Practical Personal Observations On The AR Platform .300 Blackout Truck Gun (update 5-28 with photos and minor revisions)

A personal tale about a Rat Rifle.

Are you running an AR15 platform gas impingement .300BLK?

Took me awhile to decide if I wanted to build me a 300BLK. Firstly I didn’t know what to make of the round. Kind of a caught in limbo round. Was it good for a suppressed platform, as designed, with short barreled handy little rifles? A combat grade rifle? A 30-30 with better exterior ballistics in a modern semi auto platform? A secret Squirrel ninja gun? A fad? Seems like a definite niche round? What is it?

The exterior ballistics on paper didn’t provide characteristics much different from .556×45. There wasn’t a lot of manufactured ammo choices in the beginning. I really didn’t know what to make of it, so I back burnered it in leu of other 80% builds I felt where more important to my ideal arsenal of practical combat/hunting rifles. Besides, it would be another added cost in regards to ammo. Trying to make every weapon/gear penny stretch as prudently as possible without having to sacrifice proven combat grade equipment quality and reliability.

As time went on I acquired a pretty good box of singular .mil spec AR bits, enough to consider building a Rat Rifle out of the calico collection of AR parts. Then one day I was on Green Mountain Rifle Barrel‘s site ordering some of their excellent unturned gunsmith barrel blanks, I noticed they had added a .300 BLK AR Chambered barrel blank. At, get this, $95 Bucks! It’s their super rugged high survivability 4140 high Vanadium alloy, 1-7 twist rate, mil spec chrome lined, fully chambered & head spaced, with phosphate bolt extension attached. You only have to turn your diameters, crown, thread the muzzle, drill the gas port, blue or parkerized it, and YAD: You Are Done.  Woo Hoo! I snatched one of those babies up on the spot. Like everything Ive bought from GMRB, comes wrapped and shipped like excellent old timey machine tools where shipped, on an oil skin paper wrap, inside a heat sealed bag, inside paper padding, inside a wicked rugged long narrow hvy duty box. Its like christmas to me when those guys ship me an order. Every barrel blank I have turned of theirs has turned out to be insanely accurate. Stupid crazy accurate. Like you can’t shoot good enough for how accurate kind of accurate. I don’t really know why, but I know I’m machining Thompson Center Contender 16 inch carbine barrels in .22 RF, .22mag, and .22 hornet, that shoot on a scale of accuracy I have never experienced. I’m no great shot, just a life time long good woods snap shooter.

Anyways, the barrel blank gets here in two days, (did I say the boys at GMRB are awesome?), I take it out to fondle and drool over it, get a feel for its weight and balance in a stripped upper/lower/stock-buffer mock up, and it hits me. This is going to be the rifle I replace my trusty Ruger .44 carbine I bought in highschool I’ve hunted with exclusively with for 42 years,

(The shell lifter assembly gave up the ghost 2 years ago on my little sweetheart Ruger .44 mag deer killer. I’m in the process of machining a new 16 inch barrel from a GMRB blank, and converting from the factory tube fed magazine, to a Desert Eagle pistol magazine. Whole other story right there. Later on that).

I had a mil spec 80% lower and A3 flat top upper, a phosphate .mil spec M16 bolt carrier group, an A2 full rifle stock with buffer, spring and tube assembly, from the box of extra AR bits & pieces. All Mil Spec parts, (I believe it is the only way for a guy like me to build his own combat grade/reliable infantry rifle. I’m really into that after taking small unit infantry training at Max Velocity’s school on Romney WV. All those fancy baubles and trick parts, they shit the bed right when you need them most. No thanks!),  it felt pretty good even in its straight unturned contour condition. It has a .300″ hole down the middle, thats a lot of ounces less than a .224″ hole down the middle. The way it feels rough cut in the band saw at 16.5″, I decided to run as short & close to the 16 inch BATF rule barrel and stay out of prison, because every gram out on the end you shave, it really makes a diff in handling.

Have a bunch of photo’s for this post, later on to add. For now its straight text.

Oh yeah, with no standard dimension for the gas port due to all the ways these .300BLK’s get run, I asked the boys at Green Mountain RB what they suggest for gas port size with the set up I’ve machined, they said start with a .110 diameter hole. Start there, if you need more gas to run the bolt go up a number drill size at a time till it runs reliably. So thats what I did, and it worked slick as can be. A .110 diameter gas port, and all mil spec parts, A2 stock/buffer/M16 bolt/upper, running Hornady 110 grain Black Ammo, shoots like a dream.  Perfect. No problems whats ever. It even cycles using Hornady .208 Black Ammo subsonic too much to my happiness. It just cycles completely on the .208 subs to my complete amazement. These are the only two rounds I am planning to run. I’m “keeping it simple stupid.”

On the Hornady box it gives trajectory specs, .110 grain if sighted 3.6 inches high at 100 yards you get a perfect 200 yard zero. With only iron sights this is how I’m sighted in.

I’m keeping this little rifle clean no fancy stuff. A sling, ten round PMags, beautiful back up irons, Pmag frond, Moe grip and trigger guard. Its my tractor/truck rifle. Goes everywhere. Its an on purpose rifle, my minute man rifle. To be used hard. Got it running last fall, first try took a doe at 235 paces, thru & thru shoulder shot, slated her down, she licked the wound and expired before I hoofed it to her. Tiny hole in, 6 inch out. Bagged a buck on my second ticket at 110 steps. Right thru the ticker, he jumped straight up 3 foot off the berm of my pond, and collapsed on the spot. Shot 5 wood chucks  out on my truck garden so far, dead nuts on at 100 – 200 yes, just drop down 3 inches at 100 yards, just like Hornady states on the spec tag comes on the box, dead nuts zeroed.













