OK OK I know Ive read a thread or two talking about just what I need.... but I cant seem to think of the right search terms or the search is lame..any way... I just came home from the range where on three differnt trigger pulls I got a burst, two of them were 3-4 shots and one was only a 2 shot. the lower is a new stag ( less than 400 rnds) the upper is a delton m4. any thoughts on what is causeing this
jim
PS. I was firmly holding the grip so it was not bumpfireing

There is some info part way down the page.
www.ar15.com/content/page.html?id=111


All the FAQ's are under the tacked post at the top that says "read this before posting"

Is this a new gun? What you have described is usually related to bad disconnect timing... Ive also seen it caused by a bent firing pin retaining pin (cotter pin) that would hold the firing pin forward, making the gun act like it was an open bolt setup...

Ive been thinking about this, it was a pretty cold afternoon and I havent cleaned and lubed the lower.. it still has the factory lube, could that lube have been too cold, and also I hadnt gotten the CLP I orderd, when I cleaned and lube the upper ... so I used some sewing maching oil on the BCG ... maybe it made the firing pin sluggish.
I am going to do a full clean and lube on it and see if it still does it.
Jim

Randall is right on the money.  The disconnector spring is the first thing to check.  It could be improperly installed, the wrong spring or an out of spec spring.  If cleaning it up, or reinstalling the spring does not clear the problem up, next up is to replace it.  The next two items to replace would be the disconnector, then the hammer.  In the even that you think the parts are bad, you might want to consider contacting Stag to see if they will square you away.  

I ALSO HAVE A SIMILIAR PROBLEM.I JUST BOUGHT A BRAND NEW PANTHER ARMS PISTOL WITH A 7 1/2 " BARRELL AND IT SINGLE FIRES ,EJECTS SPENTCARTERIDGE AND FULLY LOADS NEW ROUND BUT DOES NOT RECOCK ITSELF. UPON INSPECTION I NOTICED THAT I CAN HOLD THE TRIGGER AND DEPRESS THE HAMMER AND THE DISCONNECTOR DOESN'T COME CLOSE TO CATCHING THE HAMMER TO ENABLE IT TO FIRE ANOTHER ROUND.SO I GUESS I AM HAVING THE SAME PROBLEM ....BC4...CLAY

Went to the range today, and had the same thing happen, but only one time, 2 shots. I have a new Stag lower, and  a RRA 24" varmit upper, suprised the hell out of me.

Can you get in trouble for this happening? Its not like you did anything intentional, to make it happen.

I was mistaken, the lower that double fired was from Bushmaster, ordered complete in Sept. I swap uppers and lowers out so much, I thought it was a Stag. This Bushy lower has had 500+rds fired, always cleaned afterwards, and I have never had this happen. The double was on the 2nd or 3rd shot of the day, and I fired about 50rds after, with no problems.  I've never heard that an upside down spring could cause this.  The first time I put a lower together, was in Dec. I followed the instructions in the build it yourself topics. The possibility of double fire, if the the disconnector spring is installed upside down, was not mentioned, though the direction of the spring is pretty obvious. I sure as hell don't want an illegal  firearm, on purpose or by accident, I think 99.9999% of the guys here would feel the same, that may need to be added to the instructions, just in case, and to make the possible consequences clear. Again, this was on a factory assembled Bushmaster lower. I distinctly rember installing the spring in my Stag, and know it was done correctly. I'm going to call Bushmaster Mon. I don't want to take any chances of this happening again.

Quoted: Can you get in trouble for this happening? Its not like you did anything intentional, to make it happen.

Yes.
ATF calls it an MG.
They will treat it as such.
Get the problem fixed ASAP.

I cleaned it when I got back from the range, I didn't notice anything out of the ordinary. I will dig deeper, and see if  I can find anything. It may never do it again, but I don't want any chance of getting in trouble. I haven't done anything to this lower beyond cleaning, so mechanically it's  semi, that happened to double, and if that is all it takes to get in trouble, that makes me uneasy.

Quoted: If there's any chance you were popping primers in the past, look for little remnants of the anvil or cup to have been lodged down in the disconnector or trigger. These little pieces of debris do all kinds of nasty things to an otherwise perfectly functional gun. Give it a good once-over, clean & lube and function check before you go chasing down bushmaster.

Thanks for the info, I really never knew a semi, could do that.  I read a post on the ee, for a 9mm upper, that occasionaly doubled and tripled, seller said he was selling for a friend because he shot a deer from hind quarter to head with +Ps, I figured it was accidental. I'm going to tear it apart in the morning, I've wanted a 2 stage for awhile, may as well gut it, and wait on the mail man. Thanks again.........Treeman (edit) How would I know if I was popping primers, besides metel in the lower, or checking spent brass, or is there a way to know?

I've been shooting m855,  bought in bulk. I bought 2 boxes of 1000, and shot close to 1700 or so, without a problem, with 4 lowers.  Thanks again for the help, till tomorrow....Treeman

well I finaly got to fire my AR after a good cleaning and lubeing with clp, only fired 60 rounds (all I had time for) and it worked flawlessly! so I will just keep my eye on it.. but I think its fine.
Jim

ATF does NOT call this an MG. ATF allows that parts wear and break, and may result in an unintentional burst or runaway. ATF allows those parts to be repaired and/or replaced, and the owner will not be charged with possession of a NFA weapon.

" Any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger. The term shall also include the frame or receiver of any such weapon, any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machine gun, and any combination of parts from which a machinegun can be assembled if such parts are in the possession or under the control of a person."

Parts and assembly MUST be deliberate, and designed for FA. Broken parts do not meet this criteria.

Check your recoil spring for proper length. You might want to go with a heavier buffer too.

I have heard that some ammo may cause this to happen, too. A 'downloaded' cartridge will partially cycle the action, causing it to fail to lockup in the rearmost position. Does this apply to the AR as well?

I cleaned mine really good, and didn't find any metal. I took the same lower, with a different upper to the range today, and fired about 100 rds with it, and all is well. Its had close to 600 rds fired total, and 150 or so since the double. So I'm stumped? I may have bump fired without meaning too, that is about the only thing I can think of.