Sunday, June 9, 2013

Making a .300 Blackout AR.........from a shovel?

Just kidding. No shovels needed.*

This post is one of a series I will be writing about the AR pattern rifle I am building in .300BLK

For starters, some background.....

The .300 Blackout, also referred to as the .300 blk, or just blk, is the brainchild of the Advanced Armament Corporation, or AAC. It is a .30 caliber cartridge that uses the 5.56x45mm parent case, and is trimmed down then necked out for a mid-weight .30 cal bullet. It allows anyone who has a AR15 to shoot an effective, accurate, powerful round using all the same equipment as the standard 5.56x45, except for the barrel. It can be loaded to subsonic levels, for very effective use with a suppressor, or to hypersonic levels as with any  normal rifle cartridge. It has taken the AR world by storm, and for very good reason.

(I know I'm not giving all of the little details on the round, but that's what the wiki link is there for.)

So, since I have mentioned on this blog before, I have machined several AR lower forgings (I've even welcomed fellow gunbloggers into my garage to machine their own and now need to build them into functional rifles. I've got plans for each of them, from different pistol calibers to standard 5.56, all the way up to a belt fed. One of them, however, will be in .300 blk, because I don't already have an AR in .300blk and everyone needs at least one AR in every caliber, right?

Since the everything about the .300 blk interchanges with the standard 5.56 AR, I've got a pile of parts here including bolt carrier groups, magazines, upper receivers, and furniture. The one thing I don't have is an overpriced .300blk barrel, and no matter where I looked, I couldn't find a barrel for less than a couple hundred bucks. My solution was simple; make my own.

I went over to the good folks at Green Mountain Barrel** and saw that, at the time, they had some 17" long .30 cal 1:10 twist barrel blanks for about 30 bucks apiece. I called some of my buddies up and got a group order going, then we all went in on a .300blk finish reamer and headspace gauges. That way, we all got a .300 blk barrel out of it for about 75 bucks each.

A barrel blank is just that, a rifled piece of alloy steel that has no chamber cut, nor a profile. Here is a pic of the barrel blank, sitting atop my table saw.

There is no chamber cut, it is just a rifled tube.

Since we are going to put this on an AR, the barrel must be turned for a barrel extension, which is what the locking lugs on the bolt engage with when the rifle is fired.
First, the barrel was turned between centers such that the entire outside diameter was parallel to the bore of the barrel, the ends were cut square and adjacent to the bore, and a .630" long section was machined down to .825" OD

Next, that same area is threaded 16tpi to match the barrel extension.

Next, we begin the task of chambering and headspacing the barrel. The barrel extension will be screwed on and off several times so we can check headspace using a GO gauge and the AR bolt.
Not shown in these pics is the floating reamer holder.

I got done chambering and headspacing, and was turning the rest of the barrel down to profile it when a very thick and razor sharp metal shaving caught me on my thumb, and cut my thumb from base to tip, all the way to the bone. Several stitches later, I've finally stopped spreading my DNA around, but this project is on hold until I can finish healing.

There's more yet to come on this installment. More as I get to it.

*No shovels were harmed in the making of this rifle.

**I've had very, very good results from GMB barrel blanks, I chambered and profiled a 20" .30 cal blank I purchased from them, for a Remington 700 in .300 WinMag.
The barrel worked out nicely, with several < 1" groups right off the bat. More than accurate enough to start with, and the groups get better as the barrel is broken in.


  1. Oh my feakin OUCH!

    Man, I've been cut by chips and swarf before, and it's NO fun.

    Keep it sure don't want to get an infection like I had back in March/April.

    BTW...what do you think about the 300 Whisper? "American Rifleman" had a comparison a couple of months ago, but since I don't know anybody who's shot either one, I'm just curious what "the guys who know this stuff" think about it.

    Me? I'm in the Stone Age with a Garand in 30-06!

    1. Yeah, I would never characterize this as *fun*. =)

      .300 Whisper was originally a necked-up .221 Fireball case, but a lot of folks are using the 5.56 case.
      It's a wildcat cartridge, and is almost the same specs as the .300 AAC Blackout. Performance from both rounds is similar, and I believe a chamber cut for the blk will feed the .300 whisper,but not vice versa, similar to the 5.56x45 vs. the .223

      I read somewhere that the main reason the .300 whisper never took off like the .300 AAC Blackout is because JDJ, the inventor of the whisper, wanted to keep the cartridge "proprietary". I don't know how true that is, I read it somewhere on the internet.

      As I've said before, I'm no expert, just a guy on the internet with an opinion and a coffee addiction. Nothing said here should be misconstrued as actual information or advice. =)

  2. So is it a modern M-1 carbine or a modern 7.62x39?

    1. I'd say it's better than either one.
      I've read reports wherein people have loaded it up to *almost* 7.62x51 ballistics in a bolt gun, and people have also loaded it down to subsonic levels with good results.

      I'm no expert on the round, so please feel free to google the round and find some hard numbers.

    2. Ditto. For my use, the 300 Blackout was a souped up 30 M1 Carbine. I use 110 grain FMJ-RN bullets intended for 30 M1 Carbine use. They feed through my AR-15 just fine. Even function correctly using powder charges that mirror the 30 M1 Carbine.

      The advantage is the higher pressure limit and longer O.A.L. gives you the freedom to drive the bullet faster or substitute heavier spitzers. Even cheap 150 grain hunting bullets (like Remington Core-Lok) work well.

      I was skeptical to start with, but now I'm sold. I like the round enough to make a Remage barrel in this chamber for my .223 Rem bolt rifle.

  3. I'm having lathe envy. Looks like it must have a bore greater than 1" with the end of the barrel sticking out of the chuck like that. 1 1/4"? How big is the lathe?

    I've actually been thinking about making one of these, too, but probably buying a finished upper. Definitely the barrel. No tooling to do what you're doing.

    My practical side is telling me I already have .308 and .30-06, so how different can .300 BLK be?

    1. That's a 13x40 lathe. It has a 1.5" bore through the head stock.

      The big thing about the blk cartridge is that it gives me a viable 30 cal cartridge for hunting, using a very lightweight and accurate AR15 pattern rifle.

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