ar experts help!

[wun_8_seven]

i bought a armalite m15 use but appears in great condition. i took it out to my brothers land last night put in a 20 round mag, pulled the trigger one time and 18 rounds later it stop firing, i have a bushmaster ar also ,but i haven't done any kind of work on one except put in an accuwedge. any ideas on what the problem is? i took both lowers off and can't see any difference. 187

 

[Rob96]

I am no AR expert. One likelihood could be a slamfire, although I have heard more of this happening in SKS's.

 

[Kentucky Rifle]

Wun...

What type ammunition were you using? Did you check to make sure that the "gaps" in the gas rings on the bolt were NOT lined up? Check your gas tube too. It may be clogged.

KR

 

[wun_8_seven]

tried it again this morning after making sure everything is clean and lubed. samething except now it will fire a few rounds ok then all the sudden full auto with no control. i'm using winchester fmj bought from academy sports. 187

 

[COHIBA]

i would try a different approach.
stop shooting it.
take it apart.
get a friends ar bolt and try it. if the problem stops w/ his bolt. rebuild yours.
i have a feeling the gun is slam firing also. could be a lot of light primers or high primers. if it is happening w/ various ammo you could have a too long firing pin.
if dirt caused AR's to go auto most people wouldnt clean them at all. also try your bolt in your friends AR and see if it duplicates the problem. just use 3-4 rd/s at a time OK??

 

[wun_8_seven]

ok now i'm really confused. i took my armalite lower and put it on my bushmaster upper, worked fine. put bushy lower on armalite upper and it works fine. aint gotta clue. 187

 

[Badger Arms]

Two possibilities IMO. First, was the Armalite 'worked on?' Did somebody file down the sear notch or the disconnector? If I had the gun, it woud be easier to diagnose. Does the Armalite trigger pull seem really light, smooth, and/or short?

The best way to check for a slam-fire is this: Field-strip the bolt and remove the extractor pin, extractor, extractor spring, and spring support (that plastic thingy). Reassemble bolt carrier assembly minus these parts. What this does it it increases the velocity at which the bolt carrier will close and therefore increase the likelyhood of a slam-fire.

At a range, lock the bolt carrer back with an empty magazine inserted in the gun. Pointing the gun in a safe direction, insert a live round into the chamber. Press the magazine release. If the gun goes off, it's a slam-fire issue. My first thought would be to replace the firing pin, possibly with a titanium one which is lighter.

 

[Lucky Jim]

AR goes into overdrive

I bought a Colt SP1 used at a gun show years back. It did the same thing you describe. After taking the thing apart I found that someone had been using one of those drop in auto sears along with M16 parts in the lower. This was way, way before all these parts were illegal. I replaced the M16 safety selector, disconnector, hammer and trigger with factory AR15 parts and things were back to normal. The only time the gun would go into overdrive with the M16 parts was when you moved the safety selector fully to the rear. In that postion when you pulled the trigger you never knew if it would empty the mag. or fire one shot. This would only happen if the trigger was kept fully pulled to the rear. If you just pulled and released quickly it fired one shot most of the time. This overdrive function is not a good thing. I would have it checked out by someone who knows AR's inside and out. It could get you into trouble in more ways than one. Take care-

 

[Jeff White]

Did you perform a function check?

Unload the weapon. Point in a safe direction. Pull the charging handle to the rear and let the bolt go forward. Place the selector on fire. Press the trigger, the hammer should fall.

Pull the charging handle to the rear and let the bolt go forward. Place the selector lever on SAFE. Press the trigger, the hammer should NOT fall.

Place the selector on fire. Press the trigger and hold it back. The hammer should fall. Continue to hold the trigger back and pull the charging handle to the rear and let the bolt go forward. Release the trigger. The hammer should NOT fall. If it does fall, that means that the trigger is not catching the notch in the hammer.

I doubt if it's slam firing. It's more likely that someone has attempted a trigger job on the Armalite lower. It's also possible that there may be some M16 parts in the lower. Although the mere replacement of AR15 spec parts with M16 spec parts will not cause an AR15 to go full auto without an auto sear and full auto bolt carrier.

Is this Winchester white box ammo, Q3131 or Q3131A? If so it's made to USGI M193 specs. If anything the primers might be a little harder then standard commercial loadings.

Don't waste time or money on gimmicks like titanium firing pins. Your problem is in the trigger group. The parts aren't hardened throughout and are only surface hardened. So if someone attempted to lighten the trigger pull by stoning either the notch in the bottom of the hammer or the notch in the disconnector it's probable that they got through the surface hardening and into the softer steel underneath. This would account for the problem being sporadic.

If the rifle passes the function check, separate the upper and lower and repeat the function check by pushing the hammer down instead of charging the weapon. I will bet that you can induce the malfunction this way.

In my experience it's usually the small notch in the hammer that's bad, not the trigger or disconnector. But it's possible that the trigger is bad.

Jeff

 

[AR-10]

If the rifle passes the function check, separate the upper and lower and repeat the function check by pushing the hammer down instead of charging the weapon. I will bet that you can induce the malfunction this way.

If you do this, do not let the hammer strike the magazine well. It may crack the receiver. Keep your hand in the way, or put a piece of leather where the hammer will hit the receiver.

Another thing to look at is how much the firing pin sticks out as it strikes the cartridge. Not all pins are identical. I trashed a brand new pin the other day because it was out of spec, and it did not protrude enough. I was getting light strikes and failure to ignite about ten percent of the time. Yours may be overly long, contributing to a slamfire situation. The fact that you can switch uppers and lowers and eliminate the problem makes this unlikely, but it would not hurt to check it.

If the Bushmaster lower has a weaker action spring driving the bolt carrier into the forward position, you may be loosing enough momentum on the Armalite bolt carrier to eliminate the slam fire.

You're not putting oil or grease on the firing pin are you?

 

[Jeff White]

If you want to eliminate slam fire as a possibility, try this:

Lock the bolt to the rear. Point the weapon in a safe direction down range. Single load a round into the chamber. Place the selector on Safe. Keep you finger off the trigger and let the bolt go forward. If you are truly getting a slam fire, it will fire if you do this. These are the same circumstances that the M16 originally had problems with when they redesigned the firing pin in 1963.

This eliminates the trigger group from the equation. If your firing pin is too long, it will slam fire like this. Most firing pin problems are like the one AR-10 referenced...too short. Not impossible for it to be too long, but I think it's unlikely. If it was long enough to cause a slam fire, it would probably be long enough to pierce primers when it fired normally ( hit with the hammer).

Remember guys, for it to be slam firing when going full auto, the hammer wont be hitting the firing pin after the first round. The disconnector hook will be catching it and slowing it down enough for the notch on the trigger to stop it.

Jeff

 

[wun_8_seven]

well took the armalite apart. and it looks like someone has done some filing on the notch in the hammer as jeff had suggested. now to find a new hammer or should i replace the whole trigger group? thanks guys 187

 

[Jeff White]

wun,
If you're sure that someone only took a stone to the hammer, just rplace that part. If it were my rifle, I'd call or e-mail Armalite, explain the situation and see if they would cut you a deal on replacement parts.

Personally, I'd replace the hammer and trigger.

Jeff