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Light primer strikes

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  • Murfpcola

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    I have a 16" carbine gas 7.62x39 upper on a Aero Precision lower with a carbine buffer. I have had it about 2 years now. I had 10 out of the first 80 rounds with light primer strikes so I added an enhanced firing pin and it has been flawless up to 260 rounds. I have actually kept the exact round count since I knew going in that these could be problematic. I have used 10, 20 and 30 round C P Defense mags without issue as well as a couple of 30 round ASC mags also without a hiccup.

    A couple weeks ago I picked up 4 more 20 round CP Defense mags as well as 2 30 round ASC mags, all new in wrapper. I was shooting at some family land and since I had this rifle with me I figured I would try out the new magazines. I only had one 20 round box of Wolf ammo so I loaded up one of the ASC mags and started to shoot. A few rounds in and I started having light primer strikes.

    Here are a few things I tried. I swapped lowers to one with a standard mil-spec trigger. I tried 4 of the 6 brand new mags. I would run the light strike rounds back through and some would ignite and some not. I could drop the mag and pull the charging handle back far enough to cock the hammer but not eject the round and it would fire every time. If I dropped a round into an open chamber with no mag it would fire every time. I could use the bolt release and tap he forward assist a couple times an have some light strikes. If I did the half cock try with a mag inserted I had light strikes. The ammo was a brand new package that was in good condition.

    I only had the one 20 round box with me that day and I probably had at least 15 light strikes but all eventually fired so no duds. I also did not have any of my known mags with me. The rifle was filthy as it had never been thoroughly cleaned since new, only light wiping and then oiling as well as a patch down the barrel. No scrubbing. When I got home I disassembled the BCG and did some wiping and scraping on the bolt. I also sprayed some carb cleaner in the barrel extension. I figured that would be enough to get it running againn but late one evening I got a chance to run five rounds through it and the third round failed to ignite the first time.

    Do you guys think it could possibly still be dirty? I guess I need to get out some solvent and brushes. Will a .223 AR chamber brush do much to help clean my lockinglug area? I will post pics of some "scratches" on the bottom of the bolt carrier that are concerning. You can barely feel them but I know they are not normal". Also there is some marks on the carrier right near where the firing pin is.

    Any input is appreciated and thanks for reading this far. IMG_4927.JPG
     

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    Murfpcola

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    A couple more photos. I need to take some time and scrub out the firing pin cavity IMG_4928.JPG .
     

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    Murfpcola

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    The nastiness on the paper towels was just from wiping down the BCG IMG_4926.JPG
     

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    Droshki

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    Unless there's something egregiously wrong with the way the upper is locking up the round, I'd have to say (sounds like you've already got the longer firing pin) that the issue is with hammer spring strength. But if you've swapped out lowers with no improvement...? Then that rules that out?

    You did not specify age and pedigree of the ammunition? Also what's the story on the upper?

    FWIW, I've got 2 Palmetto 7.62 uppers that run like champs. Only problem I've ever had was trying to run the light (yellow) springs in a lower. Commie ammo just ain't made for that.
     

    Fear21

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    I don’t suppose you’re using Tula ammo? It’s known to be finicky in ARs due to the primers they use. What all have you ran through it?

    *edit: nevermind, missed the “Wolf” part of the post.
     

    Murfpcola

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    I may have just figured it out. If you look in one of the grooves on the bottom of the carrier, where the feed lips ride, you can see what almost appears to be a crack. It was a hard substance that did not wipe off but I did remove with a pick. Maybe that was enough to slow down the bolt and cause it to not seat as deep as it should? Maybe wishful thinking? If anyone has a new RedX brand enhanced firing pin could you mic the length for me? Maybe mine has become shorter with use.
     

    wildrider666

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    Non expert opinion on possible causes.

    Other the the crud, the New mags are the only thing that changed, making a reliable gun: intermittent. New mags even from the same Mfg can still be out of spec to function in your specific firearm. New mags have stiff springs possibly creating additional friction slowing or preventing a hard tight lockup that you get everytime without them installed. Is the bolt catch being raised slightly by the new Mags causing friction or maybe the mags seat too deeply and rub the bolt carrier. Did you try it again with the old mags? if old mags run great, you can try different new mags or modify what you have. If not the fault of the new mags, we need to start from scratch.

