I was doing a little prone practice with my Bushmaster DCM, and was a bit startled to get a chain fire. I've had chain fires with an SKS before, where the firing pin locks forward, and empties the magazine in short order because the primer is struck every time the bolt slams forward. This was not quite the same - rather, I was getting irregular two or three round bursts. Mostly it would function properly. I have a habit of holding the trigger back momentarily after releasing a shot, as part of the follow-through, so this was not the double-tap on release problem others have had.
The first thing I did, based upon my SKS experience, was check to make sure that the firing pin was free, which it was. The rifle was recently cleaned and oiled, and the problem only started after about 60 rounds. While the thought of having an accidental automatic is mildly entertaining, this is a competition rifle, and I'd very much prefer to avoid this during a match.
Has anyone else had a similar experience and know of a cause and solution?
More info please....what are you using for ammo?
Only way that it could be an Ammo problem is if he was reloading with soft (pistol) primers, or not fully seating the primers. Both of which can lead to slam fires.
My guess is that he needs to adjust/correct the disconnector. The problem could be as simple as the disconnector spring installed upside down, too as severe as adjusting it for a later release.
As noted by Dano the "proper" term is slam-fire.
Believe he's also on target with his comments about your problem.
Chain-fire IIRC is a term associated with black powder revolvers, in which a improperly loaded six gun (with powder left in the cylinder "in-front" of the lead ball) fires not only the round intended but also lights/ignites the adjacent chambers due to the improper loading mentioned.
Thanks for the feedback. I apologize for the incorrect terminology (negligent repetition). I did check the FAQs (thanks Tweak) - that's where I initially saw the information about the doubling, but realized this is a different problem, thus the explanation in the original post.
I have not explored the disconnector spring potential - I'm admittedly a newbie to the AR-15 community. If you could point me to some more information on this, it might help. As a note, I have not disassembled or adjusted anything in the lower. I do make sure that the workings in the lower are clean and lightly oiled when I clean the gun in general, but have had no need to make any adjustments - thus far, the rifle has been accurate and reliable. Perhaps I have neglected to perform some standard maintenance?
I tried manually reproducing the condition by cocking and releasing the hammer with the upper open, but could not reproduce the problem.
Regarding ammo, I've been shooting Federal American Eagle 55 Grain FMJ almost exclusively. I purchased the gun almost two months ago, slightly used, in excellent condition, and bought a case of factory loads soon thereafter. I'm nearly through the original case, but have not started reloading yet.
One additional note, which probably does not make a difference, but may, so I'll mention it - the trigger is one of the Bushmaster DCM approved two-stage triggers.
Thanks again - very grateful for the responses and help,
Disconnector function check:
Visually verify that the chamber is empty and no magazine is inserted.
While holding the trigger back, charge the weapon.
VERY slowly let off on the trigger - as slowly and smoothly as you can.
Try this several times, at least.
If the hammer falls during trigger release, you have a problem with the disconnector releasing too early (before the trigger sear is engaging the hammer). This can lead to burst fire, when you hold the trigger back, but due to recoil and/or slight muscular twitch, you're slightly retarding the hammer with the disconnector, then letting it go, and happen to be in a 'sweet spot' where it can cycle that way for a few rounds.
If you pass the disconnector check above, it's probably a dynamic problem with the disconnector, like the hammer hitting the tail of the disconnector and causing it to 'bounce' out of engagement with the hammer.
To clarify, "slam fire" is the proper term for what jeff experienced with the SKS. Slam fires are possible with ARs with the right circumstances tho they rarely repeat. A slam fire is when the piece fires when the bolt closes without any manipulation of the trigger. Soft primers and heavy firing pins will cause this in the AR as can loading rounds directly into the chamber. The extra inertia added by the lack of resistance from the magazine follower is transmitted to the firing pin in that case.
"Doubling" is when the hammer slips off, or misses, the nose of the trigger when the hammer drops from the disco. It can also be caused by the rear of the hammer striking the top of the disco hard enough to make the disco miss the middle hammer hook. This usually manifests itself as a rare double, perhaps once every 3 mags or so, unless the parts are badly out. You can get doubles that run until the mag is empty but that is rare. Usually, the piece will double until the bolt stops in battery with the hammer forward. We call the interrelationship of the FCG "timing".
