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-   -   New Bushmaster ACR Slam Fires .223 ammo (http://www.firearmstalk.com/forums/f25/new-bushmaster-acr-slam-fires-223-ammo-31555/)

IXLR8 09-09-2010 10:33 PM

New Bushmaster ACR Slam Fires .223 ammo
 
I sent the following email to Bushmaster a week ago and they have not responded. If you have an ACR I would not shoot it, and if you do, be aware that it may fire any time the bolt is closed with live ammo.

If you Google the Bushmaster ACR and slam fire, you can read about a guy who shot his big toe off with a 15 round burst, and the safety was on!

Email:
I was very very happy to receive my new Bushmaster ACR Enhanced this week, I waited a year to get it. Today I took it to the range to sight in the weapon. I loaded 1 round of Parvizan 5.56 and shot it, no problem. A precaution I take with all new firearms. I then loaded a full clip of the PRVI Parvizan 5.56 BT ammo and shot the full clip without incident. I changed stations from the 50 to 100yd benches. It setup and fired 4 more boxes of PRVI 5.56 ammo without incident.

The problem started when I loaded Hornaday .223 ammo. I released the bolt from the open position using the release button. The rifle fired from an open bolt. After releasing the bolt with the button, the bolt chambered a round and fired it in a single action. I was very surprised by this and immediately unloaded the rifle. I then removed the bolt and examined it to see if there was a stuck firing pin, or some other malfunction. Everything seemed to operate freely and properly. There was almost no gunpowder residue on the bolt, and I examined the locking rings in the barrel and they were clean and clear. I thought that this may have been a single unusual event. I loaded another single round and tried it, and it loaded and fired as expected. I loaded up 15 rounds of Hornaday .223 ammo and l loaded a round into the chamber using the bolt release button. No problem. I fired 6 rounds and no problem was detected. On the 7th round, I pulled the trigger and 3 rounds fired in quick succession. I was very surprised to fire 3 rounds with 1 trigger pull. I immediately unloaded the weapon and again disassembled the bolt assembly and did not find anything unusual. I was curious about the ammo being a problem so I loaded first a single round, then 3 clips of 10 rounds of the PVRI 5.56 ammo. There was not any malfunction or misfire through the 31 rounds.

I have the 3 spent cases from slam fire, the original ammo boxes for the Hornaday, and some examples of PRVI cases fired. Upon examining the slam fired cases, there is an indention from the firing pin, but it is not crisp and deep like the 5.56 cases, or the .223. In addition I have 3 unused .223 Hornaday rounds from the box I was shooting from.

I will wait for your reply before touching this firearm again. Please let me know what action you would like to undertake? I am very disappointed that I received a firearm with an obvious defect, after all of the fine articles I have read about the ACR. I actively blog on a number of online gun forums. I will give Bushmaster a chance to respond, before I start posting this information online. I am a fair individual, and it is possible that something I did caused the slam fire, since it is a brand new weapon to me, and the first time that I shot it. I have used both types of ammo in my Bushmaster Varminter for the 2 years that I have owned it without a single problem.

JonM 09-09-2010 11:04 PM

a lot of .223 shelf ammo can have softer primers than 5.56ammo. this can cause slam fires to happen. switching to a titanium firing pin pretty much alleviates the issue. it isnt a design flaw its just the nature of ar15 and some ammo. i use a titanium pin in my 458 socom. i stick to harder primed ammo for my 5.56/.223 ARs.

robocop10mm 09-09-2010 11:10 PM

This is a case of ammunition incompatability, not a design flaw. Note you are using the incorrect ammunition for the rifle. It is chambered for 5.56 mm and the .223 ammunition slam fired. I do not know which manufacturer's primers Hornaday uses, but it sounds like it is Federal. Federal primers are known to be softer and easier to touch off than most others. I will not use Federal primers in any ammo for my AR's.

I also use titanium firing pins to help safeguard against slamfires.

Use the proper ammo and you will not have a problem.

IXLR8 09-09-2010 11:24 PM

Quote:

Originally Posted by robocop10mm (Post 347685)
This is a case of ammunition incompatability, not a design flaw. Note you are using the incorrect ammunition for the rifle. It is chambered for 5.56 mm and the .223 ammunition slam fired. I do not know which manufacturer's primers Hornaday uses, but it sounds like it is Federal. Federal primers are known to be softer and easier to touch off than most others. I will not use Federal primers in any ammo for my AR's.

