Converting .223 rifle to .300 Blackout in 2 Steps: Part 1

The .300 Blackout is an effective round that bridges some of the wide gap between a .223 and a .308 as well as allowing an AR15 best ar15platform rifle to encroach on the ballistics territory of the venerable AK 47.  Plus the 300 BLK has the benefit of easily going subsonic making it about as quiet as possible given the mechanical noise of operating a rifle’s action.  Adding to the quiet excitement is that the difference between a traditional AR15 in .223/5.56 and one in 300 BLK is little more than a barrel swap. That’s right, everything else might be interchangeable between the two.

By Doc Montana, a contributing author of Survival Cache and SHTFBlog

Chicken or Egg?

Wildcat cartridges can successfully address niche ammo needs, but unless the specific cartridge was blessed by Sammy 300 blackout conversion(properly SAAMI or Sporting Arms and Ammunition Manufacturers’ Institute), the cartridge would not get the widespread support needed to be taken seriously by the big gun and ammo manufacturers let alone the general shooting public.  AAC, or the Advanced Armament Corporation in collaboration with Remington Defense ironed out the kinks in the wildcat .300 Whisper cartridge getting formal SAAMI joy in 2011 which is why the .300 Blackout still has that new car smell.

The .300 Blackout is a 30 caliber solution that grew from a set of needs not the least of which included pure pitcher made in usa NSF shield english 99.99 400x250 USAthe use of existing AR-style magazines while maintaining the same mag capacity, the use of M4-style platform uppers and lowers; being ballistically similar to the AK 47 round of 7.62mm x 39mm, and be a higher-mass barrier-penetrating bullet while maintaining low recoil and high performance through short suppressed barrels. Oh, and best of all, easily running both supersonic and subsonic in the same rifle with absolutely no change in the gun. In fact, it is this latter capability that 300 BLK owners find most attractive. So the .300 Blackout can drop a deer at 200 yards, or lob 30-cal lead downrange with little more noise than a cycling bolt.

AK 47 rifles are near impossible to run subsonic due to the gas system. And they are certainly not able to interchange between supersonic and subsonic on the fly. Major adjustments and tuning would be needed. In the case of the .300 Blackout, it is a cartridge deliberately made to run flawlessly in an AR rifle in both subsonic and supersonic. In fact, the high bullet weight of the subsonic 300 BLK ammo is not just to slow down the bullet (F=MA in Newtonian physics) but also to provide enough of an equal and opposite force to cycle a traditional AR bolt and buffer (Newton’s Third Law of Motion).

While the initial ballistics of a 300 BLK running subsonic are very similar to a .45 ACP, the bullet shape of a .300 Blackout provides a much better trajectory and deeper penetration. A 220 grain 45 caliber slug flying out the pipe of a handgun designed prior to 1911 is much like a forty-five caliber musket ball. On the other hand the .300 Blackout behaves more like a 7.62×39 round causing death hundreds of yards away. A .45 ACP will bounce off cowhide at distance while the 300 BLK should still shatter bone.

Walmart Test

300 BLK ammo in the supersonic variety did pass my Walmart test.  That means it is sitting on the shelf at the local Walmart right switching from 556 to 300 blackoutnow. However, I was unable to locate any subsonic .300 Blackout ammo at the any nearby Walmarts.  Of course subsonic 300 BLK ammo was available at almost every gun store and big box sporting goods store I checked so the stuff is common.  And the Walmart gun clerk did say they’ve had 300 BLK subsonic ammo in stock before, but it was elusive as 500 round bricks of .22 long rifle.

Related: 10 Basic Tools For Your Armorer Kit

The ammo choices for 300 BLK in supersonic was varied across price and performance.  I found plenty of boxes of 20 from $16 all the way up to almost $50.  Subsonic rounds hovered around $20-$25 and there was rarely more than one choice at any given store.

Presto Change-o

Changing a .223 AR 15 into a .300 Blackout can be as simple as swapping barrels.  The complete upper, lower, magazines and gas 300 Blackout vs .223system might work just fine with the 300 BLK. Usually there are a couple other parts that get changed out as well, but truly in a nutshell, it is just a barrel switch.  So a best-case conversion to turn your .223 AR into a .300 Blackout is 1) remove your .223 barrel, and 2) install a 300 BLK barrel.

