How to Rechamber Your "AR"
If you’ve decided to use a rifle based on the ArmaLite 10/15 platform for hunting, you might want to take advantage of its modularity—including the ability to convert it to another caliber. This article will guide you through how to do that.
The popular AR platform likely needs no lengthy introduction. Originally a military rifle, it has, thanks to its quality and civilian popularity, become common among hunters as well. Its advantages include reliable semi-auto operation, a potential for low weight for an autoloader, sub-MOA accuracy with reputable brands, and better ergonomics compared to traditional bolt-actions. Of course, if you’re after an ultra-precise/powerful/cheap/lightweight/maximally simple or just a classic hunting rifle, an “AR” may not be ideal—but it has strong potential as a hunting firearm. Where it truly excels is modularity. The market has driven manufacturers to flood it with endless new models and components in countless forms and price ranges, all highly cross-compatible. Think of it like a Lego kit: want a different stock, barrel, caliber, accessory, trigger—anything? No problem. And better yet, you don’t need a gunsmith! Practically everything can be swapped, modified, or even built from scratch, exactly to your needs. Some hunting rifles do offer user-changeable barrels or calibers, but these are usually proprietary and expensive. ARs require more time and tools for caliber conversion—but no other firearm offers this level of adaptability. You do, however, need to know what you're doing—and that’s what this article will teach you, focusing specifically on the trickiest part: swapping calibers.
Title photo: AR-15 barrels
“With ARs you can practically swap, customize, or even build everything from scratch—entirely DIY and tailored to your exact needs!”
Not a Single Turn Without Torque
The AR platform splits into AR-10 clones, which can be converted to virtually any cartridge that fits into the shortened bolt-action pattern for .308 Win. The smaller AR-15 platform can handle rounds up to .223 Rem in length—like .300 BLK. But there are far more options. For example, Faxon offers AR-15 barrels in twelve different chamberings. I’ll demonstrate a caliber swap using a Perun short sport/tactical rifle originally in .223 Rem, converting it into a suppressed hunting-capable special-purpose rifle in .300 BLK. However, the same procedure applies to almost any AR-15/10 rifle with MilSpec-compatible parts and a direct impingement gas system. If your rifle uses a piston-driven system, the assembly steps will vary slightly depending on the model—but the rest remains the same.
Some tools are mandatory – Delta Series Ultimate AR-15 Armorer’s Kit includes everything.You’ll need specialized tools for the swap. While improvisation is technically possible, it risks damaging components, and a poorly assembled rifle will be at best inaccurate—or at worst, may catastrophically fail and injure the shooter. So don’t take that risk. I use the “Delta Series Ultimate AR-15 Armorer’s Kit” from Wheeler Engineering. It’s a complete professional gunsmithing toolkit that lets you safely and efficiently disassemble and reassemble your AR down to the last screw—and handle nearly every conceivable upgrade or repair. From a technical and quality standpoint, I have no complaints and can recommend it. At 12,115 CZK, the price is fair considering the complexity, quality, and refinement—though still a notable investment. Wheeler also offers a more basic but still fully functional kit—the “Delta Series AR Armorer’s Professional Kit”—for 8,848 CZK. If you only need the most essential tool—a torque wrench—the XL F.A.T. Wrench used in this conversion is available separately for 4,956 CZK. Alternatively, the simpler and less versatile “Delta Series Combo Tool” sells for 1,798 CZK. You’ll also need a vise, a headspace gauge (see box), and if you don’t want to invest in a full kit, at least get a clamping fixture—it significantly simplifies the assembly process and protects components from damage. Several types exist—I used what’s known as an “action rod”.
Two types of upper jigs – bottom one is “action rod,” locks into barrel extension.Which Barrel to Choose
For the rebuild, I purchased a 254 mm (10") barrel for 8,100 CZK (from prodej-zbrani.cz), made by Aero Precision. While Faxon has a slightly better reputation, for my intended use Aero’s advantage of immediate availability was more than enough. You can order other brands too—check brownells.cz, where you'll also find complete “upper receiver” kits with bolt carrier group, barrel, and handguard ready for quick swap on an existing “lower receiver.” It's more expensive and selection is limited. When selecting a barrel, consider the optimal length for your cartridge—to achieve the desired velocity without excessive length that can hinder handling and increase weight, especially if you plan to use a suppressor. With modern manufacturing, length has less impact on accuracy and more on velocity. Many modern cartridges are optimized for short barrels—like 8.6 BLK and .338 ARC. In contrast, 6.5 Creedmoor needs at least 20" (508 mm) or more—otherwise, you lose significant energy. Also consider the barrel profile. A thin “pencil” barrel is the lightest but heats up quickly, which impacts group consistency and point of impact—though Faxon’s pencil barrels handle this well. Heavier profiles are more stable but add weight. A fluted heavy profile is a good compromise. For hunting under 300 m, I wouldn’t hesitate to use a Faxon pencil. For long-range, high-precision, or rapid-fire scenarios, go with a standard or heavy profile—preferably a target/heavy for AR-10 builds.
Check rifling twist rates before purchase. Usually, calibers come in one standard twist rate, but .300 BLK, for instance, can be found with 1:5", 1:7", or 1:8". Generally (though not always), faster twist rates stabilize longer, heavier bullets better. Research proven combos from experienced shooters. Now choose the barrel material. For “maintenance-free,” corrosion-resistant barrels, go stainless (usually 416-R). Otherwise, 4150 CMV is a time-tested classic.
