Tips and Tricks for Your 9mm AR Build

The 9mm platform is great on the AR. As a truck gun you get a versatile rifle with the ability to reload from your sidearm ammo. As convenient as they are the 9mm is not a 5.56 and the mechanics of making a reliably functioning rifle must be taken into account. 

The Gas System - Or more accurately, the lack of a gas system. The 9mm AR uses a blowback operating system. There is no traditional AR gas system. This translates into a much heavier counter-weighted bolt assembly and a very heavy spring/buffer combo. This is where almost all problems with the 9mm builds originate. The 9mm round is high pressure and transfers a ton of energy to the bolt during the cycle of operation. This energy must be tamed or you will have bolt overtravel and/or bottom out the spring travel. This tends to make the feeding/chambering process problematic. The buffer should be around 7oz at a minimum. Do not use an AR-15 buffer. They will not work. If you are running hot ammo or suppressed you will likely need to go heavier and possibly a Red Spring from SprinCo. 

If you are having feeding or chambering problems:

• Use a better magazine 
• Use a heavier buffer / spring

Secondly, The ejector. Depending on the manufacturer and what magazine they are designed for will determine the style ejector you will need. Typically, Glock-style magazines require a Glock-cut bolt. The bolts will have a much deeper cut on the ejector side. Any build can use a Glock-cut bolt but if you are using a Glock style lower you cannot use a Colt-style 9mm Bolt. Also. the ejectors are a big piece of metal sticking out of the top of the receiver. Make sure they didn't get bent during the build, cleaning, etc. and that they sit straight in the bolt groove. You will see the drag marks on the lower side of the bolt if this is an issue. 

Did I mention use a heavy enough buffer? Oh good. Please remember that. Its important. As almost every 9mm upper/ build kit includes one that is way too light.