The AR has been adopted as the North American standard for a weapon design. This platform serves in a host of various roles: long range shooting, close quarter combat, various game hunting, competition shooting, and of course self-defense. The AR’s ability to be proficient in all of these roles are shaped not just by the various offerings in different barrel lengths and other parts and accessories; but also the availability of several calibers compatible to the AR. The same lower receiver that you train with in standard 556 NATO can also be used with 22LR, .17 HMR, 9mm, .45 ACP, .308 Winchester, .458 SOCOM, and even .410 gauge.

What is the best AR conversion kit?

A singular firearm wouldn’t be the best fit for every possible shooting role, but what if one firearm could be converted to fulfill almost any purpose for the average shooter? Well, AR15 conversion kits do just that. The AR15 has amassed a large consumer base with plenty of support for conversion kits of just about every practical caliber and useful feature. Building ARs is a great way of coming to a better understanding of how your firearm functions and diagnose failures or discrepancies. The convenience of most AR15 conversion kits is being able to use existing parts that you’ve gained experience or an affinity for in new builds. This will help you to save costs from purchasing additional accessories and components that would have been needed for multiple complete rifles. Conversion kits also tend to take minimal installation time without permanent modifications to your firearm should you want to revert to a previous or different configuration. After all it’s just punching out the pivot and takedown pins, slapping on a new complete upper receiver and boom, you’re done.

AR15 Gas Piston Conversion Kit

Gas impingement systems in AR rifles utilize redirected gas from when a round is fired to push the bolt carrier group back into the buffer tube for it to cycle back and forth to create semi-auto fire capability. In gas piston systems, they instead use a BCG that has a rod connected to it which is pushed by the gas as it activates a spring loaded through a piston towards the front of the barrel. The gas piston system introduces less burnt carbon into the receiver and less acrid gas into your face.

Uppers from the HK416, Primary Weapon System rifles and the SIG516 all run on gas piston systems. Although gas piston systems are more expensive and are slightly heavier to carry than gas impingement systems, AR15 gas piston conversion kits run fantastically and help you to stay on target while keeping grime out of your receivers over longer periods of time.

If this interests you, check out Primary Arms for a complete gas piston upper to convert your AR to a cleaner running system. If buying a complete upper is beyond your budget you can also consider other AR15 gas piston conversion kit options like the Superlative Arms Adjustable Piston Systems.

AR15 Bullpup Conversion Kit

Commercial AR15 bullpup conversion kits are basically nonexistent. The buffer tube design of the AR lower forces any AR bullpup conversion kit design to accommodate the rear of the lower receiver and take up precious space that would compromise the bullpup concept. Some hobbyists have posted their personal projects of bullpupped AR15s, but these are expensive proof of concepts that are not something that can easily be machined by the average person.

In short, AR bullpups are a difficult engineering feat and have not been made commercially yet. It is particularly a feat that not many minds are inclined to develop as there already exist fantastic options for bullpup firearms. The IWI Tavor serves excellently in a close quarter combat role with the ergonomics to match. The Steyr AUG shoots well too and outclasses the Tavor when it comes to shooting past 150 yards to 300 yards.

The AR community has often found themselves at odds against the bullpup design sometimes to a rabid extent. As a Steyr AUG owner, I personally found it really difficult to shoot offhand if I were to peek from a wall or a barrier with the non-dominant side of my body. Getting used to the mechanics of reloading was also not very intuitive. The AUG’s trigger is also a lot heavier than that of most AR15 drop-in triggers as it’s much larger and is made of so much more material. But all that being said, you get to enjoy a 16” barrel in a much shorter overall package. These are the main trade offs that come with a bullpup design when it comes to function. Should bullpups ever grow in popularity within the US, it might start to receive a larger aftermarket support.

AR15 308 Winchester Conversion Kit

If you’d like to shoot .308 on the AR platform, get yourself an AR10. Problem solved. Similar to the subject of a AR15 bullpup conversion kit, there is not a commercial AR15 308 Winchester conversion kit. It’s not there isn’t a demand for a conversion kit to convert an AR15 to shoot 308, it’s more so because the magazines for .30 caliber rounds and up will simply not fit an AR-15 lower receiver’s magwell.

Best AR15 Conversion Kits For Training and Hunting Varmints

During the pre-Covid era, (about three years ago) 9mm cost as little as $0.10/round. While the market has slowly climbed down from the peak of $0.90/round to about $0.30 today... Ammo is still more expensive today than it ever has been in recent history. What does this mean for people who need the ammo to train? Well, it means we might need to start looking at other calibers as an alternative to save money. This is when using a conversion kit to swap out the upper for a lower powered cartridge would be extremely useful.

AR15 22 Conversion Kit

Out of all the packages for an AR, the 22LR conversion kit is one of the most commonly enjoyed. The popular staple name of CMMG’s 22LR AR Conversion Kit starts at the cost of $189.95 and magazines begin at $24.95. 22LR is as cheap as ammunition comes and the 22LR Conversion Kit can help you buy a lot of training time in the field with less money.

The AR15 22 Conversion Kit design provides an easy installation and comes as a drop in bolt carrier group. Installation and removal of this AR15 22 conversion kit is no different from simply swapping a BCG in and out. The low recoil of 22LR also makes the 22LR AR conversion kit a good training tool for those who are beginners at shooting.

