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Posted: 11/13/2011 12:16:31 PM EDT
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My less than 1 year old build is having some issues. When at the range I've noticed since day 1 that some rounds don't fire. Lately it seems quite regular. I'll eject the round and check the casing and now it's to the point where the firing pin hasn't struck the case.
I take the upper apart and realize that when the pin is not protruding. The retaining pin is in place but the firing pin moves and I can't keep it in the proper place. This is my first AR so I don't know if this is a common issue or not. The firing pin appears straight and with no flaws. The retaining pin appears to be in the same condition. Should I just get a new retaining pin and see if that solves the problem? |
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This is screaming a cleaning problem. Over the age of the rifle, the face of the firing pin stop collar actual gets peened by the back of the bolt, and increases the firing pin protrusion over time (working spec for FP protrusion is .032 to .037 out the face of the bolt when the FP is fully seated against the back of the bolt.
I would stay that either the problem is that the hammer is grabbing the firing pin stop collar as the B/C moves forward, and may be one of the problem. With the bolt in the forward position on the carrier, take a look at the edge of the stop collar as the FP is pushed all the way forward; it should be flush or slightly lower than the carrier ramp. 
If you find that the edge of the collar is proud of the carrier ramp, then pull the FP, chuck it up in a drill, the spin file the collar down so it's flush with the ramp when the FP is pushed all the way forward. While you are at it, push the bolt all the way in, and notice that until the bolt has fully locked up (all the way inward), the back of the bolt does not protrude to allow the FP to reach full protusion (back of the carrier blocks the FP so you do not get an out of battery ignition). Next, lets address the chamber, If you have not been using a chamber brush by hand, then chances are the last of the chamber cut is packed full of crap that you failed to get out through normal cleanings. So to sum it up, either the hammer is catching the FP collar on the way forward to slow the forward stroke down, or the chamber is so packed at the last cut, then when the bolt tries to lock up, it's not doing so, and until the bolt fully locks up, the back of the bolt blocks the FP to prevent it from fully protruding to ignite the primer. As for the FP retainer pin, you can straighten it out and get some more life from it, or just buy a couple since they are so cheap, and just change it out. As for direction of the FP retaining pin, put the slot of the pin in line with the FP. It's wider that way, and will be less likely to bending that way. |
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My less than 1 year old build is having some issues. When at the range I've noticed since day 1 that some rounds don't fire. Lately it seems quite regular. I'll eject the round and check the casing and now it's to the point where the firing pin hasn't struck the case
Just in case it was incorrectly assembled. One of the more common reasons for a light primer strike/fail to fire is an incorrectly installed hammer spring. It is possible to put it in up-side-down and have it function intermittently, but there is inadequate spring pressure for reliable ignition. Check to see that the hammer spring is installed like the one in the photo. Note that the legs on the spring come from underneath and ride on top of the trigger pin. 
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This could be caused by several issues. Are you running a factory assembled rifle or lower receiver, or is it something that you built yourself? What brand of rifle or on the upper and lower receiver assemblies? What is the barrel length, which length of gas tube (carbine, rifle, or one of the mid-lengths), carbine or full length stock, and which buffer is in the rifle? Is it a stock firing pin, or something after-market like the Titanium Nitride coated firing pins? Are the trigger components factory, or something after-market like a match trigger or lighter springs? About how many rounds through the weapon? What types of ammunition are you using, and have you tried different types? Have there been any custom modifications, or accessories added to the rifle like a compensator, muzzle brake, or suppressor? Is it happening when you manually cycle the bolt carrier assembly to charge a round into the chamber, or does it happen when you are firing the gun and the rifle through it cycling loads the new round into the chamber.
I recommend always starting with a good cleaning of the rifle. Thoroughly clean the firing pin, especially the face (front) of the large head where carbon can build up, and back of the bolt tail (where the firing pin goes into the bolt). Next using a good solvent (I like Slip2000 725 Cleaner/Degreaser) and a pipe cleaner (brissles if you can find them), thoroughly clean the firing pin channel on the bolt assembly. Then lubricate anywhere the metal rubs metal like the bolt carrier assembly and 7-bolt lugs are a few key places (Slip2000 Extreme Weapons Lubricant or the lubricant of your choosing). Reassemble and try shooting the rifle again and see what happens. As already posted above, make sure your hammer spring is put in so that it is winding tighter when the hammer cocks back, and make sure the hammer spring legs are sitting on top of the trigger pin (not under it). As already mentioned, the firing pin could be out of spec for measurement, especially if it is worn or damaged on the tip. If you have access to a firing pin protrusion gauge or micrometer, then measure it to make sure you are within proper spec. Another possibility is bolt bounce, of which I would then replace the buffer/action spring, and if necessary you may need a heavier buffer. CY6 Greg Sullivan "Sully" SLR15 Rifles TheDefensiveEdge.com (763) 712-0123 |
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As already mentioned, the firing pin could be out of spec for measurement, especially if it is worn or damaged on the tip. If you have access to a firing pin protrusion gauge or micrometer, then measure it to make sure you are within proper spec.
PN 7799735 to check for proper firing pin protrusion (minimum 0.028 in. (0.07 cm) - maximum 0.036 in. (0.09 cm)). |
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