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Ultimate Spare Parts AR-9 [Pandemic Build]

What do you do in a pandemic? Build a 9mm training rifle from spare parts, of course! Come see what I pieced together to build-out my AR-9

Like many of you, the PPT team has been stuck at home, with shooting ranges closed to the public due to COVID-19.

And like many of you, that means only one thing…

Digging through the spare parts box to see what new rifle we can make!

AR loot box
Literally how it feels to go diving in your spare parts box

For me, spare parts boxes plus a 9mm BCG from Cryptic Coating meant I was just a barrel and an upper away from a 9mm Pistol Caliber Carbine.

While this build didn’t turn out exactly as I expected, it does serve to illustrate some important points, highlights some outstanding gear, and should serve as a reminder to us all that none of us are infallible.

Table of Contents


Build Goals

A good PCC has been on my list for a while, specifically one built using a normal AR-15 lower and MeanArms EndoMags to turn my 5.56 lower into a 9mm PCC.

Pandemic AR-9 rear look

For many, EndoMags are simply the cheapest method of getting an AR-9 — but to me, they serve another purpose also, at least I hoped they would. But I would have to build it to find out.

I’m a big fan of training rifles, especially sub-caliber training rifles. For long range that means using a .22 LR training rifle. But I’ve never been a huge fan of .22 LR in the AR platform as a training rifle.

.22 LR doesn’t have the recoil, the handling, or the ballistics to really come close to simulating 5.56 NATO in any meaningful way.

endomag guts
EndoMag guts

9mm on the other hand, has some promise.

Ballistics aren’t close, but that’s actually a good thing. 9mm is safe on steel at about 10-yards meaning you can use a 9mm PCC for more CQB style training, such as USPSA or just at the range.

at Brownells

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons

With EndoMags I’m able to use the same mag carriers as my full caliber rifle, the same handling, and the same capacity. All for a fraction of the price per round.

Not as cheap as .22 LR….but with some major advantages.

Parts and Specs

Since this is a spare parts build, this might sound a little strange but…

Upper Receiver and Barrel

The receiver itself and the barrel were provided by Faxon (thanks, guys!). I used the 9mm 16-inch barrel with integrated muzzle brake. While the receiver is a standard AR-15 Faxon receiver.

Best Lightweight 9mm AR-9 Barrels
at Faxon Firearms

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons


Holosun 510C-GR-Elite…Big name for an optic.

Basically, this is their open sight green dot with a titanium shroud.

Pandemic AR-9 with EndoMags and Holosun HE510c GR Elite (2)
Pandemic AR-9 with EndoMags and Holosun HE510c GR Elite

I really, really love this thing. I bought one for my HD rifle and three days after it came, I bought a second for this Pandemic Trainer rifle.

We’ll have a LOT more info on this model in another review but bottom line — highly recommended.

The other optic I used for a while was the Bushnell TRS-26 — another solid option that I really enjoyed. (Read the complete review here!)

at Brownells

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons


Keep in mind that this is coming from my spare parts box, so we’re going with the Aero Precision 15-inch ATLAS S-ONE.

This is a great handguard. I’ve always liked the feel of them and the 12-inch version is the one I use on my HD rifle.

Pandemic AR-9
Pandemic AR-9 with the Bushnell TRS-26

To be honest, this one a bit is too long.

Since the Faxon barrel uses an integrated muzzle brake, the baffles start at about 14-inches. If you did the math, that means the muzzle is a bit recessed into the ATLAS handguard.

I could have replaced it with a new handguard, but I was committed to my Pandemic build!

Best Bang-For-The-Buck Handguard
at Aero Precision

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons

Bolt Carrier Group

Cryptic Coating is a BCG brand that readers have requested us to review, so after speaking with them at SHOT we were able to get a few in hand to try out — including their brand new 9mm BCG!

So dirty, but just a wipe down away from sparkling!

I’ll tell you now, these are amazing! Period.

I don’t know what black magic they use in their coating, but it’s by far the slickest and easiest to clean.

Cryptic Coating 9mm BCG (7)
Oh ya, that’s the stuff.

Charging Handle

I went with the same Radian Raptor LT charging handle I use for basically all of my AR platform rifles.

Straight up, it works and it’s nice. I recommend it.

at Brownells

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons

Lower Receiver

I opted for a standard Aero Precision M4E1.


Since I am a huge fan of the MBT-2, I stuck with LaRue and went for an MBT-1. Basically, the MBT-1 is the single-stage version of the MBT-2.

It’s not as impressive as the MBT-2s, but it’s proven to be a great trigger and I like the feel of it.


I kept it basic with the Magpul ACS-L. Simple, solid, and just really nice. One of my fave stocks.

at Optics Planet

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons

Buffer Weight

This will come up later but, I thought I threw in a 5-ounce 9mm buffer… but it turned out to be a standard carbine buffer at 3-ounces.

Eventually, I would change this out to the Odin Works Adjustable 9mm buffer.

at Brownells

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons

That is one expensive buffer weight, but the adjustablility will open up a range of options.

And, I felt like getting something fancy.

Grip, Safety, LPK

I threw on the Strike Industries MegaFin (because California) also the SI 45 degree throw safety and the SI Enhanced LPK.

Best Featureless Grip
at Primary Arms

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons

Range Report

Shockingly, everything ran perfectly. I bought three EndoMags and converted three Gen 2 PMAGs from 5.56 to 9mm.