    1. That pig is filthy. Obviously, detailed thorough (muzzle to buffer) cleaning it is not you favorite thing but it must be done to eliminate to see if it eliminates the problem. You do need a AR chamber brush that will get into the barrel locking lugs. When clean you can inspect for defects. Using a proper lubricant, apply appropriately where required and not excessively. Now you can functionally trouble shoot the gun. The cleaning may have resolved the issue.

    2. Sometimes we encounter bad lots of ammo or primers that are too hard for our worn springs. I believe commie mfg ammo generally have much harder primers. Take several brands and feed them with a known good (clean) Mag. If the problem occurs with one lot of ammo and not the others: you found the base problem, don't use that ammo. If you want to use the ammo (you said it will fire under certain conditions of how its loaded into the chamber. The ammo might be slightly out of spec.

    3. Headspace could be off. Just because rounds chamber does not mean its right. If its sloppy the case is not fully seated in the chamber. The firing pin uses speed and mass to ignite a primer. If energy is lost pushing the case forward the strike is weakened. Compairing fired and hit but not detonated primers can be misleading as on the instant of firing the case/primer is pressured against the bolt face and still kinetiicly forward firing pin. Only a primer only fired case will show the true strike depth. Go-No Go gauge will answer this Question.

    4. Weak Srings. I know you changed the lower but that does not mean it will overcome current resistance in the upper every time, especially a filthy upper. Test the now clean Upper with the "other" Lower. Test ammo that works and from the failed lot (virgin primer, not a dimpled prior attempted cartridge). If the second Lower works without failures: swap buffer/spring into primary Lower:test. If Primary still fails its probably the Hammer Spring or: next

    5. Drag/binding causing bolt/carrier to slow down slightly. You mentioned the scratches on the bolt carrier, any corresponding marks in the Lower?Make sure the bolt release catch fully seats (not engaged) under spring tension. We have fwd assists to help seat the bolt but you can't generate the same force as properly slamming bolt into battery. Its for hand impact, don't hit it with a mallet or hammer. Slide the bolt carrier group (BCG) inside the just the upper, from entry to lockup, anything unusuat? Remove the Buffer and Spring, assemble upper/lower: Silde the BCG full throw in both directions, repeat with a good mag (with the follower and spring removed). Remove bolt from carrier observe via mag well: gently slide carrier observe/feel gas key engagement with gas tube.

    Hope you get it figured out.
     

    Murfpcola

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    I dug this thing out of the safe again and scrubbed it pretty good and tried shooting a box of ammo the other day and still no luck. I ordered another RedX enhanced firing pin hoping that even though mine was longer than standard, it had gotten shorter with use. It had not. The new one and old one are the same length.

    It took me about a week but finally found my stash of mags. I have not tried all the things Wildrider suggested but I need to make the time. I have also not checked the headspace with go/no go gauges. Other than that I think I just need to go buy some new ammo and load up everything I got for this thing and go play at JJ’s one weekend.
     
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    Rebel_Rider1969

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    I used an h2 buffer and the psa extended firing pin. Pig break for more gas 7 inch barrel. Runs great. Asc and believe it or not the pro mags. 10 rounds work out of the 20rd gi mags. 556. I think it helps to use the heavier buffer as you are trying to move a cartridge twice the weight of the 556. I'm no expert just what works for mine.
     

    Murfpcola

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    Well I came over to my family’s property where we have a range setup and tried out the new firing pin. I shot 56 rounds through about 10 different magazines without a single issue. The ammo was five different kinds. Tula fmj and hp, Wolf fmj, Monarch fmj and hp. Just like my typical poor planning I did not bring the old firing pin along or I would have re-installed it to see if the light strikes come back. I plan to get a friend of a friend machinist to mic the two pins to find out the difference.

    I know the 7.62x39 ARs are known to be problematic but I am somewhat disappointed with the first pin lasting less than 300 rounds.
     

    USAF Sarge

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    That's how dirty it is from shooting 20 rounds? Either that's some dirty as hell ammo, or the gun needs to be stripped and clean. I've spent hours on the range and put close to 200 rounds through an AR and never had it look that dirty.

    Heck even the half-arsed cleaned ones that we got from CATM for our refresher training weren't that dirty.

    Not trying to sound harsh, just making an observation.
     

    USAF Sarge

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    The pics of the dirty bcg was after about 250 rounds of steel case. I agree that it was a dirty pig

    Ah okay, that said I apologize and retract my observations then. The way I read the first post was it appeared that was after 20 rounds.

