Help: I have an AR9 that has FTF when using 150gr ammo

Steelart99

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Some time ago I built an AR9 which functioned well with 115gr 9mm. I thought I might put a suppressor on it in the future. So, I opted to try out some subsonic 150 gr ammo (Federal Syntech Jacket Flat Nose 890 FPS, 150gr) which gave Failure to Feed and jammed brass.

AR9 Configuration:
  • Spear Arms 80% Lower
  • PSA Gen4 10.5" 9mm 1/10 Nitride 9" Lightweight M-lok Railed Upper (Note: shallow lead-in taper on barrel bore)
  • GUNTEC AR9 BUFFER AND SPRING COMBO (8.2 oz buffer, 10 oz weight) Carbine Length Teflon Coated Chrome Spring
  • New Frontier Armory 9MM/40S&W Buffer Spacer
  • Glock OEM magazines
My thought was that I needed to reduce the weight of the Buffer or change the buffer spring (to a flat coil carbine?). But, when I started to do an internet search, I got a mix of "solutions" from lowering to increasing the buffer weight, changing the spring (KAK red), changing the feed angle on the barrel bore (more sloped), etc.

I think I have some 147gr 9mm (Remington 28435) somewhere that I might be able to try also ... the search is on.

So, I'm turning to my go-to experts on all thing gun'ish. :D

I'm hoping that some of you might have a few words of wisdom on how to get this thing to not have FTF issues with heavier 9mm ammo.
 
Replying only to see notifications about what others think. I have put 115gr through my AR9 with suppressor attached. No failures w or without suppressor. Haven't tried subsonic rounds yet - I do have a few boxes of 147gr subsonic so it is on radar to try with my handgun and PCC.
 
FN may not want to play nice. Try a similar bullet weight in either TC or RN.
 
Check this site - Blowback9.com

If it worked fine with 115gr, it probably doesn't like flat-nose subsonic ammo. Try other subs and if it still jammed, you might need to re-profile the barrel cone.

Pictured is a common 9mm barrel cone on the left, enhanced on the right.

20230803_105130.jpg


Options
1. DIY - Youtube had a few vids I saw, didn't look too terrible to try.
2. You can send your barrel to Macon Armory - The premier manufacturer of AR-45 pistol-caliber conversion parts and kits in .45 ACP for AR-15 rifle platforms. - Macon Armory for re-profile.
3. You can buy an enhanced feed cone barrel and swap it out.

Confirmed enhanced feed cone. Plug and play.
Macon Armory guaranteed-to-feed barrel
Aero Precision EPC barrel
Ballistic Advantage EPC barrel – Makes barrels for Aero, often less expensive.

The total weight is dead on at 22oz+. (The PSA gen 4 BCG is at 14.2oz + Guntec buffer at 8.2oz = 22.4oz. )

For the carbine tube, what you have is fine, typically extended buffer is used for AR9 because of shorter AR9 BCG, but you have a spacer to fill that gap. Extra strength, such as 308 spring, is not needed, it does more damage than good.

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Here is my AR9 build list for reference... I only tried range ammo 115, 124 no issue so far.


Upper

Macon Armory 8.5" 9mm Guaranteed-to-Feed Barrel
Mercury Precision AR 9mm BCG 13.4oz
Davidson Defense Billet 9mm PCC Stripped Receiver
KVP XL Linear Compensator 9mm Caliber, 1/2x28
KVP AR-15 PCC Carbon Fiber Handguard 10.25"
TYRBE Ambi Charging Handle


Lower
80% Lower etch Spike Tactical
R&D Precision Magwell Conv. for Colt 9mm Mag
KAK Industry 9MM Carbine Extended Buffer 8.4 oz
Aero LPK
Radian Ambi Safety
PSA's carbine tube, spring, castle nut, end plate
PSA's brace
LaRue MBT 2 Stage
Kung Fu Grip
 
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I use 124 grain JHPs and FMJs in my 10.5" AR9 and have had no feeding issues. It is what I use in all of my 9mm toys and works great in all of them.

Rounds will be traveling faster from the longer barrel than listed on the box. My handloads pick up about 200 FPS over any 9mm handgun when fired from the AR-9.

Before looking to the gun for a fix, try another brand of similar ammo. If the problem persists, go to a lighter bullet. Some barrels will not play well with every bullet weight. Best to find an ammo that is reliable, IMHO
 
Thanks for the pointers. I saw a few YouTube vids earlier, but had not thought to look into modifying the feed ramp.

It looks like I "should" be able to knock off the hard edge between the breech face and ramp area where the flat nose of the bullet impacts, and then polish everything up. It would be a quick way to determine if I need to go to an enhanced feed cone. In essence, I'd be making an enhanced feed cone only on the lower part of the breech face.

I do know to stay away from the entry edge to the barrel bore (i.e., feed ramp to bore interface) to maintain a full contact area for the bullet base.

