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Posted: 12/24/2011 10:49:37 AM EDT
Let me preface this by saying the subject is an assumption. I just bought an AR from a relative that is built on a Doublestar Arms lower with a Rock River Arms flat-top upper. It is a couple years old, but it has never been shot.

I took it to the range yesterday to put a few rounds through it and zero it in. I got through about 15 rounds without a hitch. Upon firing the ~16th round, the gun fired just fine and seemingly cycled without a hitch. The next round seated and the bolt closed entirely, but the gun would not fire. The trigger wouldn't move at all. I could then "single fire" the gun by pulling the charging handle and ejecting a fresh round. I could then fire another round and the same thing would happen.

The bolt is most definitely closed on a good round but I'm thinking the hammer isn't being reset properly. The gun is clean and properly lubricated and doesn't seem to be exhibiting and other unusual properties. Where do I start the troubleshooting process? Thanks guys!
Link Posted: 12/24/2011 10:59:25 AM EDT
Could be a gas port/ tube issue. Sounds like the carrier isn't going back.
Link Posted: 12/24/2011 11:39:57 AM EDT
Something is blocking the trigger.

If there were movement of the trigger, but no click, then the hammer did not catch the sear and followed the bolt home.

Since you described that there was no movement of the trigger at all, then there is something blocking it. It may be that the selector switch is loose and moved in the way, or you have some sort of debris in the fire control parts which are preventing the trigger from going forward.

My guess is that it is the latter. When you cycle a new cartridge you do not have pressure on the trigger which allows it to return to its forward position since vibration helps overcome friction. When firing, you are pulling the trigger to the rear while all the vibrations of the cycle and recoil are going on. When you let up on the trigger the return spring cannot overcome the friction alone and stays to the rear.

You can test this (if you haven't already taken it apart and cleaned it) is to make sure the rifle is unloaded and no loaded magazines are present and you have a safe direction in which to point the muzzle. With no magazine in place, cycle the action and pull the trigger. It should drop the hammer normally. Now try holding the trigger back while you cycle the action. If the trigger does not go forward on its own with a slight click after the action is cycled, that will confirm the problem.

One way you can tell if the hammer is not resetting is to pull the bolt back a short distance. The hammer will provide a bit more resistance than the buffer spring alone. You should also feel and hear the hammer click into engagement with the sear. If the hammer follows the bolt back you should notice this too. Try it both with the trigger pulled and not.

Link Posted: 12/24/2011 12:48:37 PM EDT
Yes, first step for sure is to function check your fire control group to ensure there is no problem with it.

Inspect carefully to make sure you don't have a trigger or hammer pin that's walked to one side or the other, than the disconnector moves smoothly and springs back into position smartly, and that hammer/trigger engagement is good. Make sure the trigger can move through its range of motion and that there is no debris under the trigger. Put the upper on and do the standard function check as described above... safety on, does not fire - safety off, hammer drops... KEEP trigger pulled to the rear, cycle the action, release the trigger and you should hear a click, pull trigger again and hammer should fall again.

As long as the fire control is all functioning correctly you likely have a short stroking problem. First question is if you've cleaned the brown corrosion protection gunk off of the parts and lubricated the rifle - specifically, field stripping the bolt to remove it from the bolt carrier, getting some light oil onto all the parts after cleaning, then reassemble. Before you clean though, examine the BCG for a loose gas key or evidence of a gas system leak... pull the BCG and check the gas key to see if it's physically loose on the carrier or if you see any evidence of gas leaking around the key-to-bolt carrier area. Then clean and lube if you didn't already, the missing lube is likely the problem. Other than that, inspect the gas block installation on the barrel to see if you see any evidence of gas leaking or misalignment of the gas block.
Link Posted: 12/24/2011 12:53:05 PM EDT
[Last Edit: 12/24/2011 12:57:07 PM EDT by Dano523]
Link Posted: 12/27/2011 8:09:40 AM EDT
Thanks for the tips, guys! I'm a bit of a noob when it comes to AR maintenance, so I'm going to work my way through your posts one by one.


Originally Posted By ArimoDave:
Something is blocking the trigger.

If there were movement of the trigger, but no click, then the hammer did not catch the sear and followed the bolt home.

Since you described that there was no movement of the trigger at all, then there is something blocking it. It may be that the selector switch is loose and moved in the way, or you have some sort of debris in the fire control parts which are preventing the trigger from going forward.

The selector switch seems to "lock in" to both positions pretty well, although it's not as firm as my Bushmaster. The trigger works every time if I pull the charging handle first, but it doesn't work for any followup shots.


Originally Posted By ArimoDave:
My guess is that it is the latter. When you cycle a new cartridge you do not have pressure on the trigger which allows it to return to its forward position since vibration helps overcome friction. When firing, you are pulling the trigger to the rear while all the vibrations of the cycle and recoil are going on. When you let up on the trigger the return spring cannot overcome the friction alone and stays to the rear.

