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Discussion Starter · #1 ·
Been having an issue with my ACR since I bought it second hand where it seems to be extremely ammo sensitive, and most ammo will cause some pretty ugly malfunctions. It doesn't happen consistently, maybe one in 20-40 shots. Either:
  • It short strokes, failing to fully engage the hammer with the disconnector, leading to a dead trigger
  • More commonly, it malfunctions and dumps a few rounds in what I can only describe as full auto, sometimes two, sometimes more. Taken from someone else's topic on this, a video of this happening
Both of these problems I'd attribute to the hammer not catching on the disconnector, where the hammer is riding the bolt carrier group as it goes back home and when it locks back into the breach the hammer either hits the firing pin hard enough to cause a second or third round to go off, or it doesn't hit the firing pin hard enough and leads to a dead trigger.

I keep my ACR very clean, so I don't think its an issue of lubrication or carbon buildup somewhere, although I can't really get into the tiny holes of the gas regulator so maybe there's something there? At the same time, I don't think my ACR is undergassed as it kicks much more than my finely tuned XCR does.

One possibility I'm considering is that my ACR could be so overgassed that when the bolt carrier impacts the back of the receiver at the end of it's travel, a weakened disconnector spring could allow it to disengage under that final kick and allow the hammer to come free and ride home on the bolt carrier group as it travels forward?

Other than that, I'm not really sure what could be causing this. Almost every ammo I've tried causes this random issue, from Winchester M193 to crappy ZSR .223; I want to say the 62gr M855 surplus I have doesn't cause the issue but I may not have put enough through the gun to cause a malfunction.

Does anyone have any ideas?
 

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Sounds like you may have gotten one of those original problem rifles that never got fixed. Not sure what to recommend now that Bushmaster is kaput...
 

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Some of the early rifles had an issue firing more than one round per trigger pull occasionally. You can contact Bushmaster and see what they say. Maybe they'll have you send the rifle in and they'll repair it. IIRC it had to do with the firing pin.

Does the rifle have the original, unmodified trigger? Someone may have done a poor DIY trigger job before you took ownership.

If they cannot/will not help you then I'd take the rifle to a good gun smith to check the size of your gas port. I cant remember what its supposed to be but a knowledgeable gun smith would. My gun smith has a bullet catch in his shop so we can test fire rifles immediately after changing something on them.

Next, I'd install a new trigger. I never knew if my stock trigger was bad since I put a Super ACR Geissle trigger in, I highly recommend one; Super ACR Trigger (geissele.com) If you're not familiar with the AR style triggers then I suggest having the gun smith install it.

Good luck on solving this issue.
 

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Bushmaster's fix for the doubling and tripling of shots was a titanium firing pin to replace the original steel one. You can check your firing pin with a magnet to see if you have the upgraded firing pin, also make sure the firing pin return spring is installed.
 

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Discussion Starter · #5 · (Edited)
Some of the early rifles had an issue firing more than one round per trigger pull occasionally. You can contact Bushmaster and see what they say. Maybe they'll have you send the rifle in and they'll repair it. IIRC it had to do with the firing pin.

Does the rifle have the original, unmodified trigger? Someone may have done a poor DIY trigger job before you took ownership.

If they cannot/will not help you then I'd take the rifle to a good gun smith to check the size of your gas port. I cant remember what its supposed to be but a knowledgeable gun smith would. My gun smith has a bullet catch in his shop so we can test fire rifles immediately after changing something on them.

Next, I'd install a new trigger. I never knew if my stock trigger was bad since I put a Super ACR Geissle trigger in, I highly recommend one; Super ACR Trigger (geissele.com) If you're not familiar with the AR style triggers then I suggest having the gun smith install it.

Good luck on solving this issue.
Thanks for the all the information! I do have a second trigger pack with a Super ACR trigger installed in it, sadly it seems to produce the same issue. The original trigger, as far as I can tell, doesn't appear to be modified in any way, for the record. Hopefully Bushmaster can help.


Bushmaster's fix for the doubling and tripling of shots was a titanium firing pin to replace the original steel one. You can check your firing pin with a magnet to see if you have the upgraded firing pin, also make sure the firing pin return spring is installed.
Dang, you got me really hopeful about the titanium firing pin - I guess it's what mine has in it. I do also have the firing pin spring installed - could it be too weak?
 

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I would also check what gas regulator you have since there are a few versions but I think your best bet is measuring the gas port. Does the rifle have the factory bushmaster barrel or is it aftermarket? Also what is the gas system length and barrel length?
 

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Discussion Starter · #7 ·
I would also check what gas regulator you have since there are a few versions but I think your best bet is measuring the gas port. Does the rifle have the factory bushmaster barrel or is it aftermarket? Also what is the gas system length and barrel length?
Any suggestions on how I'd measure the gas port?

As far as I can tell it's the factory 16" (or 16.whatever) barrel with the rifle length gas system - marked BFI 5.56 NATO 1/7.
Air gun Trigger Gun barrel Gun accessory Wood
 

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Your rifle appears to be one of the latest models, should have all the upgrades. I would think maybe it is ammo related, I use surplus Lake City in my rifle and have not had an issue. There is a wide variety of 5.56mm loading out these and I could see light loads causing issues.

Also, Make sure your regulator is set with the letter "U" (unsuppressed) facing up ("S" is for running a suppressor and bleeds off more gas from the operating system). Make sure your barrel is tight as well.
 

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Gotta remove the gas block to check gas port diameter, but it should be fine as it looks like you have the updated gas regulator and the 1/7 would means it's a newer bushmaster barrel, since the first ones came with a 1/9 twist. Have you pulled the gas regulator out and cleaned it, the gas block, op-rod, and checked the spring or op-rod support? made sure there is no binding?
 