Those are real nice ballistics for a brush and small field hunting carbine using Troy’s fine gauge quality combat iron sights. In a rugged combat grade package.

Man am I ever happy made this little honey. It is so much quicker handling than the same set up in .556 16 inch bbl with A2 .mil flash suppressor. A world of difference. I threaded the muzzle for a can if I ever get the bucks to spring for one. Instead of a flash hider or brake, I machined up a thread protector that gets out far enough to protect the 11 degree muzzle crown. Thats it. Plain & simple. I might get the Trijicon BAC Green triangle Tritium powered 1-4 scope, with a quick release monolithic scope mount. Its a beauty of a scope. Shoot both eyes open and you get some mag power for reaching out accurately in a hurry in all light and dark. But for now, KISS, keep it simple stupid.

Here’s a couple pics after I completed the build. There’s a simple 2 point combat sling on it, and its acquired a slew of scratches and dings from hanging from the tractors roll bar and riding in the truck or at the ready on my back and a myriad of places leaning up on wire fence, posts, trees, or on the ground. Not a single problem has happened. Total Zero Fail so far after a fall, winter, spring, and now summers use, pretty close to everyday. I’m always shouldering the carbine and acquiring a sight picture on every sort of target. My premise here is to make it a natural. and everyday practical extension of my body.

I like the set up so much I ordered another GMRB chambered blank in 5.56, to duplicate the 300BLK by converting an existing AR over. Have a cash rathole building up to get Trijicon’s Accupoint 1-4 powered Fiberoptic/Tritium powered BAC green triangle reticle scope. Works like a red dot at 1 power and a medium range both eyes open scope at higher mag. Love that great green triangle, along with the superb glass, it appears like a hologram on your target. Uncanny effect.


What the 80% lower looks like milled & drilled.



Left side: A2 rifle stock tube buffer & spring, ten round Pmag, Troy iron sites. Moe grip and forend, enhanced combat trigger, Green Mountain Rifle Barrel Chambered Blank turned to 16+ inches. It gets a rattle can woodland camo paint job next.


Right side: Its said less is more. Keep It Simple Stupid. A basic, mostly Mil Spec tuff rugged carbine, thats practical, no worry or fuss. Keep it cleaned & oiled, don’t need to be concerned with it banging around. Hard to say exactly what the parts cost, maybe around $350-$400. And thats a bargain in my book for a combat tough effective weapon. Its all “analog”. No batteries. No expensive optics to get banged up or fail. I can carry the essential bolt survival and spring/pin kit in the moe grip. The butt stock has that excellent trap door to store lube, a bore snake, an extra small LED flash, knife, and tritium wrist compass, a Sawyer water filter, fire starter and Bic lighter, length of Para-cord, and chest seal with an extra SOFTT tourniquet. Going to try one of those nylon stock mag holders, if it holds 2 ten round Pmags and doesn’t detract from the quickness of handling, that should complete the set up.

I could not be more delighted so far, and satisfied with my choices. Everything worked out swimmingly. And that GMRB barrel. Telling you, those boys up in CowHampshire, where I’m originally from, they make quality reasonably priced barrel blanks.

With iron sights, prone in combat position, quick as I can put the front pin on the target & squeeze I average 2-3″ with ten round Pmags I have for this rifle. When I ranged in the sights to zero, careful precision slow fire shots in combat prone I was getting a bit over 1 inch consistently on white painted steel with a 3 inch black center.

Thats it if you have considered building your own 80% 300BLK rifle. Its said less is more, simple sometimes is best. Its true. This is a keeper for me. I’ll never sell it off for parts or give it away. Its one of those things. Its just right. It points like a dream. Close to zero recoil, stupid fast re-aquire of your sight picture. And quiet. So far every game shot has been without earplugs, and my 10/22 on standard velocity target ammo seems louder than the Hornady Black .110 ammo.

I highly recommend the carbine configuration with the A2 rifle stock. It produces a perfect weight bias muzzle to butt. That, I think why there’s barely recoil, and it points lightning fast. Plus the blistering quick sight pic acquisition. The rifle buffer creates a looong slooow push. There’s no spring/buffer twang, the longer return stroke strips those rounds out of the magazine like Bam Bam wants his dinner. The buffer gets quite the head of steam on the return stroke. Own yeah, I’m running a regular rifle weight buffer and spring. Might get a small rugged LED flashlight for it. I’m liking the Fenix rigs that run one AA or two 123 batteries. They have a nifty quick detach clamp and tape switches for reasonable money also.

And I’m not bullshitting about this sweet being a tractor/truck gun. Its already got scrapes, scratches, dents and wear rubs all over. The sling is for hanging it from the ROPER cage on the tractors for quick access, so it will swing and bang over the bumps, it goes muzzle down on the tranny hump in the pick-up, same for when running the side by side. Lot of banging and sliding around. I’m not abusive to it, but it ain’t no safe queen either.

If you got any question. I’d be honored to answer best as possible.



One thought on “Practical Personal Observations On The AR Platform .300 Blackout Truck Gun (update 5-28 with photos and minor revisions)

  1. Pingback: Practical Personal Observations On The AR Platform .300 Blackout Truck Gun (update 5-28 with photos and minor revisions) – Deft Systems, LLC

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s