9mm AR Upper
The AR platform is very versatile, and it can be chambered in a variety of calibers. In the past decade, pistol caliber carbines and AR pistols have become quite popular, and most of them are chambered in 9mm, which is the most popular pistol caliber in the world.
These 9mm ARs have a lot of advantages. They are small and maneuverable, making them an excellent home defense weapon or PDW. Moreover, the AR platform allows the shooter to get the most out of the 9mm caliber and shoot it more accurately and effectively. Even at the range, shooting a 9mm AR is much more cost-effective, and some people even use 9mm ARs for competition shooting.
To build any high-quality AR rifle, you need to have a good lower and upper receiver. Several companies make really good-quality AR uppers and a variety of configurations.
You can opt for the carbine configuration, like a standard AR-15, but with a different caliber. It is legally a rifle and hence needs to have a 16-inch barrel. This configuration has several advantages. It allows the shooter to stretch the effective range of the 9mm round because of the longer barrel, and the stock allows for easy shouldering and accurate shots.
You can also opt for the pistol configuration, which can have a shorter barrel. However, they have to use a pistol brace instead of a stock. They are also very maneuverable and more controllable than a pistol as well.
What To Look For In A 9mm Upper:
The general idea is that the longer the barrel, the better the ballistics. However, a long barrel can be problematic with a caliber like 9mm, which does not have anywhere near the power and range of a 5.56 round. Moreover, the increase in speed and range with the increase in barrel length isn't that significant after a certain length.
According to most people, about 7 or 8 inches is the ideal barrel length for 9mm barrels. After that, the increase in performance is very insignificant. Moreover, if you keep increasing the barrel length, it may even have a negative impact on the ballistic of the 9mm round.
Barrel Twist Rate:
Almost all gun manufacturers agree that the 9mm round gives the best performance with a barrel that has 1:10 twist rifling. This pattern stabilizes the round and allows it to be the most effective and accurate.
The BCG and Magazine Preference:
Firstly, unlike AR-15s, a 9mm AR upper has a direct blowback operation system; hence, the bolt is also different. Also, when selecting an upper for your 9mm AR, ensure you get the bolt carrier group that actually works with your lower.
Some lower feed from Glock magazines, whereas some may use colt style 9mm mags. Therefore, get the 9mm AR bolt that works with your preferred style of magazines.
The handguard is also an essential part of any rifle. It allows the shooter to control the firearm and add accessories to it as well. You should look for a 9mm upper that comes with a modern modular handguard with M-LOK slots or Picatinny Rail so that you can add attachments to it as well.
Apart from all these factors, you should also ensure that you are getting your 9mm upper from a reliable manufacturer with experience in making 9mm pistol caliber carbines and is known to use high-quality parts.