:outta here:
 
I use 124 grain JHPs and FMJs in my 10.5" AR9 and have had no feeding issues. It is what I use in all of my 9mm toys and works great in all of them.

Rounds will be traveling faster from the longer barrel than listed on the box. My handloads pick up about 200 FPS over any 9mm handgun when fired from the AR-9.

Before looking to the gun for a fix, try another brand of similar ammo. If the problem persists, go to a lighter bullet. Some barrels will not play well with every bullet weight. Best to find an ammo that is reliable, IMHO
The only reason why I was testing the >147gr rounds was to be subsonic when adding a suppressor. I've run both 115 and 124 in my AR9 without issue ... but I don't recall if I'd only tested round nose or if any were flat nose or JHPs. I'll have to go back and check what I was running back then.
 
I dug out some FN 147gr ammo to try today and loaded up with 3 rounds. First round fired and cycled correctly. The second and third round fired off one after the other on the single trigger pull. .... sigh ... but there was no FTF issues :rolleyes:
 
I believe that it was a Rise Armament Super Sporting Drop-In Trigger Group (Single Stage). I'll check a bit later to confirm.
Most likely it is the trigger, light triggers have been known to be problematic on AR9's. Lately, there have been more reports of Rave 140 trigger causing issues.
 
Most likely it is the trigger, light triggers have been known to be problematic on AR9's. Lately, there have been more reports of Rave 140 trigger causing issues.
Interesting. I hadn't heard that. I'll dig through my supplies and see what triggers I have available to try out. At the very least, I have some stock triggers that are a bit heavier.
Thanks
 
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Well, I opened up my AR9 to see what trigger I'd put into it ... and appears to be a standard trigger configuration (I don't know if it is "Mil-Spec"), that was a part of the Spear Arms LPK that I purchased at that time. I did take the time to polish up the various contact surfaces; without grinding away material. I do have several "spare" triggers (again, I don't know if they are "Mil-Spec") that I'll try, to see if that nixes the double strike.

Of note, Spear Arms doesn't carry LPKs anymore ... guess I'll ask them anyway
 
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Well, I opened up my AR9 to see what trigger I'd put into it ... and appears to be a standard trigger configuration (I don't know if it is "Mil-Spec"), that was a part of the Spear Arms LPK that I purchased at that time. I did take the time to polish up the various contact surfaces; without grinding away material. I do have several "spare" triggers (again, I don't know if they are "Mil-Spec") that I'll try, to see if that nixes the double strike.

Of note, Spear Arms doesn't carry LPKs anymore ... guess I'll ask them anyway

I would look at this as three possible scenarios and eliminate them one at a time...

1.) Did you actually fire the 2nd round and didn't realize it? That's not a personal knock on you, I've done it myself. If you get the trigger pull weight low and minimize the over-travel, the act of releasing the pressure on the trigger to reset it, then applying it again to fire might get lost in the inertia of the rifle's recoil. That's the core of the "bump firing" technique.

2.) Did the firing pin get stuck in position in the breech face? Since an AR9 is blow-back, it looses that little safety feature brought about by the cam action of the rotating bolt in the bolt carrier like a normal AR bolt carrier group. So the only thing retracting the firing pin back into the bolt is inertia and hopefully a spring around the firing pin. I don't know if all blow-back AR's have a firing pin spring, so it is worth examining the bolt to make sure the firing pin isn't getting stuck.

3.) Poor engagement of the dis-connector and hammer. It could be too little engagement or at the wrong angle. It might have caught the hammer and held it while the rifle was cycling, then released it when the bolt came to a sudden stop against the cartridge in the chamber.
 
I would look at this as three possible scenarios and eliminate them one at a time...

1.) Did you actually fire the 2nd round and didn't realize it? That's not a personal knock on you, I've done it myself. If you get the trigger pull weight low and minimize the over-travel, the act of releasing the pressure on the trigger to reset it, then applying it again to fire might get lost in the inertia of the rifle's recoil. That's the core of the "bump firing" technique.

2.) Did the firing pin get stuck in position in the breech face? Since an AR9 is blow-back, it looses that little safety feature brought about by the cam action of the rotating bolt in the bolt carrier like a normal AR bolt carrier group. So the only thing retracting the firing pin back into the bolt is inertia and hopefully a spring around the firing pin. I don't know if all blow-back AR's have a firing pin spring, so it is worth examining the bolt to make sure the firing pin isn't getting stuck.

3.) Poor engagement of the dis-connector and hammer. It could be too little engagement or at the wrong angle. It might have caught the hammer and held it while the rifle was cycling, then released it when the bolt came to a sudden stop against the cartridge in the chamber.
1.) I only loaded 3 rnds when first trying the 147gr since I was only (at the time) trying to determine if I'd still get the FTF as I had with the 150gr rnds. With the original 3 rounds, the first one fired properly, while the second trigger pull resulted in the dual round firing. Then, I opted not to go down the path of "hold my beer" as I tried it again ... until I had some time to evaluate what might have happened.

