One Rifle to Rule Them All: AR-15

Folks like me who are into guns can talk without end about firearms and cartridges. Many of those discussions are also thought experiments. A common one that I’ve blogged about before is the “if I could only have one rifle” discussion and it was that same thought experiment that led me to write this post. As time passes, one gains experience and opinions evolve. With that additional experience, perspective changes and sometimes we can see the same problem or topic in a different light. My intent here isn’t to rehash that thought experiment, rather it’s to explore how I see the AR-15 now.

In the past, I tended to associate the AR-15 with .223 Remington (.223 Rem)and 5.56x45mm NATO (5.56) cartridges and mostly neglected other cartridges it can be chambered for. I suspect many, if not most, folks hold a similar association. After all, .223 Rem and 5.56 are the usual suspects when it comes to the AR-15. While they are very popular and highly available cartridges, they excel in some applications and are sub-optimal for others. However, when we consider that a single AR-15 lower receiver can be attached with a myriad of upper receivers chambered for a wide array of other cartridges, the AR-15 becomes a single rifle that can be configured, or perhaps, optimized for a much broader set of applications. Let’s take a look at several options.

Starting with the mainstays of .223 Rem and 5.56, these cartridges are highly available, generally affordable, and are great choices for a handful of applications. Applications include self defense, competitive shooting, and hunting varmints or small predators with an expanding projectile. There are folks who will also use these cartridges for hunting deer at close ranges and for hog eradication. However, as I’ve mentioned before, I’m of the opinion that these cartridges are sub-optimal for hunting medium sized game like deer and hogs. These cartridges are also great options for introducing folks to shooting rifles and the AR platform due to their low recoil.

One thing to keep in mind when it comes to .223 Rem and 5.56 is that it’s safe to shoot .223 Rem from rifles chambered for 5.56. However, the opposite is not true as 5.56 produces a much higher chamber pressure that rifles chambered for .223 Rem may not be able to handle safely. For a long time, this made me think that there was no reason to acquire an AR-15 upper receiver chambered for .223 Rem since it limited one to a single cartridge with no additional benefit. That belief was short sighted because due to the subtle differences between the two cartridges shooting .223 Rem from a 5.56 chamber hamstrings the precision potential of the .223 Rem cartridge. This has been addressed recently with the introduction of upper receivers with a .223 Wylde chamber which can handle the pressures for both the .223 Rem and 5.56 with sacrificing the precision potential of the .223 Rem.

Several manufacturers offer a wide variety of complete upper receivers chambered for 5.56 and .223 Wylde with several barrel length options for AR-15 rifles, short barreled rifles (SBR), or pistols. For example, the selection of upper receivers from Palmetto State Armory range from 7″ to 20″ in length. The selection variety means more ways to optimally customize the AR-15 for a specific application.

Another very common cartridge for the AR-15 nowadays is .300 AAC Blackout (.300 AAC). Like the 223 Rem and 5.56 mainstays, the .300 AAC Blackout is an excellent choice for self defense and hunting varmints or small predators with an expanding projectile. Unlike the mainstays, the .300 AAC tends to be less available and more expensive. So what is the point of the cartridge given the application overlap with .223 Rem and 5.56? A valid question indeed. In my opinion, .300 AAC shines when paired with shorter barrels or when shooting suppressed. This is because .223 Rem and 5.56 are loaded with very light projectiles (common weights range from 55 to 77 grains) which makes them highly dependent on velocity to achieve the terminal performance optimal for their applications. Whereas .300 AAC uses a heavier projectile (commonly ranging from 125 to 220 grains) that allows to achieve better terminal performance at lower velocities. Furthermore .300 AAC cartridges loaded with heavy projectiles are usually designed to remain at subsonic speeds which results in improved hearing safe operation when paired with a suppressor. One could argue that these characteristics make the .300 AAC a more optimal configuration choice for AR-15 SBRs and pistols with short barrels especially for defensive applications.

While not quite as common as the cartridges discussed so far, an upper receiver chambered for .22 Long Rifle (.22 LR) along with a conversion kit allow an AR-15 to be optimally configured for hunting small game with an expanding projectile. This configuration is also an excellent choice for practicing and improving fundamental rifle shooting skills or introducing somebody to rifle shooting or the AR-15 platform.

That covers the most common cartridge configurations for the AR-15 I’m aware of. However, there are plenty more cartridges that are common enough to readily find complete upper receivers and ammunition for. Let’s look at those.

The 6.5mm Grendel (6.5 Grendel) is an interesting cartridge choice for an AR-15. Commonly loaded with a 123 grain projectile boasting a fairly high ballistic coefficient (BC) ranging from .420 to .510 and a sectional density (SD) of .252, this cartridge is a relatively flat shooting cartridge that is suitable for hunting medium game animals including cougar and black bear with an expanding projectile out to 500 or so yards. This cartridge can also remain supersonic out to approximately 1100 yards making it a good option for long distance target shooting. Not to mention, it can also be used for hunting self defense and hunting varmints or small predators. Personally, I think .223 Rem, 5.56, and .300 AAC are more optimal configurations for defensive use especially for home defense scenarios given that the barrel lengths for upper receivers chambered for 6.5 Grendel are more commonly found in 18″ to 20″ variants. In terms of price, 6.5 Grendel ammo tends to be priced similarly to .300 AAC.

