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Posted: 8/3/2013 11:15:46 AM EDT
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I'm usually pretty good at solving problems with the AR platform but I'm stumped.
I built my 9mm SBR a few years ago. It uses Spikes upper/lower receivers and uses a spikes BCG. The gun has about 3k rounds thru it. The gun was flawless for along time when about 6 months ago it would fire , chamber the next round but would not fire again with out manually cycling the charging handle. After the first round was fired the trigger felt like it was not reset. So I took it apart to find the problem. Upon removing the Timney trigger I found that it was broken. I sent the trigger back to Timney and they fixed it free of charge. I reinstall the trigger and the gun was good for a awhile. Friday I took it out to shoot and it starts doing the same thing again. It would shoot the first round, chamber the next round but would not fire again with out cycling the charging handle. This morning I removed the Timney. I can't see anything wrong with it. Instead of putting it back in the gun I installed a new Geissele SD3G in it. The gun still is doing the same thing. Upon seeing this I decided to give the BCG a look. I removed the firing pin and cleaned everything. Put it back together with no luck. I am really stumped and need some advice. I did notice the BCG has some wear marks on it where it contacts the hammer. Can a BCG get worn enough to not reset the trigger all the way? I want to say my guns problem is that the hammer is not being reset 100%. Here's a few picture of the wear marks- 
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It sounds like when you fire, the bolt carrier is comming back far enough to eject the fired case and pick up a new round out of the magazine, but not cock the hammer.
Shoot the rifle, then if it will not shoot again, pointing it in a safe direction open up the upper, and look at the hammer in the lower reciever. If it is not cocked, that is the problem... |
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Quoted:
It sounds like when you fire, the bolt carrier is comming back far enough to eject the fired case and pick up a new round out of the magazine, but not cock the hammer. Shoot the rifle, then if it will not shoot again, pointing it in a safe direction open up the upper, and look at the hammer in the lower reciever. If it is not cocked, that is the problem... The problem is the between the BCG and the hammer. I can see that the Geissele SD3G isn't tall enough to fully engage the 9mm carrier. I installed a GI trigger this afternoon and the gun is flawless. I understand why the Geissele has issues but I'm still perplexed as to why it quit working with the Timney that has been in it for years. |
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I have the same dilemma.
I built an "a la-carte" supressed SBR and can not get the weapon to fire a follow-up shot. I used a CMMG ramped bolt with the Geiselle SD-3G as well. Manual charging,... the shot fires and a round is fed. However, the hammer is not reset and in order to get another shot, I have to manually charge the weapon again, (and pick up the good round that was ejected). I swapped hammer/trigger groups between my AR-15 and my AR-9. In the AR-15, both the standard AR hammer/trigger (it is a notched hammer) AND the Geisselle SD-3G work as advertised. BUT, in the AR-9, neither hammer/trigger group works. (Manual charge only). I typed CMMG and the tech said they've not had any problems with their bolts. After a few back and forth e-mails it came down to "they'd have to see the set up in order to figure out a fix". I also typed Geissele and was told to hold the stock tighter into my shoulder and not "feather" the trigger. I guess I didn't fully describe the same symptoms you've had, nor the troubleshooting I've tried. But I guarantee, the weapon is tight in my shoulder and I'm NOT feathering the trigger, I'm holding it back frimly waiting for a successful hammer reset that never comes. This past weekend I had a shop TIG some material onto the bolt in hopes of taking out the ramp and allowing the bolt to push the hammer down far enough to catch. I haven't fired it yet, but a manual test STILL shows the same problem....good first shot (hammer fall), but no reset. My next thought is to have some material TIGged onto the tip of the hammer. I see your GI trigger fixed your problem. I hate the idea of having to swap that sweet Geiselle hammer/trigger for a GI set up,... and do ??? with the SD-3G? Has anyone ever added material to a Geiselle set-up and gotten it to work? Thanks, Flash |
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Lets start with the ramped bolt, and work backwards.