Tool Side

Changing barrels on your standard direct impingement AR is fairly straightforward, but does require some tools. The undeniable magpul armorers wrenchtool is a barrel wrench which is usually part of a multi-function armorers tool like the Magpul Armorer’s Wrench.  But in order to turn the barrel nut, you must remove the gas tube. And in order to remove the gas tube, you will need to remove the gas tube cross pin using a 5/32nds punch (gently push it out from left to right).

With the gas tube removed, you can unwind the barrel nut freeing the barrel from the upper receiver. You can reuse the gas tube if its in good shape and the right length, and maybe even reuse the gas block as well assuming it works with your barrel and handguard.  In my case, I opted for a new low profile gas block because I am going from a Magpul MOE polymer handguard mounted on a 5.56 barrel with an A2 (triangular) front post.  The Midwest Industries free-floating handguard I’ll be shrouding the 300 BLK barrel with will need a new gas block. So it was Yankee Hill to the rescue.

Also Read: How To Trick Out A Cheap AR15

Backing up for a minute, there is an essential tool that makes barrel removal and installation every so much easier and that is an converting a 556 rifleupper receiver vise block. The vise block is a blockish clamp that wraps the upper receiver like a glove allowing the whole unit to be clamped in a vise without concern of damaging or warping your upper receiver. Add a torque wrench to round out your toolset and you’re as good as done.

Grunt Work

The .300 Blackout went into military service in July of 2015 when the Netherland’s Dutch Convert 300 BLKMaritime Special Operations Force (NL-MARSOF) ordered 195 carbines chambered in 300 BLK.  According to an uncited Wikipedia article on the .300 Blackout, it has an effective supersonic combat range of about 500 yards. Flying subsonic, 200 yards is pushing the limits of effectiveness outside of threats made of paper.  Now before anyone goes all sniper on me, most folks, and let’s be honest here, are not able to shoot reliably to 500 yards even under ideal conditions. In fact, 200 yards is a very reasonable and ethical hunting distance. In my particular case, I intend on hunting with this rifle in thick woods where a 50 yard or less shot is common. I grew up hunting in such places with a Winchester Model 94 30-30 which is an excellent “brush gun” as we liked to call them. Iron sights were plenty good at these distances.

I also intend to hunt with a suppressor, or silencer if you want to retain the original name that its epic water filtersinventor Hiram Maxim called them back in 1902; the “Maxim Silencer” to be exact. On a side note, a movie in 1946 was made about Hiram’s life and titled “So Goes My Love.” But reading about the movie, it doesn’t sound like there is any gunplay in it, let alone any silenced fire.

Quiet Down

Factory loads of 300 BLK come in several popular bullet weights. In general, those bullets over 200 grains slide down the pipe How to convert to 300 Blackoutunder the 1100 feet per second speed of sound while anything lighter breaks the sound barrier with a boom. Since most of the powder is burned within the first nine inches of barrel, near total performance can be achieved in very short barrels. To avoid paperwork and a tax stamp and months of delay, I opted for a 16” barrel literally off the shelf at a local gun store.

I already had a SilencerCo Omega suppressor so adding a can to this build was a no-brainer. In fact, that Omega is most of the reason I went down the 300 BLK road in the first place. A suppressed subsonic .300 Blackout literally is only as loud as the bolt cycling and bullet impacting.

Related: Firearms Maintenance When SHTF

All this is not without a problem. And it’s potentially a big one. A .223 or 5.56 round will cycle into a 300 BLK barrel, and possibly 300 BLKthe reverse is true. This means you have to practice proper ammo management. At no time can you risk mixing up or mixing together your mags or your ammo.

There are various solutions and products to keep your ammo act together. The Blackout Band is a silicon bracelet you wrap around your 300 BLK mags. Some folks run different colored mags, while others mark their mags in personal ways. I chose to dedicate Magpul’s sand colored mags to my .300 Blackout with the intent to dye them later to a more fun and useful color. So at the moment, white mags for the Blackout. No exceptions. There is an ever growing number of tales where someone had a loose 300 BLK round that found it’s way into a .223 mag only to blow the gun apart when it was stripped off the top of the mag by the bolt and the trigger was pulled.