You can test this (if you haven't already taken it apart and cleaned it) is to make sure the rifle is unloaded and no loaded magazines are present and you have a safe direction in which to point the muzzle. With no magazine in place, cycle the action and pull the trigger. It should drop the hammer normally. Now try holding the trigger back while you cycle the action. If the trigger does not go forward on its own with a slight click after the action is cycled, that will confirm the problem.



Honestly, I didn't try pushing the trigger back forward to see if it might be an issue with a return spring. I'll have to try that next time I can get out to shoot, but I don't think that is an issue.

I tried the function test you noted and everything appears to work properly. I racked the charging handle, pulled the trigger, cycled the action again, and let go of the trigger. It moved back to its fully forward position and clicked into place, and worked properly once again when I pulled the trigger.



One way you can tell if the hammer is not resetting is to pull the bolt back a short distance. The hammer will provide a bit more resistance than the buffer spring alone. You should also feel and hear the hammer click into engagement with the sear. If the hammer follows the bolt back you should notice this too. Try it both with the trigger pulled and not.

Everything "feels" the same with both firearms when I walk through this process. If I hold down the trigger, I get firm resistance throughout the entire pull of the charging handle. If I don't hold down the trigger, I get a "two stage" effect with a light pull from about 1/3-1/2 the travel distance and a firmer pull for the rest of the distance. Is this because of the buffer tube?


Link Posted: 12/27/2011 10:06:50 AM EDT

Originally Posted By Gamma762:
Yes, first step for sure is to function check your fire control group to ensure there is no problem with it.

Put the upper on and do the standard function check as described above... safety on, does not fire - safety off, hammer drops... KEEP trigger pulled to the rear, cycle the action, release the trigger and you should hear a click, pull trigger again and hammer should fall again.
I performed the function test shown here: http://www.ar15.com/content/guides/functionCheck/ Everything checked out okay. I cycled the action and pulled the trigger and heard the hammer drop, then held the trigger while cycling the action. I heard the click as I released the trigger, and it dropped the hammer again upon pulling the trigger.



Inspect carefully to make sure you don't have a trigger or hammer pin that's walked to one side or the other, than the disconnector moves smoothly and springs back into position smartly, and that hammer/trigger engagement is good. Make sure the trigger can move through its range of motion and that there is no debris under the trigger.

I checked all the pins on the firearm and everything appears to be in place. The disconnector moves smoothly through its limited range of motion. While I had the upper removed, I manually engaged the hammer and it locked into place and dropped just fine when I pulled the trigger. I didn't find any evidence of debris (primers, dirt, etc) anywhere within the firearm. Everything appears to be cleaned and lubricated properly.


As long as the fire control is all functioning correctly you likely have a short stroking problem. First question is if you've cleaned the brown corrosion protection gunk off of the parts and lubricated the rifle - specifically, field stripping the bolt to remove it from the bolt carrier, getting some light oil onto all the parts after cleaning, then reassemble. Before you clean though, examine the BCG for a loose gas key or evidence of a gas system leak... pull the BCG and check the gas key to see if it's physically loose on the carrier or if you see any evidence of gas leaking around the key-to-bolt carrier area. Then clean and lube if you didn't already, the missing lube is likely the problem. Other than that, inspect the gas block installation on the barrel to see if you see any evidence of gas leaking or misalignment of the gas block.

Short stroking was my first guess, but I thought that would impact the ejection and loading of a fresh round? It ejects and feeds properly, it's only the trigger that seems to be problematic. The gas key is not physically loose and I don't see any damage or gas fouling anywhere on the outside of the BCG. I removed and stripped the bolt and didn't see any obvious issues there either. The gas rings are intact and are not aligned, and I don't see any signs of warping. The bolt did have an excessive amount of oil on it, so I cleaned it up a bit.




Link Posted: 12/27/2011 11:19:44 AM EDT
Link Posted: 12/28/2011 9:25:31 AM EDT
[Last Edit: 12/28/2011 9:33:34 AM EDT by Meta4]

Originally Posted By Dano523:
Either the disco spring was not installed correctly, the disco is binding in the trigger slot, or you have a piece of blown primer hiding under the trigger.

Pull the FCG out of the receiver (Hammer, trigger, disconnector), clean the parts, and while you are in there, look in the bottom of the receiver void for a piece of blown primer, and that the disconnector spring was installed large coils side down into the back of the trigger slot under the disconnector.

Before you go to reinstall the parts, dry fit all three parts to the pins, and each other outside of the rifle. Some times you can get a bur on the sides of disco or trigger pin channel on the inner groove slot sides for the disco, which does not allow the disco to free move under spring tension, leaving it sticking backwards/leaving it wide open to not catch the hammer when the hammer sear pushed it back at rear cock position.

I had never torn apart the FCG before as it looked a bit intimidating, but I ran through the process using the guide you posted and everything went quite smoothly. Everything appeared to be installed correctly and I found no debris in the bottom of the receiver. The oil on the parts was a bit thick, so I stripped and re-lubricated everything.

Sadly, I forgot to remove the disconnector spring from the trigger to ensure it was installed properly. I'll pull everything apart again when I have a chance tomorrow to double-check this.