Out the gate, the 9mm is a blow bay action, so if you ramp the bolt, then since you pulled the geomitry of the flat back bolt to hold it forward a tad longer, then you need to gain this loss of hammer spring tension to the bolt, back at the recoil spring. So step one, change the standard recoil spring out with extra tension recoil spring. Not only will this keep the bolt forward longer at ignition, but will slow down the stroke to semi normal as well. Next comes the issue of the bottom of the carrier getting the hammer back down far enough for the disco to catch the hammer disco sear. Most FCG lower the hammer down to be caught by the disco, but as stated from the start, if the cycle speed is too fast (from the ramped bolt, and even compounded by changing out the hammer spring to less tension unit), the although the hammer may be lowered back down far enough for the disco to catch the hammer, the cycle speed is so fast, the disco may not have time to recovered to catch it. Also to point out here, since a heaver buffer still may have floating free weights, trying to use a heavy buffer may not solve the problem since the weight in the buffer may be rearward, allowing the bolt/buffer to move a good distances before the added buffer weight is applied. To bottom line it, Blow back AR rifle love to snap pins. The reason why is the blow back action moves cycles quickly/brutally, and the hammer is slung back against the top of the trigger to cause the stress (even un-ramped). Ramping does help to remove some of that rougher hammer sling, but only if you go with stronger springs since you just remove the flat angle of the hammer against the back of the bolt that was slowing the normal unlock down to begin with. Simply, most of your current problem is the ramping to try to save the pins, combined with reduced hammer spring as well, and you never compensated either of these will heaver mass or stronger recoil spring tension to equal everything back. So I'm not saying that the Jewel trigger will not work, but that since it uses a reduced tension hammer spring, then you have to equal the works back out with a stronger recoil spring, and maybe even a heaver than 9mm buffer as well. Once you have the stroke corrected, then this should give the disco enough time to recover, and retain the hammer once it over cocked past the disco spring. |
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Quoted:
Lets start with the ramped bolt, and work backwards. Out the gate, the 9mm is a blow bay action, so if you ramp the bolt, then since you pulled the geomitry of the flat back bolt to hold it forward a tad longer, then you need to gain this loss of hammer spring tension to the bolt, back at the recoil spring. So step one, change the standard recoil spring out with extra tension recoil spring. Not only will this keep the bolt forward longer at ignition, but will slow down the stroke to semi normal as well. Next comes the issue of the bottom of the carrier getting the hammer back down far enough for the disco to catch the hammer disco sear. Most FCG lower the hammer down to be caught by the disco, but as stated from the start, if the cycle speed is too fast (from the ramped bolt, and even compounded by changing out the hammer spring to less tension unit), the although the hammer may be lowered back down far enough for the disco to catch the hammer, the cycle speed is so fast, the disco may not have time to recovered to catch it. Also to point out here, since a heaver buffer still may have floating free weights, trying to use a heavy buffer may not solve the problem since the weight in the buffer may be rearward, allowing the bolt/buffer to move a good distances before the added buffer weight is applied. To bottom line it, Blow back AR rifle love to snap pins. The reason why is the blow back action moves cycles quickly/brutally, and the hammer is slung back against the top of the trigger to cause the stress (even un-ramped). Ramping does help to remove some of that rougher hammer sling, but only if you go with stronger springs since you just remove the flat angle of the hammer against the back of the bolt that was slowing the normal unlock down to begin with. Simply, most of your current problem is the ramping to try to save the pins, combined with reduced hammer spring as well, and you never compensated either of these will heaver mass or stronger recoil spring tension to equal everything back. So I'm not saying that the Jewel trigger will not work, but that since it uses a reduced tension hammer spring, then you have to equal the works back out with a stronger recoil spring, and maybe even a heaver than 9mm buffer as well. Once you have the stroke corrected, then this should give the disco enough time to recover, and retain the hammer once it over cocked past the disco spring. I never had a Jewel trigger, I had a Timney trigger and fired 1000's of rounds with out 1 issue. The timney did break and get fixed my the manufacturer for free. I reinstalled it, shoot a few hundred rounds and it quit functioning again. I have removed the timney and can find no issues with it. I understand now that the Geissele doesn't work with ramped BCG's. |
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If you compare the bottom of a 9mm bolt (even ramped), to a 223 bolt, the bottom of both are the same depth.
The difference between the two is the speed in which the 9mm comes back instead. So unless you have something out of spec on the lower or upper receiver, then any AR hammer should be lowered enough for the back hammer sear to get under the disco sear line by the bottom of the bolt, and the question at hand is if the Cycle speed is so great, that the disco is not coming back up closed fast enough to catch the hammer instead. Again, by ramping the 9mm bolt, you lose the holding closed tension that the hammer used to have against the back of the square edge bolt that help keep the bolt foward at ignition, and this has to be compensated for with both the ramped bolt and (bolt now coming back a lot faster at ignition) or your going to tear up FCGs/pins pretty quickly. As for NM type FCG's, all pretty much use some sort of reduced tension hammer spring to get a lighter trigger pull, so in the case of a Ramped 9mm bolt, using a stronger recoil spring to compensate lose of tension that is holding the bolt forward at ignition becomes the norm. If the stronger spring tension alone does not do the trick to slow the bolt speed back down so it not killing FCG parts, then the standard 9mm buffer is swapped out to a even heaver forward weight kept type of buffer instead. Simply put, ramping the bolt helps to band aid the pin breakage problem to a degree, but only if you compensate for lost tension that the non ramped bolt use to apply to hold the bolt forward. |
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Referencing my post above...
I swapped the Geiselle SD-3G FCG with a DPMS round-hammer AR-15 set up. So far, with a manual charge, everything works as it should. I'm trying to work in a range day to see if live rounds will cycle the system like they're supposed to. Thanks for the tips. Flash |
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