The Troubleshooting Checklist covers checks of the FCG.
Just to add on to Circuits post, since the rifle is used and running a two stage FCG, The area to concentrate on will be the disconnector and corresponding hammer rear hook.
The simplest way to attack the problem after function check is to just confirm that the all the sear surfaces/ledges are still sharp (not screwed up by a novice trying to re-work the trigger), and that the disconnector spring was not clipped (someone trying to wrongly lighten the secondary pull after disconnector engagement).
To walk you threw the FCG, since the two-stage triggers are not listed in the manuals,
At trigger rest/ hammer set, the front sear of the hammer is retained by the front sear of the trigger. As you start to pull back the trigger, the back hook of the trigger (disconnector) presses against the rear hook of the hammer (above the sear surface). This engagement is the secondary pressure of the trigger that you feel just before the hammer releases. On a stock trigger, you will have some creep after the extra pull weight, which is a safety factor in order for the FCG to work in a wide variety of rifles with out needing to be smithed/ tuned from the start. This added creep factor is that in order for the hammer to make it’s way threw the front and back hook to be released, the trigger must be fully released in order for the hammer to be released from disconnector hook, caught by the primary sear (front), then the trigger needing to be pulled to release the hammer off the front hook (with tension from the back hook adding trigger pressure).
When doing a trigger job on a two stage FCG, the disconnector is worked to so that once the hammer disconnector sear touches the rear hook (above the hammer sear) the next movement of the trigger, with the added tension of the disconnector, should be the trigger/hammer releasing (like breaking a thin glass rod). Polishing/stoning the sears to set the FCG to a specific rifle that it is being used in will accomplish this ideal functioning of the FCG. Ideally, the first stage of the trigger should be around a 2 lb with creep of about ¼”, then the 4.5 lb resistance of the secondary pull weight (no creep just added resistance of the disconnector) with the clean break of the hammer.
Now if someone tried to do an untalented trigger job on the FCG and screwed it up, they may have rounded the sear edges, then set the disconnector with not enough retention. This lack of retention would be that the gap between the trigger hooks would be set too wide, and the hammer can be danced out of the trigger sears with just the slightest of trigger dancing (trigger finger movement at recoil bouncing on the hammer). Or, they may have stoned too much surface off the disconnector sears, and the FCG lacks any rear two stage effect (hammer can make it way out of the hooks with not trigger movement due to the hammer disconnector sear slipping past the disconnector sear if the trigger is held in just the right position.
To sum up this post, I could have just told you from the start that you need to take the rifle to a smith to check it out (rework the FCG if possible), but like the rest of us, you want to fix the problem yourself. If the fire control group (FCG) were just the normal unit of a standard rifle, this would not be a problem and most of the guys here could instruct you on adjusting/tuning a standard disconnector. But, since the trigger group is a two-stage unit, and takes a talented smith to set one up correctly (match tuned, and not just sloppily functioning), chance are you may total destroy the FCG in the process of trying to correct it (hacked trigger job). My guess is still that the disconnector is set incorrectly, and hopefully you can find a local smith that can correct the problem with out needing to replace the entire trigger/hammer group.
Thanks again, all.
Dano, your disconnector suspicians were right, and your description of the mechanism helped very much (I did mention I'm a newbie, right?). There haven't been any modifications, so I know some of the suggestions couldn't be possible, but this did point me in the right direction.
Based upon your comments, I examined the disconnector more, and found that it is worn to the point of intermittent failure. This was very subtle, and not immediately apparent. The best visual clue was a wear mark on the top edge of the hammer hook where it contacts the disconnector. There is a clear line at the limit of where the disconnector should contact, and the actual point of contact was much nearer the edge. Based upon the fact that the problem would only occur after about 50 rounds, I suspect the heat may have been just enough to alter the geometry or friction such that the hook would not catch.
So what I did was stone the bottom of the front edge of the disconnector just enough so that the spring could push it into place and line up with the old wear mark. This seems to have solved the problem. I actually did this shortly after your last reply, but wanted to test it enough to verify that this actually fixed the problem. I've put about 200 rounds through with no problem, including a 500 point NRA high power league match (my first high power match!) , and it has functioned perfectly so far (knocking on wood).
Again, thank you all very much - I'm grateful to have a resource like this board, and for everyone's willingness to help.