improvement with no downside.