And as I noted in my review of the Magpul D-60 drum magazine, not all ammo containers for the .223/5.56 platform are completely interchangeable. In fact, some are downright dangerous. But since .300 Blackout ammo is easily twice to three times the price of .223 rounds at a minimum, getting sloppy with Blackout ammo shouldn’t be a popular problem.

Survival Apps

It should go without saying that a 30 caliber subsonic suppressed round with a 200 meter range should have endless uses. Hunting is obvious as is protection. But let’s put a finer point on that protection thing. A bolt cycling is noisy but only within a very limited sound radius. Add snow or thick brush or trees and the noise of a buffer spring boinging and bolt clanking will not travel far. And the thump of bullet impact is evidence that it’s too late to do anything about it.  Unfortunately, the 30 caliber bullet leaving the muzzle under the speed of sound drops like a mountain pass after a hundred yards, and like a double-black diamond ski slope at 200 yards. Beyond that it’s ballistics curve would be a boat anchor.

The Downside of Loud

And speaking of sound suppression, if you ever plan on popping off a round indoors, you will want to minimize the bang or risk convert from 5.56 to .300 BLKtemporary disorientation and permanent hearing damage. Sorry to be a buzzkill here, but I do have trouble taking seriously anyone who plans on using a short-barreled AR or AR pistol with spiked muzzle brake as a home defense weapon. One boom and it’s all over for most involved. Better get that first shot right because you will be too stunned for a follow up shot. Kind of like flash-banging yourself and loved ones.

Also Read: Review of the Glock 42

Just as a gas-powered generator makes an unwanted and unavoidable racket, and a campfire makes unwanted smoke and smell, firearms make unwanted noise. So having a silent solution with more umph than a pistol is a good thing. And when things do go noisy, you have a 500 yard solution at your index finger’s fingertip.

Given that the 300 BLK is still young enough to have spots it’s not surprising that reloading your own brand is the go-to option for the more-than-curious. There are limited factory ammo options for subsonic bullet designs often leaving the big game hunter to settle for either throwing projectiles faster than sound or launching barely hollow-pointed varmint rounds downrange to settle the score. But the big news here is that there is actually a selection of subsonic 300 BLK ammo on the shelves of the big boxes. So something’s going in the right direction these days.

Let’s see how this all works out. Stay tuned for Part 2.
All photos by Doc Montana

This article is for informational purposes only, please consult a gunsmith before you make any changes to your rifle.

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16 comments… add one
  • alfred uhrich September 5, 2016, 6:17 pm

    I would love to have a 300 blkt ( and maybe will have one in the near future ), but right now I don’t want to add one more to the mixture. And being on a fixed income doesn’t give me much leeway. But it does sound ( or the lack of sound ) is interesting.

    • Angelo February 19, 2017, 12:07 pm

      Yeah. I came here looking for info on the blackout because im building my first AR. I have since decided my first will be the 300. As i only will have the one for a while the expenditure for ammo will be the only drawback.

      • Robert Pinto September 13, 2018, 3:15 pm

        I am surprised usually the first go to caliber would be the 5.56 or the .223, due to the fact that the ammo is fairly cheap and plenty of it. I also am on a fixed income and would love to have a 300 Blackout for more stopping power against a big animal. right now I have settled for the 5.56 until I can put some funds together

  • Scott September 6, 2016, 9:23 am

    Building or purchasing an entire 300 Blackout upper allows a quick switch between that and the original 5.56 upper. This essentially gives you two rifles in one, the 300 for close up shots and the 5.56 for longer range shooting. I keep the 300 Blackout upper set up with a red dot site for home and personal defense. The 5.56 upper is set up with a 3-9x scope ready in case I need to switch out and make some longer distance shots.

  • Drew September 6, 2016, 9:42 am

    I’ve shot a buddy’s .300BLK supressed SBR with 200-grain gas-checked cast lead bullets – and the trajectory at 1100 fps makes a .45-70 look like a varmint rifle. The difference in POI between subsonic and supersonic ammo at distance is enough of a PITA for me that I probably would gear the .300BLK rifle for one load and leave it set up.

