My Oly doubled on me the other day.  This is the first and only time this rifle has done this.  I function checked the FCG and everything seems OK.  Time to replace the FCG anyway?

Hoppy

No, just time to pull the FCG and give it a good cleaning, and maybe adjust the disconnector if needed.

Since the double happened only once, chances are that something (read fouling) got under the disconnector/pad, and held it partially open.

Do this check:

1.  Clear the weapon.
2.  Pull back the charging handle and let it fly.
3.  Pull the trigger to the rear, hold it there.
4.  Pull back the charging handle and let if fly again while holding the trigger to the rear.
5.  Slowly, in a smooth motion, let out the trigger.  The hammer should not fall.  All you should feel is the "click" of the hammer being released by the disconnector and engaging the trigger sear surface.

Do this again and again and make sure the hammer does not fall.  Certain lower parts kits were released with INEXCUSABLY WRONG DIMENTIONS.  The disconnector was letting go of the hammer before it was engaged by the trigger sear surface.

That's the pertinent part.

Hoppy,

Unload the rifle.
Remove the upper from the lower.
Ease the hammer forward.
Hold the trigger to the rear.
Cock the hammer onto the disco.
Push the hammer to the rear as hard as you can.

--Describe the relationship between the middle hammer hook and the tip of the disco.

Continue to hold the trigger back and release pressure on the face of the trigger.

Strike the face of the hammer with a glancing blow to the rear using a no mar mallet. Strike it quickly and firmly. Repeat several times.

--Does the hammer fall?

Thanks folks for all the info.

The FCG is pretty clean as I had only shot about twenty rounds before this happened.  I was rapid firing the last ten rounds or so from the mag and the doubling occured at around shot number four.  I finished the remaining rounds in the mag then opened the rifle for a check.  Not seeing any problems I fired another 100 rounds or so without any problem that day.  I've done multiple function checks and everything seems OK.  I won't have a chance to check it again until Sunday so I'll get back with ya'll on what I find.

Thanks again.

Hoppy

You know, this happened to me. I had ordered a lower parts kit from Bushmaster to replace a very worn FCG on my Sporter II.

I later had LOTS of double and TRIPLE fires...
It turned out that the Disconnector that was sent in the replacement kit was a M16 Disconnector that was modified into a AR15 Disconnector. It was cut all off angle from the facotry.

Thankfully, the rangemaster had a new DPMS disconnector for $6 and not a problem since.

A lot of semi discos are converted from auto discos. The conversion does nothing to change how it functions as a semi. A bad disco is bad no matter its origin.

no intent to bash manufacture.
When we took out the bad disconnector, placed it side to side, there was a big difference.

I just happen to get the "one in a million" bad part.

CB

AH, understood CB1.

BTW, bad mouth manufacturers all you want on here as long as they deserve it. I encourage naming names.

Small gap between middle hammer hook and disco tip.

Completed the instructions and the hammer did not fall when hit repeatedly with a wooden hammer handle.  

Hoppy

OK, is the gap horizontal of vertical? The two hooks should separate vertically but only very slightly horizontally.

Are there impacts from the rear of the upper hammer hook on the top of the disconnector?

Looks like there is.  Somethings taken the blue off the top of the disco.

Hoppy

OK, with the trigger forward, rotate the hammer into the cocked position then a little further. How close can you get the tip of the disco to the middle hammer hook?

It looks like less than 1/32 of an inch and the large hammer hook will come completely down on the end of the disco.

Hoppy

OK, this is how it appears to be. The disconnector needs more engagement, that is almost always the case. But, you've only got about 1/32 of an inch available.  1/32 may or may not be enough change to fix the timing. You don't want to rotate the disco (by relieving it under the nose) past the point where tip of the disco and the middle hammer hook are ALMOST touching. Worse, as you relieve the underside of the nose of the disco the rear of the disco will sit higher. Unfortunately, the rear of the disco is already being struck by the upper hammer hook. When this happens the disco may be knocked to the rear and slowed down just enough to not be able to catch the hammer on the battery stroke.

So, you can try;

Fiddling with the disco by relieving the underside fo the nose AND the top of the tail.

OR

Fiddiling with the disco by relieving the underside of the nose and the back of the upper hammer hook.

OR

Drop in a new set of internals and see if the timing sets up better.

OR

Send it back to OAI and let them handle it.

Per usual I vote for the last one.

I vote for never surrendering and having to send the rifle back to have something as simple as re-setting/re-timing a disconnector.

1. Since the hammer tail is striking the disconnector tail hard enough to leave a line (hope I read you right), grind .040 of the tail of the hammer (not the sear) to allow the hammer to be lowered father down into the receiver before it contacts the back of the disconnector (tail/behind the hook). The hammer/disconnector may not be a present problem, but is a sign of the hammer/carrier binding on the rearward cocking-stroke (with the trigger held back).  Where the problem of binding will rear it ugly head is when you start to shoot light load, which the binding may cause problems with short stroking.

2. With a rag/piece of towel in front the of the hammer and receiver/hammer contact point if the hammer slams forward, hold the trigger back, lock the hammer onto the disconnector, then slowly release the trigger until the hammer is released by the disconnector and caught by the trigger front sear.  Note the distance of the trigger/hammer release point, and the amount of distance that the trigger still travels forward before it stops all the way forward.  The trigger free run after release should be .020/.040 of forward travel (read two thick black hairs).  Now if you have more free run than this, you remove metal from the bottom front portion of the disconnector (where it rides/contact the front of the hammer.  By removing metal here, it cams the hook forward on the trigger, and retards the release of the hammer from the disconnector during the forward movement of the trigger.

P.S. Granted that you don't need to set the disconnector release point this tight/short, But since I will tweak the sears during a trigger job, the disconnector retaining the hammer until just before it comes to rest allows the trigger sear to get into the catch position as far as possible upward before the hammer is released.  This allows the trigger sear to retain the hammer on re-set, and not be deflected off the trigger sear (read very little sear contact), causing the rifle to double/fire on trigger release.

Your problem could be solved by simply replacing the dissconnector spring which may have gotten weak.  I've seen this problem four or five times(older M16A1's), and have fixed all of them with new springs or new disconnectors.  Of course there are other causes, but these have been the main two.

Thanks for all the suggestions folks.  The talent here is amazing!

I’m going to order a replacement FCG.  It’s only about $30. in parts.  I’m not the first owner of this rifle so I don’t really know how many rounds have been through it.  I did replace the springs about 500 rounds ago as the pins were walking on me.  This will probably tighten the FCG up all round and the info I received on this post I can use to check the new parts for fit and timing.

Tonight I took the FCG apart.  The parts don’t look bad, but do show some wear.  I’m going to dress up the disco, relieving the underside of the nose and the top of the tail a little with a stone.  This should get me by if I have to feed my addiction for wasting some ammo before my new parts come in.

Thanks again.

Hoppy

Hoppy,

I have found that when ordering parts, more is better.  It's always a nice feeling to know if you got all your parts together in the same pile, you could build up at least 3 rifles out of them.

P.S. Just don't keep all of the parts together in the same room or boxes, the wife gets a little pissed knowing that you have more rifles that you told here you bought.  It also helps to sell her on the idea that when you buy a complete parts kit, your just doing so as a saving than on just buying the few parts that you need.  You can tell her that my spending just one hundred dollars more, you have saved hundreds in the long haul, and can sell the extra parts not needed off at a profit (like that's even going to happen).

My list has already gotten bigger with "while I'm in here" and "saving on shipping" parts. [;)]

Hoppy