At MCS Gearup, we sell a wide range of complete, assembled uppers that builders can use to customize or build complete sporting rifles at home.
Gone are the days when you had basically two choices in AR uppers, being 5.56 and 7.62 NATO. Today, there is a wide range of variety in caliber and cartridges.
In this article, we’re going to cover some of the more popular AR complete upper receivers by caliber, and what they’re good for.
Old Reliable: 556 Uppers
There’s a reason that the 556 upper has been a staple for so long among AR fanatics. It’s because it’s a great cartridge that is highly versatile and has lots of different applications.
Even though the 5.56x45mm NATO might be a small cartridge, it is excellent for high volume target shooting, long-range shooting, competition, training, defensive applications, and hunting small and medium game.
This is because it is light, easy to carry and handle, low on recoil, has a flat trajectory, and offers decent ballistic performance. It also is generally widely available (especially if you account for military surplus ammo) and pretty cheap.
On top of that, the classic AR platform has been built around a 556 upper for so long that there are thousands and thousands of parts and accessories compatible either with the cartridge or with builds made for it - so there are all your reasons and then some to consider a complete 556 upper for your next build.
A Compromise: 223 Wylde uppers
Are you a fan of both 556 uppers and the .223 Remington? Then a 223 Wylde upper is your compromise.
The 5.56 is cheap and widely available, and the .223 is slightly more accurate and there are more premium loads available. But here’s the thing you need to know if you don’t yet. They’re not quite the same cartridge.
Sure, they look it, but technically, they’re not identical and on top of that, 5.56x45mm NATO produces higher chamber pressures than .223 Remington. The result? You can fire a .223 in a 5.56 chamber, but if you do the opposite and try to fire a 5.56 in a .223 chamber, you might destroy the gun and injure yourself or bystanders.
A 223 Wylde upper is the solution. This is a hybrid chamber that can feed and fire both .223 Remington and 5.56x45mm NATO cartridges without any issues, so you can have your cake (low cost, accuracy, wide availability, etc.) and eat it, too.
7.62x39 Uppers
Familiar with 7.62x39mm? Don’t confuse it with 7.62x51mm NATO! They’re not the same cartridge even though the calibers agree.
The latter is the NATO cartridge developed to replace .30-06; the former is an older Soviet cartridge, one you might be familiar with if you’ve ever handled and fired either an SKS or AK-47.
Also known as 7.62 Soviet, the 7.62x39mm is a cartridge that sits between the 5.56 and the 7.62 NATO in terms of power and performance. It is more powerful than the former but less powerful than the latter.
Actually, the 7.62 Soviet offers trajectory and performance that’s not so different from the .300 BLK (see below).
Anyway, if you’re interested in building a rifle with a 7.62x39 upper, know this. It has solid stopping power at close ranges, but has a badly drooping trajectory so beyond 100 yards performance is spotty.
Also, 7.62x39 ammo is usually pretty cheap because it is still widely manufactured and imported.
Just make sure you get appropriate parts for your 7.62x39 upper.
300 Blackout Uppers
If you’re looking for the power of a 30 caliber cartridge but you don’t want to stray too far from the 5.56 and what it offers, then the clear answer for you is to build a rifle with a 300 Blackout upper.
The .300 Blackout, also known as .300 AAC and .300 BLK, is basically a necked-out 5.56 cartridge loaded with a 30 caliber bullet.
It offers far superior stopping power at close ranges and much higher muzzle energy than the 5.56, all without producing too much more recoil.
The other good thing about building with a 300 Blackout upper is that you can use many of the same parts you would with a 5.56 rifle, including the lower parts and the magazines. Just make sure you never chamber a .300 Blackout cartridge in a 5.56 chamber, so keep the parts clearly marked even if you do interchange them.
There are a few tradeoffs to using .300 Blackout, though. One is that it can be a little expensive and in some instances hard to find. You might be able to counter this by loading your own, though.
The other thing is at extended ranges, it’s just not good. Like 7.62 Soviet, it has a seriously drooping trajectory, so it’s only good for engagements within 100 yards or so. Still, for defensive applications and hunting at close ranges, it can be an excellent choice.
350 Legend Uppers
Last but not least we have the option to build a rifle with a 350 Legend upper. This is a straight-walled cartridge, making it a good option for hunters in states with straight-wall-only restrictions.
The 350 Legend cartridge produces around a third more muzzle energy than the 5.56 and has a slightly higher sectional density, too (which equals better penetrating power), making it suitable for hunting game around as large as whitetail deer.
It also produces only a little more recoil than 5.56, but it has a much less effective range. Beyond 100 yards is questionable for .350 Legend. Like the .300 Blackout, you’ll have to keep ranges somewhat close if you build with a 350 Legend upper.
The other thing is that .350 Legend can be pretty expensive and hard to find, but if you only use the rifle for hunting, that shouldn’t matter. One box of cartridges could theoretically last you through several seasons.
A Successful Build Starts with an AR Complete Upper from MCS Gearup
Ready to get started building or converting a rifle with an AR complete upper? Get in touch with us here if you have any questions at Sales@MCSGearup.com. We carry everything you need to complete or customize a build, including lower parts kits and build kits as well as parts including gas systems, charging handles, bolt carrier groups, and a variety of different uppers/barrel lengths, so if you have questions, just ask.