I also use titanium firing pins to help safeguard against slamfires.

Use the proper ammo and you will not have a problem.

I hear what you are saying. The ACR manual specifically lists 5.56 & .223 without any qualifications. The Hornaday ammo is considered premium .223 ammo, and should not cause a problem.

Three occurances in 27 rounds is a problem. Two different types of .223 were tried.

Bushmaster added a spring that was supposed to keep this from happening...

IXLR8 09-11-2010 12:02 AM

Pictures of the Primers on the fired .223 rounds
 
2 Attachment(s)
Attachment 18967
These rounds "slam fired" ie trigger not pulled, but bolt was in battery

Attachment 18966
Unfired .223 rounds from the same box for comparison

What do you think?:eek:

Bigdog57 09-11-2010 12:38 AM

Looks like VERY sensitive primers on the Hornady loads. My Bushmaster M17S Bullpup (NOT an AR-15 platform) will slamfire using Winchester 45 grain "Varmint" loads, but anything else runs fine.

IXLR8 09-12-2010 02:57 AM

BCG examined
 
I removed and inspected the BCG. I am not an armorer or a gunsmith and do not proclaim to be one. The ACR is one of the easiest guns I have ever worked on. The parts can be removed without tools, as long an you have something resembling a bullet tip to push out pins.

The Bolt does not appear to have any damage or wear. The bolt face moves smoothly in and out of the bolt. When in the retracted position, the firing pin is fully retracted (by a spring). I removed the firing pin and inspected it. It had a little bit of gunpowder residue on it and a little rem oil that sprayed on the action before firing it, but wiped clean without anything but a clean rag. The spring was intact and very clean. There were not any nicks or abrasions on any parts that were inspected. All parts moved freely. Before and after assembly I checked the operation of the firing pin by pressing the back of it while in the BCG. It moved freely and returned to position without assistance. The recoil spring was clean, and moved freely along the axis of compression. The was not any visible grease on any parts. The locking ring on the barrel was clean and free of debris.

By all appearances everything is as it should be. Since I do not have the specifications for the springs, I could not test to see if they had the proper compression force. When the action was reassembled, I cycled the action several times and it appeared to operate correctly. The action has a positive feel and the trigger has an amazingly crisp breakover. I recall being amazed at the lack of play in the trigger, it has truly an awesome feel.

If requested I will photobucket the pictures of the BCG in various states of diassembly. I am sure you will want the high resolution pics to verify my observations. Unless someone has a better way to post a 800meg picture.

At this point I am not sure what Bushmaster can do. The weapon appears to meet the operational design.

I have not tried removing the barrel yet, but it looks like fun. A click and a twist, on off she comes.

Any questions?

IXLR8 09-12-2010 03:15 AM

Help please, from the experts.

It appears that the failure mode is fully automatic fire. When cycled the hammer is cocked and stays in a ready to fire position. However when the bolt moves forward the firing pin is not blocked in any way from hitting the primer. It is free to move within the BCG. If the weapon discharges, it will continue to do so until a primer withstands the force of the firing pin hitting it, or the magazine is emptied. The safety on the weapon is only attached to the trigger. In this mode, the safety can be ON since neither the trigger or the hammer are operating and still fire. It appears that if the barrel is struck against a hard surface, that the firing pin is not impeded from striking the primer, that an unintended discharge will occur.

The firing pin appears to have very little mass. I do not know if it is titanium or not. I have been told that reducing the mass of the firing pin will lessen the chance of this happening.

Is this the same way an AR Stoner mechanism also works?

Bigdog57 09-12-2010 04:34 AM

The picture of the fired cases tells the story - too-soft primers. The weapon is not at fault. Those Hornady shells fired from a very light primer strike.
Easy fix - do NOT fire THAT ammo in THAT weapon.

M14sRock 09-12-2010 06:52 AM

Send the ACR back to BM. If it is rated to use 5.56 or .223, it should handle whatever you feed it. Hornady is good stuff, and should be expected to be used by lots of shooters.

Bushmaster needs to step up.


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