Building a 300 BLK Short Barreled Rifle (SBR)

I was about to be a first time dad…and in some strange way, the 300 AAC Blackout arrived at a perfect time for me. 

I had decided to build a compact, suppressed short barrelled rifle (SBR) for home defense…but I also wanted something that could reach out if things ever got super, super weird in the neighborhood. 

I might have been having a male version of “nesting”…but with obsessions about protecting my family from harm vs. cozy baby bedroom conditions.

This urge to protect was also due to the fact there was ongoing large-scale civil unrest happening in my very immediate area.  Several tense nights spent listening for updates on a police scanner app had me thinking about upgrading my home defense battery.

So, what exactly was my “need” for the build? 

Suburban defense…5 to 50 yards, I figured.

Yes…it’s goofy to admit now, but I had visions of Red Dawn/Invasion USA scenarios in the suburbs.  Paranoid?  Perhaps. Hindsight is 20/20 but at the time, rumors of what might be coming our way were rampant.

I was not completely unprepared by any stretch.  had my trusty Colt AR15A2 HBAR topped off with 62gr. green tips close by…but with it’s 20” barrel, I concluded that I wanted something more discreet that I could tuck in an bag or under a jacket that could be employed quickly, and be devastating in close…but also reach out to 150 yards if needed. 

This tennis racket case probably tipped off “gun” more than an actual case would have…

Small was key, though. I also wanted something small I could walk out to the car every morning without alarming the neighbors…something that could also easily sit in the passenger side floorboard during my morning drive through areas that had seen unrest the night prior. 

Finally, I wanted something quiet. 

If marauding bands of Mad Max wanna-bes in pickups flinging Moltovs were rolling into my subdivision…they were not going to pinpoint the source of incoming if I had any say about it. 

Again, this was an odd time. Looking back, what I probably needed was a nice, simple AR pistol…a vacation…or a Valium.

Frankly, visions of a suburban nightmare could have also just been my mind rationalizing my deepest desire to spend a lot of money on something that looked really, really cool. 

Let’s be very honest…You’re hard pressed to find much that looks cooler than a short barreled rifle (SBR). 

Have a look at this rather rare KAC-635 for example.  Seriously…look at that thing!

More on this later….

What to Build?

My initial thought was to build a 10.3” MK18 clone.  Maybe even just an off the shelf Daniel Defense version.  Seemed practical…and very, very proven.

Example of a Costa Upper.
A fairly accurate example of the 11.5″ “Costa Upper” made popular in the Magpul “Art of the Tactical Carbine” video.

Well, that plan soon evolved into an 11.5” 5.56 gun with a KAC Carbine length URXII rail.  Chris Costa’s cool looking AR in his Magpul video really did a number on me…as did it to others, with it now being sometimes referred to simply as the “Costa Upper.”

Now, I’m not here to bash 5.56.  But the more I read about its performance from short barrels, the more I started leaning 12.5”.   Basically, the 5.56 round was causing me confusion and angst.

Just as I was about to give in, stop worrying, and learn to love the 5.56, and “settle” on a 10” Mk18 clone (in the end, size did matter) I was tipped off to a website called, a spiritual cousin of, which has long been one of my favorite websites.

It didn’t take long to realize just how special this new 300 AAC Blackout round was, especially for the exact purpose I felt I required.  

To counteract any second-guessing and force my own hand, I immediately ordered a very large quantity of what was for a short time, the ammo of choice…125gr. Atlanta Arms Pink-tips. 

Atlanta Arms 125gr Pink Tips.  There was a moment in time where this was the go-to round for hunting and self-defense.

With these in hand, there was no going back.  I’m not going to return ammo…I would be forced to build the gun.

I then doubled down even further, ordering several cases of the since discontinued Remington 110 gr. UMC.

Around this time, my brother, also bitten by the Blackout bug, began reloading.  I tripled down…lots of time spent procuring cut down 5.56 cases, powder, and thousands of 147 gr. Surplus .308 heads commenced…as did my build.

So, what did I build? 

Being both potentially paranoid and very unoriginal, I decided to do an almost exact knock-off of that SR-635 above, but chambered in 300BLK…which pictured was chambered in 6 x 35mm.

Final product. 
  • I went with an off the shelf lower from Knight’s Armament featuring their ambi controls, their 2-stage trigger and the SOPMOD stock.
  • I went with a Noveske upper receiver paired with of their original 8” barrel offerings. (They have since changed the twist)
  • For a bolt carrier, I used an old Daniel Defense, pulled from a DDM4.
  • I topped it off a Knight’s URX II rail, an Aimpoint T1 and a Rugged Razor suppressor.

A Fast Favorite

The end result has been one of the most fun shooting and reliable firearms I have ever owned.  It has become my go-to firearm.  As my first SBR, it will always hold a special place in my heart.

Since I built it, 300BLK has taken off, and more and more manufacturers are offering their own take on the 300BLK SBR upper.

At times, I’ve even been tempted to flip that original offering for something a bit newer and lighter, but I just don’t have the heart to get rid of it.  It’s here to stay.

The same can be said for the caliber itself.  With news that elements of our military have adopted the round, it really does seem like it has officially made it.  As such, I can safely say that a 300 Blackout SBR build most certainly belongs on your short list, should your state laws allow it…Even if you don’t have overblown fears of a suburban apocalypse.

A Subsonic Footnote

Arthor’s defensive round of choice…the Barnes VOR-TX Ammunition 300 AAC Blackout 110 Grain TAC-TX

Finally…A lot has been made of how it’s a great rifle to use suppressed, and I do run a suppressor, but I’ve been having a tremendous amount of fun with it without even running subsonic. Any buzz you hear that this is just a caliber that’s useless if its not suppressed is just woefully inaccurate.

I’ll run a few of my brother’s hand loaded 208’s or 220’s through it every now and again for the ‘wow’ effect, but otherwise, I keep it almost strictly supersonic…running my hand loaded 147gr. or my 110gr. defensive loads.

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