    I wonder how much of a horsepower difference there is between a .300BLK with a 190-200 grn bullet at 1100fps and a 9mm carbine with a 147 gran bullet at 1100fps? Enough to matter at under 100 yards?

  • Roger September 7, 2016, 9:02 pm

    Or, you could just get you a good semi- or bolt-action .308 rifle and not have to mess with barrel changes since it will do all that the .223/300 BLK combo can do and better! Having an ability to change calibers by changing barrels sounds good but 300 BLK is not a common caliber nor is it likely to be in the near future. If something breaks that is common to both calibers such as the trigger group, you have NO rifle! IMHO, two rifles in the same mid-range to high power caliber is a better choice, stuff happens, usually at a bad time! GLAHP!

    • jim- September 30, 2016, 5:35 pm

      If you put a 300 blk barrel on an ar15 everything is ar except the barrel. Same trigger group same upper same lower same bcg. So easy to fix anything on the black lego rifle.

  • alfred uhrich September 9, 2016, 10:49 pm

    Another thought comes to mind about the 300 bkout, seeing as how the 300 is a fairly new setup, what is or will be the availablity of more or extra ammo when and if the poop does hit the fan and who will have it? this is another reason that I myself would stay with the available ammo 223, 308, or 30-06 that has been around a lot longer. But then again, each to their own. Just saying.

    • Doc Montana September 9, 2016, 10:58 pm

      Good questions Alfred. One thing to keep in mind is that while 300 BLK ammo is considerably less abundant compared to many other common calibers, it can be manufactured using .223 brass and standard 30 caliber bullets (but remember that 7.63×39 AK bullets are not 30 cal).

      Those so inclined have built assembly line jigs in their garage using wood, then using basic shop tools to cut the brass to the right size. Then use a reloading press to reform and enlarge the neck followed by a touch of reaming and polishing. The rest is standard reloading. In fact, many .300 Blackout aficionados request used .223 and 5.56 brass from their AR 15 friends so they make more 300 BLK which is much more cost effective than buying it by the box.

  • google September 13, 2016, 8:38 pm

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  • jim- February 20, 2017, 9:27 pm

    The best way to correct the availability of ammo is to load your own. 300blk is a re-loaders dream, you can start with 223/5.56 brass and cut it down and reform it to speck. Any weight projectile from 110gr to 220gr (have seen some 230gr cast) Powder and primers per loading manual. Sub or supers, Just takes time and money. After all it is a hobby for most of us.

  • Thomas Gray April 22, 2017, 3:04 pm

    Just got my 300 upper, and bolt separately, saved $300, any way, here is what happen, now with reloads, so far, not factory yet, ejection great , but not picking up new bullet in cycling, triggers resets and all, but not picking up bullet, now few things i am going to try are different mag. One i used was given to me, used it because of size, using lead sled, different powder in loads/and or factories, you can see where bolt slides across brass, I was told to try lighter buffer,

    • jim- April 22, 2017, 4:11 pm

      Thomas: You need to run some rounds thru it to break it in and use plenty of lube on the bcg.

      • jim- April 22, 2017, 4:15 pm

        Also, check out 300blktalk dot com

  • Thomas Heilbrun August 18, 2018, 10:34 pm

    I have an Ruger 556 that I would like to convert to a Blackout. Can you use any 16 inch barrel to do this? Reason I ask is that the prices on line are as little as $ 79 and the Ruger website has a kit that cost nearly as much as the as entire gun. ($ 400+) Anyone know what it would cost for a gunsmith do this for you?

    • Sam b May 28, 2019, 7:32 am

      I also have the same gun and am doing the same thing. From what I’m seeing and what I was told, the only major change is the barrel. The bolt, bolt carrier group, and mags are all the same. I’ve heard some say they had to make some albeit minor changes just for cycling but besides that not much changes. This is entirely what I’ve heard and not what I have done. Like I said I’m just doing this myself for the first time, was always more of an Ak guy (not knocking the Ar) hence the want for a little more “oompf” out of my Ar.


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