Starting with an assembled upper, whether you are building a micro ARP or a full-size rifle, is a great way to make the whole project a lot easier on yourself.
But it does not answer the question of “What caliber or cartridge should I build this gun around?”
Having broached that subject, let’s assume you landed here because you are thinking about building a true micro ARP - one that is 5” in length or shorter.
What is the best caliber for you? This guide will break down some of the more common offerings.
Some Common Chamberings
This post will cover 5 of the most common chamberings in micro ARP builds. There are others, but these are probably the most popular.
.223 and 5.56
All else being equal, .223 and 5.56 are probably the two most cartridges around which AR platforms are built, and that includes ARPs.
Part of this is probability due to habit and familiarity, since the original AR was built around these cartridges over half a century ago. Because of that, demand for both cartridges remains high, and supply is high to match it.
Even if that were not the case, as an intermediate rifle cartridge (both of them), they are low-cost, produce reasonable stopping power, little recoil, and are suitable for a wide range of disciplines, such as hunting, competition, and long-range shooting.
For a micro ARP, they offer rifle-sized firepower in a more compact package, and since the cartridges themselves are fairly small and light (and least by centerfire standards) they remain popular.
.300 BLK
The .300 BLK was designed for AR platforms and shoots very well from short barrels as well as suppressed barrels. In fact, propellant load data is such for this cartridge that in some cases it can fully deflagrate even from very short barrels.
This actually gives the .300 BLK a bit of an advantage with respect to micro ARPs, as well as with AR pistols in general. On top of that, it produces much higher stopping power than .223 and 5.56, and performs well in suppressed platforms.
Because of this, it’s a good option for ARP builds and it has its advantages for close-range defensive engagements. But there are some shortcomings of which you should be aware.
One is that it is somewhat hard to find, and another is that it’s fairly expensive. Just be aware of that if you do choose this cartridge for your build.
9mm
Not just the king of handgun cartridges, 9mm is also probably the king of pistol caliber carbine cartridges, and that makes it positively ideal for micro ARPs, especially those with extremely short 4.5 inch AR pistol uppers.
There are some things that give 9mm an advantage if we’re talking about ultra-compact micro ARPs, and many of the 4.5 inch AR pistol uppers we sell here are built around 9mm.
For one, it, like 5.56 and .223, is low cost. In some cases even more affordable, and just as accessible. It produces low recoil, is optimized for performance from a short barrel, and though it produces less stopping power, at close ranges, the heavier bullets perform very well. It might also produce a lower risk of overpenetration.
Also, since the cartridges are smaller and more compact, it is easier to carry the ammo, or magazines, resulting in another potential advantage.
.22 LR
Some micro ARP uppers are built around .22 LR, which is also interestingly an excellent option, with a caveat.
This cartridge is generally seen as unsuitable for defensive applications because of the abysmally low stopping power it produces. That said, it is very cheap, produces very little recoil, and is the most widely accessible cartridge mentioned here.
If your plan is to build a micro ARP for target shooting or plinking, then this is actually one of the better options at your disposal, if not the best. You could burn through tons of .22 LR ammo without making a huge dent in your wallet, and on top of that, .22 LR performs reasonably well from short barrels and hardly kicks at all, two things you need with an ARP.
7.62 Soviet
Lastly, we’ll take a closer look at 7.62 Soviet, or 7.62x39mm, which actually wasn’t designed for AR platforms (it is a Russian cartridge that was widely paired with AK-47 and SKS rifles) but which is paired with AR rifles commonly nonetheless.
Also an intermediate cartridge, like .223 and 5.56, the 7.62 Soviet offers better power, especially at short ranges. It’s abysmal at long ranges, but if you need a cartridge that hits hard, 7.62 Soviet is a good option.
Here’s the thing to keep in mind. The 7.62 Soviet offers similar stopping power to .300 BLK, but it is also likely to overpenetrate. On the flipside, it is a lot more affordable than .300 BLK, so consider those two things when making your decision between one of the two, if it comes down to them.
Explore 4.5 Inch AR Pistol Uppers and Parts Here
Whether you’re here for a complete 4.5 inch AR pistol upper or an assembled upper for a rifle build, we have complete options as well as parts kits and more. Check out our full collection and get in touch with us at Sales@MCSGearup.com if you have any questions before you buy.