    Could you be getting debris in the firing pin channel? Or could it be that the carbon buildup is causing issues during shooting? IE getting to much carbon buildup while shooting. I know my AR had light strikes once, I apparently had buildup on the pin, previous owner didn't clean the pin properly. I clean it like I was taught in the AF, take a brass brush to the pin and scrub it, haven't had any issues since.

    Again my apologies for misreading the original post........


    ETA: Just thought of another possible cause, too much lube or wrong lube, causing more carbon to get caught instead of being blown away, leading to excessive build up and fouling.

    My son a former USMC firearms coach also added this.....

    Also with the light strikes, it could be the gas tube not getting enough pressure from being dirty and not letting the trigger reset all the way causing a poor strike. Also I'd look at the trigger more than the firing pin.
     
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    Murfpcola

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    Thanks Sarge. When this problem first occurred (which was over a year ago) I switched to a completely different lower that day with no change so it should have nothing to do with trigger or buffer assembly. This problem appeared out of the blue on a day I was trying out 4 new mags but did not have my proven mags with me. The only thing I have done since the dirty pics were taken is clean the gun, clean it again a time or two and replace the firing pin. It is possible that during some of the cleaning that I dislodged some carbon that was keeping the firing pin from going fully forward.
     

    Murfpcola

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    I got the two firing pins back out and started measuring them again with my calipers. The new pin still seems to measure the same length but seems to be smaller diameter. When I set these calipers to the diameter of the old pin. The new pin seems to spin much more freely. Not enough that I can see a change in measurement on the caliper but it is different. Maybe that difference in thickness along with carbon build up in my bolt was enough to slow down the firing pin speed causing primers to not ignite?

    Any harm in soaking my bolt in a bottle of Hoppe’s 9 overnight?


    Thanks 568CF437-7F20-4B64-AC16-AD182BD27568.jpg
     

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    Murfpcola

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    The saga continues...

    I had a friend measure both my pins with digital calipers and they are the same identical length but the new pin is 1/1000 smaller in diameter on both the small and large section.

    I did some more shooting yesterday and the light primer strikes started again with both firing pins. On round number 64 it clicked so I went to eject the round and the charging handle would not come back all the way. I looked in the chamber and there was not a round in it. I separated the upper and lower and heard a rattle. When I removed the buffer I saw that it had come apart. The polymer end was threaded in instead of pinned.
    71A2889C-86DC-4126-A660-A7781A5DAAE5.jpg
    I am not sure exactly where this buffer came from. The three options are Aero Precision, PSA, or one that came with a Shockwave Blade assembles from a questionable dealer in central/south FL from a few years ago. This is not in a KAK tube but may have gotten swapped around at some point.

    I installed another carbine buffer I had and shot another 20 rounds and still had light strikes from both pins in several different magazines

    DA749773-88ED-472A-90F8-CBC58612A98C.jpg
    I plan to try some of the things suggested earlier and also need to get with someone with a set of “go/no go” gauges
    Total round count on the rifle is now at 434.
     

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    Rebel_Rider1969

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    I'd guess shockwave buffer. If you have the h2 buffer and spring I'd try that. The x39 has way more recoil than the 5.56. This is the pin I used, solved my issue.
    20191029_130418_1572382044771.jpg
     

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    wildrider666

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    Post scrub down & buffer fix (coincidental failure IMHO), any misfire/light strikes with U.S. or other BRASS cartridges? Just Web search tulammo hard primers. There has been debate on firing pin strike force in AKs vs ARs platforms but there are reported primer strike failure to ignite with Tulammo pistol cartridge cartridges too.

    Take apart the bolt/carrier. Insert the firing pin into the bolt head, press firmly and see how much protrudes. That's potential travel and should exceed what you see on primer strikes. Now, how deep is the dent in the primer of non firing cartridges? How deep on cartridges that fired?

    If the depth is different: Hard primers.
    If the depth is the same it could be the difference between BOXER and BERDAN primer construction necessitates a harder/deeper strike:
    Hammer spring.
     

    FrommerStop

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    When one eyeballs firing pin protrusion on an AK it looks considerably more than on an AR. It is protrusion that you must measure and not total length. I have an AR or two that I am setting up in 7.62x39. In the first one I put an extra power hammer spring an a longer firing in it an for hundred or so it working well. Just got another up on a different lower and maybe I need to do the same.

    One can increase the protrusion of your firing pin. I think I will do more than just + 0.001

    IMG_20140407_082455.jpg
     

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