I frankly do NOT have as good an understanding of the firing mechanisms in the AR9 blow-blowback configuration. I'll try this again and ensure that I fully release the trigger without applying pressure again.

2.) Ahhhh ... good thought. I'll pull out the bolt and do a close examination of the firing pin / spring activation and operation.

3.) That's gonna be a hard one to evaluate. I'll have to think on that and see if I can figure out a way to evaluate that operation without being able to see what might be happening as the bolt is cycling under pressure. Hmmmmmmm


Thanks for those thoughts
 
3.) That's gonna be a hard one to evaluate. I'll have to think on that and see if I can figure out a way to evaluate that operation without being able to see what might be happening as the bolt is cycling under pressure. Hmmmmmmm
Actually, pretty easy to evaluate. Take the upper off, and using your thumb to operate the hammer. You are looking for that hook on the backside of the hammer getting caught by the hook on the top of the dis-connector and holding until you let go of the trigger.

When it is working, the trigger lets go of the bottom of the hammer, the rifle fires, the BCG goes back and re-cocks the hammer. In the process of being cocked, the hammer pushes the dis-connector back out of the way. The BCG comes forward and stops pushing down on the hammer. At that moment, the dis-connector should catch the hammer and hold it. When you relax your finger on the trigger, the tip of the trigger will catch the hammer and finally the dis-connector will let go of the hammer. That is a normal reset of the trigger mechanism. (and the source of that loud "click" you hear when you let go of the trigger after every firing.)

If for some reason, the dis-connector lets go of the hammer before the tip of the trigger is back in place to block the hammer, the rifle will fire another cartridge on it's own.

On every rifle I have assembled, I repeat the cycle of the trigger group by hand a bunch of times to ensure the dis-connector catches the hammer every time.

Some potential reasons for failure: The spring in the trigger is not right, the dis-connector or hammer hooks are damaged or misshaped, the pin holes in the lower are not parallel.
 
I'll walk that through on the trigger tomorrow. I have cycled the trigger by hand ... a bunch. And, I never had issues with 124gr. My brain says that trigger mechanical operation should be the same regardless of the velocity ... but then as I'm sitting here thinking about it, I can see how that could be an incorrect assumption. Thanks for pointing me in that direction. It'll be fun walking this through to see what is happening.
 
In my experience firing the 9mm blow-back AR, recoil is noticeably more than with 5.56 because there is no locking mechanism on the bolt to delay it's rearward movement. Firing pins do have springs in 9mm blow-back AR bolts.

Perhaps you were not holding the trigger back fully after firing and upon recoil, it reset causing a second trigger pull that you did not notice.

Whenever I check a new build for the first time, it begins with one round in the mag. Fire it and check if the bolt locked open.

Next, two rounds in the mag and check to insure only one round at a time is fired for each trigger pull and the bolt locked open.

At this point, I usually load 5 rounds and work on sight adjustment constantly checking for positive trigger reset in between shots and maintaining muzzle discipline.

Anything that does not feel or sound as it should is an indication to stop firing, clear the weapon, and address the issue before continuing.

Check the depth of the fire control portion of your lower. If it was not milled deeply enough, the trigger cannot operate fully and could cause the disconnector to malfunction and an unintentional discharge. I experienced this with my first build. The pocket was .001" too shallow. When I chambered the first round, it went bang. My hand was nowhere near the trigger and I was thankful that only one round was in the mag. On a positive note, the bolt did lock back.
 
I frankly do NOT have as good an understanding of the firing mechanisms in the AR9 blow-blowback configuration. I'll try this again and ensure that I fully release the trigger without applying pressure again.
Newton's 3rd law states that every action has an (equal but) opposite re-action. That law is fiddled with a bit in the example of a cartridge and burning powder within it.

With an AR-15, the burning powder (gas) expands until it pushes the bullet out of the case, into the barrel, towards the muzzle and follows it down the barrel where a tiny bit is siphoned off at the gas block, the siphoned gas travels rearward towards the bolt, through the gas tube, into the gas key, unlocks the bolt and pushes the BCG rearward out of battery where remaining rearward force is absorbed and stored by the buffer and spring. The gas has "impinged" upon the BCG. The spring then pushes the BCG back into battery.

With an AR-9, the burning powder (gas) expands until it pushes the bullet out of the case, into the barrel, towards the muzzle and follows it down the barrel. In this case, the bolt is not "locked" and more force is able to go rearward into the case pushing it out of the chamber and the BCG backwards towards the buffer and spring. The gas has "blow[n] back" the BCG. The spring then pushes the BCG back into battery.

A .380 pistol or .22LR anything work on the same blowback principle...
 
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