I have learned that I love building AR15s, so recently I was reading about the challenges of building a stable, reliable 762x39 AR15. Challenge accepted!

I just got the build done and am planning to go to the range and shoot it for the first time today.

I have both brass ammo and Tula steel ammo to test both. There will be lots to tweak.

The superlative arms adjustable gas block never been adjusted, so the gun may not cycle the first time, but I opened it up mostly to start.

The scope I put on it is a cool combo scope with a red for and laser, I am sighting in the laser for 10 yards and refit for 10-20 yards. The scope of course handles 20+, but my range only has 25, so I will zero there for now.

It’s possible the gun will cycle differently with brass vs steel casing, but hopefully not different ;that's the goal.

I am starting with an A15 spring and H3 buffer 5.6oz , but I have an H2 with me just in case.

Anyway, I will report back on the range experience. Wish me luck. And please tell me about your experiences with A47 project you may have done.

Build List in next post.

My AR15 762x39 Build

03/26/2024 - Finally Built
3/30/2024 - Going to range for first time

Summary
Rifle is all black, based upon Aero Precision stripped uppers and lowers, and Magpul ACS-L furniture. Combo scope, laser, light, red dot. My goal, to have fun, but also have a functional reliable 762x39 AR47. Trigger pull weight is 7.5lbs.

Build List

Upper:
- Aero Precision M4E1 Upper
- Use Aero Precision Enhanced Handguard MLOK
- Aero Precision barrel nut
- Faxon 762x39, 16" barrel, mid length gas from
- Aero Precision Adjustable .750 gas block  
- mid length gas tube
- ASR mount flash hider
- Omega 36m silencer (still in ATF jail)
- Black Rifle ARMs (BRA) New M16 Enhanced Nitrided Bolt Carrier and Key with Polymer Buffer Technology
- Black Rifle ARMs (BRA) 762x39 bolt, Extractor, Enhanced firing Pin, Cam pin
- 4-16x50AO Scope, Light, Laser, Red Dot Combo (needed something cheap for testing at a 25-yard indoor range; it worked very well for that purpose)

Lower:
- Aero Precision M4E1 lower
- Aero Precision M4E1 Lower Parts Kit
- Aero Precision Enhanced buffer tube
- Magpul ASC-L stock 12.4oz
- 2x DuraMag 28rd 762x39 from Black Rifle ARMs (BRA)
- H2 buffer weight 4.6oz ($11) dollars on amazon
- H3 buffer weight 5.6oz - ($24) dollars for weights on amazon
- Anderson Mfg (AM) stainless trigger kit
- JP Enterprises reduced power trigger spring  (used instead of what AM kit had)
- BRA power hammer spring (used instead of what AM kit had)

Sounds like a cool build, good luck. That scope sounds like hot garbage, though.

Yea, kills the whole thread for me.

Sounds like something from Amazon.

I like 5.56 and have several.  I think that the 6.5G beats it in many categories except price.

I've built several AR-47 rifles, been happy with all but the first one which had a carbine gas system and a crappy bolt.

Once I replaced the broken bolt and went to an H2 buffer it was decent.

These days I use a 16" mid-length barrel from Black Rifle Arms, paired with their matching bolt and firing pin which can go in any standard carrier if you already have one.

I run this with a bone stock carbine buffer and spring, no adjustable gas block needed.

It also has a Primary Arms 1-6 FFP ACSS scope for 7.62x39/300BO.

Steel or brass work fine, though I have never tried anything subsonic.

This is my deer rifle down here in the south.


Attached File



Long term plans are to build a 300BO upper to go with this also as ammo pricing and availability change.

My son has a CMMG 22LR conversion kit for use in a 5.56/.223 upper, its great for plinking or teaching new shooters.

Long, long term plans are to build a 6mm Arc upper to swap on my SPR, will also need new glass for that.

It is definitely hot garbage, but i did get the garbage to zero and hold for now. The laser and red dot work great under 20 yards. It was 116 on amazon and I just needed something functional.

That is totally the one.

Range Results 3/30/2024

Summary: The brass ammo worked 100% the steel ammo worked 0% in terms of cycling

Results:

• The 762x39 brass is very reliable; there are no failures after gas was properly tuned..


• The 762x39 steel-cased ammo will not cycle. The bullets get stuck, feeding into the chamber. They sometimes get stuck halfway in. Other times, they never fed out of the mag. There were about 10% light strikes even with the default hammer spring.


• The 762 lower trigger spring pin walked a lot. I had to fix it once per 30rds..


• Red dot and laser zeroed very well at 10 yards.


• Scope zeroed at 25 yards very well (the adjustments on the scope didn't respond well as expected, but did ultimately get dead on.)

My Actions:

• Don’t buy more Tula ammunition for now, as they will not cycle. Not one cycled, which is unusual not to get lucky even once. The Tula ammo seems to get stuck feeding. .


• Buy anti-walk pins for the trigger in particular.


• Buy more brass 762x39 ammo, since they fire every time. I tried Winchester and Rifle Line on this occasion.


• Though the 25-yard scope zeros are done, the scoped rifles must be sighted at 50-100 yards.

The Black rifle in the picture is my 762 while the FDE is my 556/223 [found out I needed gold member to post images]

I will have to look into the Primary Arms 1-6 FFP ACSS scope for 7.62x39/300B. Regarding buffer, my rifle fired 100% with brass 762x39 using both H2 and H3 buffer weights, but it will not cycle steel Tulammo. I'm using the Duramag 28 round magazine, which works well for the brass. I needed the gas block to make adjustments for shooting suppressed. I have an ASR flash hider as my Silencer is due back from ATF at any time now.

