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Posted: 10/21/2007 10:06:36 PM EDT
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A friend's AR presents the following situation: A popular smith with hundreds of trigger jobs to his credit does a trigger job with a stone on an AR to lighten the pull to 4 lbs. The first 200 rounds ran perfectly, but then the rifle starts doubling at least once per magazine. Sometimes the firing pin hangs up on the hammer notch in a hammer follow/aborted doubling situation, sometimes the double fire occurs. Assume the hammer tail has been chopped a bit so hammer wedging is ruled out, and the hammer/disconnector distance during reset is nearly nothing (around 0.003"). Would the trigger group be toast even if a function check fails to show any problem? This gun (Colt) has the red colored disconnector spring, which is the stiffest I've ever seen. It is far stiffer than a standard Rock River disconnector spring for example. Is it possible that an AR can cycle fully without engaging the disconnector if the disconnector spring is super stiff? |
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The hammer's top edge of the firing pin contact pad dictates the amount/degree that the carrier passing over it at cycle will lower the hammer. The retention of the hammer at disco hold is dictated by the disco and hammer secondary sear conditions, and even the spring tension of the disco spring itself. The free gap spacing of the hammer's secondary sear to the disco primary sear primarily at rest dictates the hammer's reset/release distance when the trigger is being released to the at rest position, but less free gap also increase the tensioning force of the disco retention as well. Bottom line is if the disco release free gap is in the .001 to .003 range, and the hammer/trigger sears is still within working/correctly contacting condition (which passes not only a slow trigger reset test but the rest of the testing as well), your friend may just be bumping the trigger (pull, release and pulled again before the bolt relocks) as he is shooting the rifle. |
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Thanks for the quick reply Dano. I considered whether the whole affair was just unintentional bumping. I shoot a 3 lb Timney, and I've never bumped my AR. I did have a double when firing his though. The whole gun is very light (cav arms lower) and may be rebounding off the shoulder very quickly. After the first double, we made sure to concentrate on the follow through and had no malfunctions. Also, given the extremely stiff yet unimpeded travel of the disconnector, I doubt it is losing its grip on the hammer. After only 400 rounds total through the trigger group, it certainly isn't worn out. If it is a bump fire, it's an extremely fast one, as you note, given that the bolt isn't even all the way forward when the hammer locks up the firing pin. I'll see if installing some weights in the buttstock or using a tight hasty sling will prevent this embarassing situation in the future. |
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The strange part is with the .003 D/H free-gap, the trigger needs to return dam near to the front at-rest position before the hammer is going to be released from the disco. The only thing that comes to mind is that there is an over travel screw installed (including the take up set screw), and the creep after break may be set too minimal (not enough total trigger movement for an auto loader). Also, take a good look at the hammer's firing pin pad and just above the hammer disco hood on the backside. If the hammer was speed mod'd (tail removed), just how much of the top of pad was rounded over. It could be that as the carrier mated (now riding slightly higher in the receiver), it's not cocking the hammer all the way down to allow the disco to catch, or even worse, with a lightened hammer spring and no back tail, the hammer may be slamming off the top hook of the disco, bouncing the disco open at time of retention and the disco questionably retaining the hammer at times. |
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I believe the doubling trigger is a Colt, as it is large pin. There are no adjustment screws to set the free gap, and it's very small. The hammer appears to have a minimal amount (maybe 1/32") taken off the tail, probably just to insure against hammer wedging. Hammer top profile wasn't changed at all. I'll take a look at the overtravel after break. I saw that the smith had welded a bead on the very back of the trigger to ensure zero trigger movement when the selector is in the "safe" position. It's possible that this bead is limiting overtravel too much. Just about the only thing I can find that's different is the very strong disco spring. I'll replace it with a Rock River spring next time we get to the range. It occurred to me that there may just be enough slop in the carrier position to cycle the action and ride over the hammer, but not overcome the strong disco spring. It's an exposed firing pin carrier (colt half moon). Strangely enough, it never doubled on full power mil spec ammo, just Wolf, almost as if the full power ammo had the guts to overcome the disco spring and engage the disconnector and the Wolf did not. I just have to double check the geometry and see if it's possible to cycle the action without actually engaging the disconnector. |
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