To remove barrel/nut: use torque wrench, vise, and upper jig.
Aligning the gas block before tightening.Lapping
Lapping process of upper receiver face.
Other Components
The gas block directs expanding gases from the barrel port to the gas tube. Some Faxon barrels come with pre-installed gas blocks, but usually you’ll need to install it yourself. It’s easy—slide it over the port, align it, and torque the screws (25–30 fl/lb) onto the flat on the barrel. If no flat exists, align it visually. Some models also use a cross-pin, which requires a compatible barrel or additional drilling. Better to stick to set screws and a low-profile block that fits under the handguard. You can upgrade to an adjustable gas block for tuning (more on that later). Our barrel was .750", so we used a .750" gas block. Then choose a gas tube—available in various lengths (pistol, carbine, mid, rifle). Match the tube length with the barrel specification. If compatible, you can reuse the block and tube from your old barrel.
If switching between .223 Rem, .350 Legend, and .300 BLK, you won’t need to change anything else. But if your new cartridge doesn’t share the .378" case head (like .22 ARC, 6 mm ARC, .338 ARC, 6.5 Grendel [.441"], .458 SOCOM, .450 Bushmaster [.475"]), you’ll need to swap the bolt. Disassemble the BCG as for cleaning and swap bolts. Sometimes a special magazine is needed too—e.g., for .350 Legend or .338 ARC. On the AR-10 platform, most common calibers share bolt and magazine compatibility.
AR-15 set up for .300 Blackout hunting.
Let’s Do This!
First, ensure the rifle is unloaded. Push out the takedown pins, separate the upper receiver and remove the bolt carrier group. Next, remove the handguard—this will vary by model but is generally easy. Lay the upper on its side and support the gas block. Using a punch, knock out the gas tube retaining pin. Slide the tube out through the gas block and remove it forward. Clamp the rear of the action rod in a vise, slide the upper fully onto it, and rotate it with the handle to lock in place—this secures it to the barrel nut. Then use the wrench to loosen the barrel nut clockwise, remove it, unlock the jig, and pull out the old barrel. Now’s a good time to lap the receiver face (see box).
Insert the new barrel with pre-installed gas block and barrel nut into the upper. Align the indexing pin into the receiver slot, lock it into the jig. For better accuracy, some shooters use green Loctite, shims, or even perform a “Thermal Fit.” Some swear by it, others say it’s unnecessary—both have valid points. Personally, I believe with quality parts and proper torque, it's solid enough—but it’s good to know the alternatives.
Lightly coat the barrel extension and threads with grease. Torque the nut to 35–80 ft/lb—55 ft/lb is generally ideal. If using a GI-style nut (with gas tube notches), you may need to adjust torque to align the gas tube path. Insert the tube back through the receiver, seat it in the block, and drive in the roll pin. Reinstall the handguard, ensuring it doesn’t touch the gas block—this would affect accuracy. Insert the BCG, reassemble the upper and lower, push the pins in—and the barrel swap is done!
What Is Headspace
A critical step when installing a new barrel is checking the headspace. In short, it’s the distance between the bolt face and a specific point in the chamber when locked. Manufacturers aim to stay within tolerances and defects are rare, but issues can still occur—if so, the barrel should be returned. If the headspace is too small, the bolt won't lock. Too large, and the case won’t be properly supported—risking rupture. For your safety, use a headspace gauge set ("go" and "no-go").
Remove the bolt carrier group, strip the bolt of extractor and ejector, and ensure both chamber and bolt are clean. First, insert the “go” gauge and try to lock the bolt. It should lock with mild resistance. Next, use the “no-go” gauge—it should not lock. If it does, headspace is excessive.
You can improvise by trying to chamber a new, unfired cartridge. Then insert a paper shim between the primer and firing pin. If the bolt still locks, the headspace may be off. This method is not precise and not recommended—but still better than skipping the check entirely. Many shooters skip gauges due to price—but that’s risky.
Headspace gauge sets.
Different ARs → different nuts → different torque heads.
Tools for gas tube & block work.
Torque spec: 35–80 ft/lb, typically ~55 ft/lb.
Properly installed barrel with alignment pin – next step: barrel nut.Some Notes on Tuning
Your rifle may need tuning to run optimally after the change. Most issues relate to gas pressure—either it's overgassed (too much), or undergassed (too little). Watch the ejection angle. If brass ejects between 12–3 o’clock → overgassed; 4:30–6 o’clock → undergassed. Optimal is 3–4:30. You can tune this with either a heavier buffer or an adjustable gas block. Adjustable blocks are more expensive but highly useful—especially if you switch between suppressed and unsuppressed fire. Without tuning, gas blowback through the ejection port can be unpleasant.
Buffer tuning involves changing weight. Heavier buffers mitigate overgassing. Most carbine buffers contain three steel weights—you can swap them for tungsten or buy a heavier buffer (H1, H2, H3 types). You could write a book on tuning—this is just a starter. Beginners are best off with a gas block regulator—it makes the process much easier.
The “Delta Series Ultimate AR-15 Armorer’s Kit”, as well as other Wheeler Engineering products, can be purchased at the STROBL.cz s.r.o. store (see strobl.cz)
Photo source: Author’s archive
Author: Tomáš Prachař
Originally published in Lovec magazine by Extra Publishing