I actually built the 22lr, it is 100% reliable and very accurate. I started with CCMG conversion too, but quickly transitioned to a dedicated 22lr barrel. I still use the CCMR bolt, but I added a 22lr bolt head to fit the dedicated 22lr barrel properly. Bore buddy accessories were added to the bolt to stop bolt bounce and a bore buddy buffer plug was. I use S&W magazines, and an AR15 magazine well insert, so "last round open" works. I will dedicate an entire thread to just this build.  

I agree. I did the 556/223 and 762x39 partly because they are highly available. I may add 6.5 Grendal at some point, but right now, I just don't need what it offers. I'm just an enthusiast-plinker and home-defender.

My Range USA shooting area is a maximum of 25 yards, so I couldn't see myself buying a 500-dollar scope for that. I just needed something to work with in that scenario. If I actually find a range over 25 yards, then I will definitely consider a different scope. For what it is worth, for 116 dollars the scope part worked great at the yards tested, and the glass is clear. The adjustments of course were not crisp clicks, and so forth but it did zero and hold. The laser and red dot also zero'd at 10 yards.

The build wasn't about the scope though. Try to ignore that for now as you don't have to use the scope i did.

It’s not hard for a scope to look clear indoors at 25yds. You’d be miles ahead by returning it and getting a Romeo 5 red dot for less than you paid. Also, a 10yd zero sounds like a pretty awful trajectory. No one is trying to be mean, just trying to help you out on the gear front.

I would set the gas block to wide open and try a standard carbine buffer with the steel ammo, if it works then use the gas block to turn down the gas for the better brass, etc.

Generally Tula is going to be lower powered compared to most other 7.62x39 ammo, even most other steel runs better.

Or you could just not use Tula, your choice for your situation.

You'll want the enhanced/extended firing pin for shooting steel cased ammo. $10 or less and they definitely make a difference in igniting the Combloc primers. They're slightly longer in the tip to allow for more positive contact with the berdan primers.


I see you have one, missed that in the parts list on the first skim through

Good on ya OP.

Combination of light strikes, and trigger pin walking out has me wondering if the hammer spring is installed in the correct orientation, and position. (Yes, I know that light strikes are not uncommon with 7.62x39 ARs.)

I thought the AR47 was some company's take on a version that used AK-47 mags?

anyways, my 7.62x39 AR-15's feed steel ammo no problems.  I think it has something to do with having deburred & polished the feed ramps on the barrel extension, before assembly.  smooth, no sharp edges.  Never shot no brass through it, purpose of builds was to consume cheap steel ammo, back when it was really cheap.  Before Russia sanctions.

No worries.

Some people missed the point. The thread wasn't called "how to mount the best scope ever." lol

Some folks responded with something meaningful, which was appreciated. It is what I expect and what I'd give back. A few of the other comments didn't add anything.

Back to point of the thread. I figured out why the steel wasn't cycling but brass was. As I suspected there was drag being created by the feed lips on the mags. I just grabbed the lips an very very slightly bent them up. That worked. Now steel 100% cycles. My batch of Durham may have had a very tight tolerance. The feed lips were literally cutting into the steel shell casings. The brass was too slippery which is why I suspect it fed so reliably.

Any way problem solved. This ends my AR project. I am kind of sad now. After a lot of dollars spent in 16 months I probably should slow down.

You are correct about the orientation of the hammer spring possibly causing the trigger spring to walk. I installed 4 triggers that day and brain farted on the orientation of that one.  Funny though it actually fired perfectly fine and the pull weight was 1.5 lbs lighter. Had it not been for the walking pins I would've not looked at it again. Two minutes to reorient a hammer spring, so that is fixed.

Good point. My 556/223 and 300 b/o use adj block and H2 so I figured I'd start there. I like your idea though given I had issues with steel.

i also deburred those sharp edges on the mag feed lips, so that they are no longer gouging out the cartridge as is slides forward.  I used a file, and sandpaper to smooth those edges.  The stamping process for the sheet metal mags leaves some pretty sharp edges.

you could also deburr and polish the barrel extension lugs at the 3 o'clock position, the expended brass drags along those edges as they are extracted and kicked out.  you can also do some romancing of the extractor.

you do all of these, and you can get to the point that your expended brass has minimal scratches or gouges.  it's an OCD accomplishment.

all detailed here:

https://www.ar15.com/forums/Precision-Rifles/Hellbender-s-a-k-a-Lostinthewoods-Guide-to-the-Ultimate-AR-fixed-/4-6919/

I've also found Tula to be weak, enough that i had to lighten up the buffer one step, e.g. H3 to H2, to get it to cycle.  so I avoid Tula.

7.62x39 in a AR is a hoot to shoot.

Great Hog Hunter.

Good link. Thank you.

Now that you said it the Tula steels did seem weaker than the brass.

Indeed!

Hogs, deer, bobcats, coyotes, etc...its great for any smallish to medium predators & game.

Every time I take one to a public range it seems to garner a lot of attention, I've sold several uppers like that lol.

Wait till you throw your suppressor on it

I had steel case ammo not eject or cycle properly on a 7.5" x39 pistol while my brass case reloads functioned 100%.

I think the brass expands better to seal at the chamber walls providing more gas to push back the BCG and eject the empty case.

Seems you figured out your issue with the steel cases not feeding... feed lips digging into the steel cases. What mags are you using?