One convenient feature to note about the AR15 22 Conversion Kit is that it doesn’t need the buffer tube to cycle. The buffer for the conversion kit is actually built into the conversion kit so you won’t need to worry about cycling or reliability.

AR15 .17 HMR Conversion Kit

.17 HMR stands for Hornady Magnum Rimfire which is an upgrade from the 22LR that comes in the package of a necked 22 Magnum casing fitted with a .17 grain projectile about 4.5mm in diameter. It’s quite the capable rimfire round known to travel with a pretty flat trajectory and punches very hard because of how fast the bullet flies. Reputable complete uppers in .17 HMR by Alexander Arms can be had for $792.95 - $1141.95.

Best Pistol Caliber Carbine Conversion Kits

The market for AR PCCs have grown substantially in the past years and shooters now have many options for a pistol caliber carbine conversion kit for ARs. Within this class of blowback action pistol caliber carbines, resides the popular AR9 and other ARs converted to a pistol caliber such as .45 ACP or 10mm. Although parts for pistol ARs have not reached the scale of aftermarket support on the level of the standard AR15, reputable pistol caliber carbine conversion kits are certain to add some fun to your range days or may just be what you need to give you the leg up in local competitions.

9mm Conversion Kit

9mm is one of the most popular calibers among AR pistols. There are many reliable 9mm conversion kits available for 9mm Glock magazines and 9mm Colt magazines. Previously, we’ve talked about Stern Defense’s AR 9mm Conversion Adapter and Hahn Precision’s Dedicated 9mm Adapter. For more information on other options for a 9mm conversion kit, read up on our AR9 Parts Guide.

.45 ACP Conversion Kit

When it comes to deciding to shoot .45 ACP it usually involves a question of whether or not you hate money. If the answer is a resounding yes, then this is one of the perfect calibers for you. If you’d like to feel somewhat closer to shooting a HK USC45 with a .45 ACP conversion kit, try out the MVB Industries’ MVB 45 Adapter to be able to use USC magazines. The major drawback to MVB’s .45 ACP conversion kit is that the only USC magazines you will probably find currently are by ProMag which have been reviewed by most of the internet to be a pretty unreliable brand. If you don’t mind foregoing USC magazine aesthetics for better practicality, then you could try out the 45 ACP Magazine Well Adapter kit by Macon Armory.

Large Caliber Conversion Kits for AR15

Conversion kits are great as they can save you a good deal of money, but what if you’re kind of a masochist and want to suffer more than a P90 owner that has to buy 5.7x28mm at $1.00 or more per round? Well, we’ve got just the thing for you.

AR15 .458 SOCOM Conversion Kit

After fighting with Somali forces in 1993, US special forces described their experiences alongside Malaysian special forces of needing multiple successions of hits to make sure opponents stay down. .458 SOCOM was the solution to introducing a round that can feed through a standard AR lower and make sure that the three guys standing behind your target stay down as well. Born from the similar roots of 300 BLK, .458 SOCOM had been designed with some suppression capability in mind as well as M4 compatibility. .458 will run on your AR buffers, magazines and lower receiver without any modifications. That’s why a conversion kit for .458 SOCOM lies in just the upper - specifically one with an ejection port wide enough and thicker walls to accommodate this large caliber round.

If the $2.00 per round price tag for feeding a .458 SOCOM doesn’t make you flinch, check out Tromix Lead Delivery Systems. Tromix has been specializing in .458 SOCOM and has gotten it right since 2001 for all your complete uppers and 458 SOCOM conversion kit needs. Additionally, Cross Machine Tools has an excellent side charging upper, AR-15 Side Charging Upper Receiver.

AR 410 gauge Conversion Kit

If you’ve ever wanted to skip the reliability and cost of a normal 12 gauge shotgun and wanted to shoot the same shell through an AR instead then the .410 gauge might scratch that itch. The .410 gauge conversion kit gives you a shotgun experience, but through the AR platform which is awesome! However, it may require some tweaking to get reliable and consistent performance. But if you get yourself a reliable complete AR 410 upper, that won’t be much of a concern.

It’s a really cool concept that greatly expands your shooting capabilities in North America when combining a normal AR15 and a 410 complete upper. With an AR 410, you get to shoot skinny shotgun shells out of a magazine with a semi-automatic function which is always fun. Expect to be purchasing shells at over $1.00/round though.

To give the .410 gauge some credit, it is an option worth exploring for those who found themselves averse to the 12 gauge’s weight, noise, and recoil. .410 is rather skinny in comparison to 12 gauge but still has the capability to be able to effectively bag deer, hogs, and even home intruders. If you’d like to try out an AR-15 .410 Conversion Upper let us know how it goes in the comments below!

Buy Your Next Conversion Kit from Modulus Arms

Have you found an AR conversion kit that stands out to you? If you’re ready to get started with your new build for the holiday season, start out with our Router Jig Extreme. With it, you’ll be able to mill out 80 percent lower receivers and build out your guns without the legal and unjust barriers. Whether it’s for an AR15, AR10 or AR9 our jig is easy to use for users with any level of experience and will set you up for a lifetime of building your AR projects.