The first 600 rounds or so saw zero malfunctions of any kind. I was running sketchy Wolf or Red Army Standard 115-grain ammo for the majority of that 600 rounds.

My green dot Holosun worked amazing! I highly recommend this optic.

The TRS-26 also did outstandingly well and even survived a drop test of about 5-feet onto Steve the optics testing rock.

Everything was perfect.

Or so I thought…

Human Error

One of the main upsides to buying your rifle whole is that you have a process in place to protect you and your rifle from problems. Quality control is critical in everything.

When putting together your own firearm, it’s important you do your own quality control. If you don’t, bad things can happen.

Borat Bad

Remember the buffer weight I mentioned earlier? Yeah.

The AR-15 buffer, in a normal 5.56 AR-15, is a fairly unimportant part mechanically speaking for a semi-auto rifle.

In fact, several very early models of the M16 didn’t use a buffer weight but instead had just a spring guide.

Best AR-15 BCGs
You’d think after testing so many BCGs we’d be pretty pro at this whole thing…oops.

This is not the case for the AR-9. When using a simple blowback action the only thing that slows and stops the BCG is the weight of the BCG itself, the strength of the buffer spring, and the weight of the buffer.

While most 5.56 NATO buffers are 3- or 4-ounces, a 9mm buffer should be at least 5-ounces and up to 7-ounces.

So what happens when you use the wrong buffer weight in an AR-9? You run the risk of damaging the rear of the gas key, for one.

Cryptic Coating 9mm BCG (9)
Uh oh…is something amiss?

I say gas key even though, technically, a 9mm BCG doesn’t have a gas key. I’m not sure what it’s actually called so I’m sticking with gas key. I mean, that’s what it is on a normal BCG so…

While cleaning the Cryptic Coating 9mm BCG, I found peening and damage on the rear of the gas key.

Further inspection found matching damage on the buffer tube.

Cryptic Coating 9mm BCG Receiver Damage
Cryptic Coating 9mm BCG Receiver Damage

As soon as I saw the matching damage, it clicked in my head that I was using the wrong weight buffer.

To be clear — this was my fault. A failure of quality control when assembling the rifle.

Fixing the issue was simple, just ordering a new Odin Works Adjustable 9mm buffer and installing it.

at Brownells

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons

I could have played around with the weight, but I decided to just put the heaviest ones in and go for it. If the weakest ammo can still work with that, all the better.

But this also raised the question — was the reason that the gun ran perfectly with crappy ammo because I had the weight way too low?

Only way to be sure was to take it back to the range.

Range Report, Part II

Another day, another 300 rounds, and still zero malfunctions. Even with the new weight my little pandemic AR-9 just works.


lego batman first try
Building a new rifle and trouble shooting it feels exactly like this.

Since the iffy ammo I normally shoot is generally underpowered, I needed to make sure my new weight still worked even with ammo that had more oomph.

Thankfully, I stocked up on 9mm before pandemic prices hit, so I was able to splurge and run 100 rounds of 124-grain +P ammo on top of the cheap-o Russian steel cased stuff.

Again, perfect function and no added damage to the BCG or buffertube.

By The Numbers

Reliability: 5/5

Before and after the fix, everything ran 100%. I was worried about the EndoMags since conversion kits rarely work super well — but these are just amazing. Absolutely perfect and very easy to install. Love ’em!

Ergonomics: 4/5

It would be nice to be in a state that doesn’t make me use a fin grip, but it is what it is.

No Right to Bear Arms

Accuracy: 5/5

Faxon’s barrel really delivers! From 10- to 50-yards, I was on steel with no issue. Pushing past that gets a little wonky with my uber-cheap tier ammo but the rifle is still able to do it when fed right.

Customization: 5/5

It’s an AR-15 build, of course, it was customizable.

Value: 5/5

Spare parts builds are some of the most fun for me, partly because we have a fairly constant stream of parts coming through the office. But even before landing this wicked cool gig, it was still a great time.

Sometimes you end up with something awesome, sometimes you end up with lipstick on a pig, sometimes you just learn a lesson.

But if you were ever the kid that liked to build stuff with Legos, then a dive through your spare parts box is for you.

Building Legos

Overall: 5/5

I’m really pleased with how this build came out.

I have a great training rifle and I discovered I love green dots. Oh, and let’s not forget, I learned a valuable lesson in building.

Pandemic AR-9 with EndoMags and Holosun HE510c GR Elite (1)


If you’re building your first AR-15, spare parts builds might not be the style to copy. But when you’re on your 5th, 10th, or 20th build — you start to look outside the box. Or, in this case, inside the spare parts box.

Seven Box

For me, this marks build number 15-ish for complete rifles and I’m really excited about how it turned out. I look forward to taking it to some 2-Gun matches once they open again.

Do you have a sub-caliber training gun? What have you learned from your builds? Let us know in the comments! Want more PCC goodness? Check out our round-up of the Best Pistol-Caliber Carbine: 9mm and Beyond.

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1 Leave a Reply

  • Commenter Avatar

    When I went through Air Force Basic Training in 1981, we qualified with M16a1's with .22 lr conversion kits. Because a majority of Air Force career fields are technical, not combat, to save money the Air Force used .22's to save money. If you were in one of the few career fields that was likely to use one, they got to fire a real one in tech school.

    December 24, 2020 3:57 pm
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