Switching Calibers in the AR-15

The AR-15 is the Lego of the firearm world. In its original 20″ barrel configuration, the AR-15 was a handy tool for target shooting, predator hunting, small game, and plinking. Shortening the barrel to 16″ created a carbine version that made it more user friendly in tight spaces and reduced weight without sacrificing too much performance. We gun owners had to deal with the threat of losing our beloved AR’s to liberal politicians bent on gutting the 2A and we acquired as many as we could afford. Sometimes these were complete firearms, but many of us stocked up on stripped lowers and parts so we could have a supply for generations to come.
Caliber Change Beginning 16 inch and stripped lowers
For the next four years at least, the threat has been diminished and now we have multiple lowers or complete guns all in the same configuration sitting in our safes and storage cabinets. Nearly all of them are chambered in 5.56mm/.223 Remington, but it doesn’t have to be that way. Since variety is the spice of life, let’s take a look at quick caliber conversions that you can do at home and increase the fun or performance factors. Here’s quick reference guide to the most common and popular caliber conversions.

.22 Long Rifle Conversions

The easiest caliber conversion is to the ever popular .22 Long Rifle. Ammunition is now becoming more available is some areas or online at reasonable prices. Gone are the days of paying close to $0.15 per round and now burning through a brick of rounds won’t break the bank. There are two main kinds of .22LR conversions: The Atchisson-style conversion kit and dedicated upper receivers.

Atchisson-style Conversion Kit

Comparison of conversion bolts

The normal .223 Rem bolt at top and the CMMG .22lr Conversion bolt below.


This style of conversion kit was invented by Max Atchisson decades ago and are currently produced by a number of different companies. It’s the easiest and cheapest way to convert your AR-15 to .22LR. High quality versions are manufactured by CMMG Inc. The unit replaces the standard 5.56mm bolt, and has a cartridge shaped extension that fits the chamber. It only takes 30 seconds or less to install. Clear the firearm and insure that it is unloaded. With the bolt forward, slide the rear takedown pin to the side, open the receivers, pull the charging handle and bolt out of the firearm. Replace the bolt with the conversion kit along with the charging handle, close the receiver, push the takedown pin back into position and the firearm is ready to use with .22LR AR-15 magazines.

Dedicated Upper Receivers

Dedicated .22LR uppers are also available that give you better accuracy and reliability. The standard twist rate is not ideal for .22LR, and near match grade quality can be achieved using a dedicated .22LR upper receiver. Again, the process of converting is easy. In this case, simply remove the entire 5.56mm upper receiver and replace with the dedicated upper receiver of your choice. The Atchisson-style magazines can be used with most dedicated uppers.

A few companies offer .22LR designed to function specifically in conversion kits for the utmost in reliability such as Winchester M-22 and CCI Tactical.

AR Cartridges

By swapping your upper you can switch your AR between .223 Remington, Subsonic .300 BLK, Supersonic.300 Blackout, Subsonic 9mm, Supersonic 9mm, and even .22 Long Rifle.

9mm Conversions

Pioneered in 1982 by Colt, the 9mm M16 is a simple blowback design that uses either a permanent or removable magazine block inside the standard AR-15 magwell, replacement bolt and 9mm barrel. Magazines are straight and hold up to 32 rounds. 50 and 100 round drum magazines are also available.

From long 20" barrels, to carbines, to SBRs and pistols, the AR offers you lots of options.

From long 20″ barrels, to carbines, to SBRs and pistols, the AR offers you lots of options.

9mm AR’s are a blast to shoot and saves money on ammunition. While recoil is about the same as a 5.56mm, it’s with the muzzle blast that many feel is uncomfortable indoors. They also are a favorite with suppressor users when used with heavy subsonic 9mm ammunition. Dedicated uppers simply replace your current upper. The heavy 9mm bolt replaces the standard two-piece 5.56mm bolt. It uses the same charging handle and buffer, but many users may benefit from a heavy buffer that holds the bolt closed a bit longer for better performance with a suppressor.