I noticed two things during the disassembly and reassembly. First, the safety really firmed up when I reinstalled everything. It wasn't necessarily loose prior to stripping the gun, but it didn't have the firm "click" that I'm used to with my other ARs. Although everything appeared okay when I stripped the gun, I've noticed the safety has really firmed up and now gives me a positive click when I change from safe to semi or vice versa.

The second thing I noticed may be a non-issue, but I don't have a basis for comparison until I can strip down another AR. The disconnector had some very rough machine marks on several parts of the radius. In the first picture, it almost looks like rust pitting, but the whole thing was blued and had plenty of lubricant out of the box.





Again, I'm not sure if this is expected or not (especially considering the gun has fired less than 20 rounds). The left and right sides of the disconnector are smooth as can be, it's just the radius that looks rough. Could this be part of the issue?

Dano, I ran out of time to complete the checks in your second post. I don't have a micrometer so measuring the gap between the hammer and disconnector sears is going to be difficult. I'll take a stab at it tomorrow if I can get some free time. I assume a sheet of paper should be about the right thickness?
Link Posted: 12/28/2011 8:25:26 PM EDT
[Last Edit: 12/28/2011 8:28:36 PM EDT by Dano523]
Link Posted: 1/4/2012 9:32:57 AM EDT
Thanks for your help, guys. I took the AR out yesterday and was able to get through a full mag without a single hiccup. It was snowing like crazy so I couldn't do much.

I'm still not sure what caused the problem, but the selector feels much better than it did previously. I've ordered a replacement disconnector and will post the requested detailed pics of the FCG once it arrives.

Thanks again guys!
Link Posted: 1/4/2012 1:21:51 PM EDT
I have had the same thing happen a time or two with new builds and cheap parts for friends. From playing with them each time the trigger would never reset and the hammer would stay locked to the disconnector. Sometimes it would go through 10-15 rounds and stick sometimes 2 or 3 mags and then stick. The hammer had a slight error in maching and was catching the disconnector and the lip would stick under it. Its hard to explain but when this did happen could you physically hear the trigger reset?

Take your lower. Pull the trigger and it should fire. Hold the trigger to the rear and push the hammer down. Let go of the trigger it should reset the hammer... If your hammer doesn't come off the disconnector it will feel just like a stuck primer under the fire control group. You may have fixed it already but if it does it again you have two choices buy a new hammer/disconnector. Or take a small file and lightly file the catch on the back of the hammer where it catches the disconnector. This is where you will see the slight machining error as well. I know a lot of people have chimed in and I hope it worked for you. But I have dealt with this a time or two and it was the stupid hammer about 90% of the time. The fact you pulled the bolt back and it fired again is why I am going this direction. It puts enough tension to decock the hammer from the disconnector..
Link Posted: 1/4/2012 2:02:28 PM EDT
Your disconnector does look like crap but the sudden onset of issues sounds like the carrier key is working loose. Torque the screws properly and stake.
Link Posted: 1/6/2012 12:19:32 PM EDT

Originally Posted By bloodsport2885:
Your disconnector does look like crap but the sudden onset of issues sounds like the carrier key is working loose. Torque the screws properly and stake.

I checked the gas key when I tore down the BCG. It was on tight and the screws were staked (albeit not an amazing stake job). There was no sign of any gas leak anywhere on the BCG, so I don't think the gas key is the issue.
Link Posted: 1/6/2012 12:33:47 PM EDT
My new disconnector arrived today. I ordered it from Brownells on Tuesday evening and paid for the cheapest postal shipping option, and still got a 36-hour turnaround. I also have to give Brownell's props for the best packing job ever. The box they shipped it in could have held several hundred of these bad boys. Amazon could learn a thing or two from them. Thanks Brownells!



While I had the FCG torn apart, I took a look at the disconnector spring and it appeared to be installed correctly. Is there a secret to removing this spring without bending it all to hell? I couldn't see an easy way to get it out, so I left it untouched. It's definitely locked in place as the assembly guide suggests.



The DPMS disconnector appears to be vastly superior to the Double Star disconnector in terms of smoothness and uniformity. The following picture shows the Double Star disco on the left and the DPMS disco on the right. On a side note, should I be concerned about the corrosion on the disco spring? You can see the gunk (copper corrosion?) left behind on the Double Star disco. I'm also surprised by how much wider the "tail" of the disconnector is. Note the difference in height from this angle:



Here is another shot of the Double Star disconnector on top of the DPMS disconnector, illustrating the differences in length for the "tail" behind the disconnector spring pocket. The corners of the tail are also much sharper on the Double Star.



One last shot of the disconnectors with the Double Star on top and the DPMS on the bottom...



I'm going to try to get out tonight and put a few rounds down range. I'll let you know how it goes. Thanks again guys!
Link Posted: 1/7/2012 7:45:16 PM EDT
I recently started having trigger issues with a Colt with less than 1,500 rounds through it. Susprisingly it was a bad disco spring. Anyone can let a substandard part slip through on occasion.
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