Another uncommon cartridge chambering for AR-15 uppers is the 7.62x39mm (7.62×39). For a while this has been an interesting option for quite some time because of the ballistic and application similarities between 7.62×39 and the .300 AAC and the historically low cost of 7.62×39 or AK ammo. Given 7.62×39 ammo prices have started to increase due to recent US sanctions on Russian ammunition imports, I’m not sure how attractive this option will remain in the future since folks who have built up an inventory of 7.62×39 ammo also tend to have access to AK platform firearms that use it and this cartridge doesn’t optimize an AR for anything else that the .300 AAC doesn’t optimize for already. I suspect that if the sanctions of Russian ammo imports are lifted in the future (and driving 7.62×39 ammo prices down) then this configuration option will regain any lost interest.

The .224 Valkyrie caught the eye of folks who were interested in an AR-15 platform cartridge for long range shooting years ago. Admittedly, I haven’t kept up with it and it seems like interest has fizzled out. Even so, a handful of manufacturers continue to offer upper receivers chambered for this cartridge. Given the small caliber and light weight projectiles (which are only marginally larger and heavier than .223 Rem and 5.56), this cartridge has the potential to over all of the same applications the .223 Rem and 5.56 mainstays cover with the addition of being suitable for medium game animal hunting out to approximately 400 yards when loaded with heavy enough expanding projectiles. Even though the potential is there the mainstays and the .300 AAC continue to be better candidates for defensive applications given .224 Valkyrie uppers tend to only be available with 20″ barrels. I doubt shorter barrel options are likely because the cartridge was plagued with stabilization challenges in its early days. Additionally, ammunition market support for this cartridge is limited to mostly a small handful of options for target shooting applications given it has the potential to remain supersonic out to distances in the 1200 to 1300 yard neighborhood.

Yet another uncommon chambering for the AR-15 is the 6.8mm Remington SPC (6.8 SPC). Application wise, the 6.8 SPC covers almost the same gamut the 6.5 Grendel covers. However, 6.8 SPC tends to be loaded with lighter projectiles that have lower SD and BC values than the 6.5 Grendel. As such, this makes the 6.8 SPC sub-optimal for cougar and black bear hunting applications and requires the use of a heavy enough projectile to be optimal for other medium game hunting applications. In terms of range, the 6.8 SPC is potentially a 300 yard deer cartridge (with a heavy expanding projectile) and stays supersonic out approximately 800 yards or so. Like the 6.5 Grendel, 6.8 SPC uppers tend to be available with 18″ to 20″ barrels making it sub-optimal for some defensive applications. In terms of price, it’s comparable to the .300 AAC and the 6.5 Grendel.

A newer kid on the block is the 6mm ARC, which is currently holding my interest. In some ways, I think this cartridge is what would result if the 6.5 Grendel and .224 Valkyrie were to mate and produce an offspring. It’s high BC and SD values make it suitable for hunting medium game animals including cougars and black bears out to 600 yards and can remain supersonic just beyond 1300 yards. Those characteristics make it an excellent long range medium game animal hunting and long range target shooting cartridge. Like the 6.5 Grendel and the .244 Valkyrie, upper receivers for this cartridge are currently only available with 18″ or longer barrels making it sub-optimal for some defensive applications. Furthermore, support for this cartridge is limited in both the ammunition and AR upper markets. In my opinion, it’s a promising cartridge, but time will tell if it is here to stay or interest will fizzle out.

AR-15 Uppers can be found or built to support even rarer, some arguably boutique, cartridges. Some of the ones that come to mind include the .300 HAM’R, .350 Legend, .375 SOCOM, .450 Bushmaster, and .458 SOCOM. There are others to be sure. However, the very limited market support for the uppers and cartridges has proven to be challenging when it comes to consolidating enough ballistic information in order for me to provide a good description for each of them. However, I’ll attempt to summarize them in the following table.

With all the options available, is it possible that an AR-15, given multiple upper receivers, could be the one rifle to rule them all? In my opinion, no. However, all of the options available make the AR-15 a very versatile platform that can be optimized for a broad spectrum of applications. For example, starting with an AR-15 with a .223 Wylde upper covers self defense, competitive shooting, varmint hunting and small predator hunting. Adding a 6.5 Grendel upper receiver to the mix covers medium game animal hunting (including cougars and black bears) and long range target shooting applications. The introduction of a .22 LR upper receiver and a conversion kit to the mix adds small game hunting and harvesting capabilities. Want to push the limit and cover a bit of large game animal hunting? Consider picking up an upper receiver chambered for .450 Bushmaster or .458 SOCOM. Feeling the need for something ultra compact optimized for self defense? One can always pick up an AR-15 pistol or SBR with a short barrelled .300 AAC upper receiver. That’s a lot of capability with one or two AR-15 platform weapons.