Since we’re discussion conversion, we’ll skip the Colt-style magazine block that requires drilling holes in you lower receiver. Removable magazines adapters are simple to install. They either drop in from the top or inserted into the lower receiver’s magwell. This Hahn Precision bottom loading magazine adapter is put into the lower and is held in place by a roller cam behind a hex screw. Top loaded magazine inserts require the removal of the bolt release to install. While not hard, it does add to the time and tools necessary to install.

Building a 9mm upper receiver is very simple since there isn’t a gas system to worry about installing. 9mm AR’s use the standard upper receivers, and most builders use upper receivers without the forward assist since there isn’t the necessary notches in the 9mm bolt for its use. Install the barrel into the stripped receiver, torque the barrel nut and finish with the handguard/rail system of your choice. (The same procedure works for building .22 Long Rifle uppers)

9mm AR’s really shine as pistols or SBR’s.

.300 Blackout

multiple calibers

With so many options, why limit yourself to just .223/5.56?


The .300 Blackout gives .30 caliber performance similar to the Russian 7.62x39mm, but feeds better than the Russian cartridge in the AR-15 due to the brass design. There’s two completely different thoughts on the .300 Blackout; heavy sub-sonic round for use with suppressors, or supersonic hunting ammunition. Heavy projectiles can exceed 200 grains, much more than the standard 55-62 grain 5.56mm ammunition. Supersonic ammo is usually in the 115-125 range, with high performance special use on either side of the spectrum.

The beauty of this round is that it requires only a barrel change. The .300 Blackout uses the same magazine, upper receiver, bolt, and buffer system of the standard .223/5.56mm version. Many 9mm suppressors can be used with sub-sonic ammunition, but check with the manufacturer before using your pistol can on your rifle. Supersonic ammunition will destroy a 9mm suppressor.

The easy conversion is with a dedicated upper that is already built using a .300 Blackout barrel. You can use the bolt, charging handle, and magazines from your standard AR-15. Building your own upper requires installing the gas system and handguard. The .300 Blackout was designed for use with short barrels around 9 inches, so you’ll find a number of pistol or short barrel rifle (SBR) versions available.

Does this have you thinking about switching calibers in your AR-15? What caliber would you switch to? Let us know in the comments below!

10 Comments On This Article

  1. 6.8 SPC, is one of my favorite rifles in the AR platform I own. Shoots very well and accurate with similar performance to a .308 but on a .223/5.56 lower. All though I think my next build will be the 300 blackout. Thanks for article, good info. Now I need to scrape up some extra money for it.

  2. All together now, 6.5 Creedmore. I was under the impression that a 6.5 Creedmore upper would fit a 5.56mm receiver. Am I wrong?

  3. I like the 7.62X39. I built one on a DPMS upper with a 16″ ER Shaw barrel. Lower receiver is Anderson Mfg with Geiselle SD3G trigger. Originally with DI which worked perfectly! But, I changed out DI to Osprey gas piston. Trouble there with extraction and ejection. After fiddling with the feed ramps I destroyed my upper then replaced with an Aero Precision M4. That helped but not perfect. Next, opened gas port on the barrel to 1/8″. No help. Last, changed out the buffer with a shorty still with carbine length spring. For good measure, bought C-Products mags. Now, finally, I have a great shooting AR. I do not necessarily recommend this round for your AR but I love the way it works when configured correctly. The shape of the case is primarily the problem but gassing is an issue too. In any event, the AR 7.62X39 is comparable to the AK but definitley is an AR. The trigger helps a bunch. My favorite (not a leggo) is a 7.62X51 I built which had similar build issues but I profited by building the 39 first. Long ago I had a ’57 chevy that could be modified any way you can imagine. The AR is the same idea and almost as much fun.

  4. I got a Mag tactical multimag for cheap from a family member. Its currently set up for 9mm. 1st thing off can i use a 50 to 100 drum on this rifle second is i wohld like to be able to change back to a .223 at some point. A little advice would be much appreciated.

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