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I want to say the 62gr M855 surplus I have doesn't cause the issue but I may not have put enough through the gun to cause a malfunction.

Does anyone have any ideas?
This leads me to believe you may have the smaller diameter gas port designed for 5.56 vs the larger gas port designed for consumer 223. I could be wrong but again its hard to trouble shoot over the forum. I would start by tearing the gas system apart if you haven't, then the bolt carrier group.
 

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Discussion Starter · #11 ·
Your rifle appears to be one of the latest models, should have all the upgrades. I would think maybe it is ammo related, I use surplus Lake City in my rifle and have not had an issue. There is a wide variety of 5.56mm loading out these and I could see light loads causing issues.

Also, Make sure your regulator is set with the letter "U" (unsuppressed) facing up ("S" is for running a suppressor and bleeds off more gas from the operating system). Make sure your barrel is tight as well.
Yeah, I'm definitely on the U setting. It'd be a shame if it's that ammo sensitive, I think M193 is a pretty ubiquitous and normal configuration.

This leads me to believe you may have the smaller diameter gas port designed for 5.56 vs the larger gas port designed for consumer 223. I could be wrong but again its hard to trouble shoot over the forum. I would start by tearing the gas system apart if you haven't, then the bolt carrier group.
I disassemble and clean my ACR thoroughly after every shooting session, including the BCG, regulator, piston, and gas block; it's definitely not a matter of it not being clean enough, everything seems to run quite smoothly. I do have some digital calipers, but I doubt that's going to be accurate enough for measuring the tiny gas port in the regulator.
 

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I disassemble and clean my ACR thoroughly after every shooting session, including the BCG, regulator, piston, and gas block; it's definitely not a matter of it not being clean enough, everything seems to run quite smoothly. I do have some digital calipers, but I doubt that's going to be accurate enough for measuring the tiny gas port in the regulator.
I was more wondering if it was a binding issue, these things can run dirty as hell so its not a cleanliness issue! If you are confident/competent enough to pull the gas block you can use drill bits to ID in place of pin gauges, although I'm not sure if there are specs posted for the 556 vs 223 gas port diameters for a 16" barrel. In leu of that you could just run a few hundred more 5.56 pressure loads to confirm you don't get short stroking then re-test 223 pressures and confirm that is the ammo you short stroke on, but this is really only looking at the short stroking issue not your double firing issue.

anyways good luck!
 

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Discussion Starter · #13 ·
Afraid I'm not confident enough to tap out the pins holding the gas block onto the barrel.

This might be a stupid question, but is the orientation of this cut in the piston important? Should it be in the 6 o'clock position in the gas block, for example?

Tool Auto part Engineering Font Metal
 

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Been having an issue with my ACR since I bought it second hand where it seems to be extremely ammo sensitive, and most ammo will cause some pretty ugly malfunctions. It doesn't happen consistently, maybe one in 20-40 shots. Either:
  • It short strokes, failing to fully engage the hammer with the disconnector, leading to a dead trigger
  • More commonly, it malfunctions and dumps a few rounds in what I can only describe as full auto, sometimes two, sometimes more. Taken from someone else's topic on this, a video of this happening
Both of these problems I'd attribute to the hammer not catching on the disconnector, where the hammer is riding the bolt carrier group as it goes back home and when it locks back into the breach the hammer either hits the firing pin hard enough to cause a second or third round to go off, or it doesn't hit the firing pin hard enough and leads to a dead trigger.

I keep my ACR very clean, so I don't think its an issue of lubrication or carbon buildup somewhere, although I can't really get into the tiny holes of the gas regulator so maybe there's something there? At the same time, I don't think my ACR is undergassed as it kicks much more than my finely tuned XCR does.

One possibility I'm considering is that my ACR could be so overgassed that when the bolt carrier impacts the back of the receiver at the end of it's travel, a weakened disconnector spring could allow it to disengage under that final kick and allow the hammer to come free and ride home on the bolt carrier group as it travels forward?

Other than that, I'm not really sure what could be causing this. Almost every ammo I've tried causes this random issue, from Winchester M193 to crappy ZSR .223; I want to say the 62gr M855 surplus I have doesn't cause the issue but I may not have put enough through the gun to cause a malfunction.

Does anyone have any ideas?
Hey brother welcome! The double and triple firing could be because the hammer spring has been installed upside down. A gun-tuber had installed his Super ACR trigger himself and installed the hammer spring (I think..it's been a while and I'm old af) incorrectly and he had the same problem. I'm aware that you had issue with stock trigger but if THAT spring was also incorrectly installed and you installed it the same way (you have to use stock spring with Super ACR do to the bend) you may have transfered problem. I say this because Bushmaster had some QC issues at the end. I hope this helps.
 

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Discussion Starter · #16 ·
Hey brother welcome! The double and triple firing could be because the hammer spring has been installed upside down. A gun-tuber had installed his Super ACR trigger himself and installed the hammer spring (I think..it's been a while and I'm old af) incorrectly and he had the same problem. I'm aware that you had issue with stock trigger but if THAT spring was also incorrectly installed and you installed it the same way (you have to use stock spring with Super ACR do to the bend) you may have transfered problem. I say this because Bushmaster had some QC issues at the end. I hope this helps.
Not sure of the best way to show this on camera, but I'm pretty sure both are installed correctly from what I've seen around online - feel free to correct me, though:

 

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Your double and triple firing has zero to do with gas issues. It has been a known problem with some of the newer ACRs. Like others have said Bushmaster said the switching to a Ti firing pin fixed the issues, I don’t know if I believe that though.

Your issues are 100% trigger related. If I was you I would reach out to Dana at Templar Precison and send it off to him. He will fix it. The best way to reach him is through Instagram.
 
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