Mrs.CMO's rifle shot about 200rds yesterday and she had about 4 light primer strikes. Federal XM193 was used in her and my rifle. I had no issues (Smith & Wesson M&P15).  So I don't think it was the ammo, because I shout about 120rds from the same batch of ammo.  This is the first time she has had this issue.  She has probably about 3K rounds in 3 years maybe.

Her rifle was fully cleaned prior to shooting (BCG, chamber end, barrel, etc...).  1 guy at the range mentioned I need to oil the firing pin, which oddly enough, I didn't do this time, but I normally do.  IDK this guy really, so IDK how accurate he is.  Everything else was oiled normally.

I googled and stuff that came up was issues due to a new trigger installed wrong, new BCG, new buffer tube, etc... None of these apply since her rifle was g2g all this time.  I found 1 post that mentioned to measure how much the firing pin protrudes, but I do not have the tool to do that. 

When 1st building the rifle, the Elf trigger she had, had this issue. But it was every round had a light primer strike. We contacted Elf and Brian (owner) was the one who picked up the phone and he sent another one.  It has worked fine all these years.

Thoughts?  And could it be the firing pin oiling?

Don't know anything about the Elf trigger, but I know if you (or someone) put the hammer spring in backwards, you can get light primer strikes.  That's pretty common.  Look up the Geissele video in installing the SSA trigger.  Looks like the Elf is drop in trigger, so shouldn't be that. 

Never heard of oiling firing pin. 

Recommend trying to isolate variables.  Make a list of what was changed then go back to known/trusted parts one by one and see if that helps.  Check to see if things drift/move.  Changing of springs can often lead to unintended issues. 

Don't know anything about the Elf trigger, but I know if you (or someone) put the hammer spring in backwards, you can get light primer strikes.  That's pretty common.  Look up the Geissele video in installing the SSA trigger.  Looks like the Elf is drop in trigger, so shouldn't be that. 

Never heard of oiling firing pin. 

Recommend trying to isolate variables.  Make a list of what was changed then go back to known/trusted parts one by one and see if that helps.  Check to see if things drift/move.  Changing of springs can often lead to unintended issues.

Haven't changed anything since the gun was built.  I didn't get to clean her yet because we got home late, but I will in the next few days.  I will double check to make sure nothing came loose.

1 post mentioned make sure the firing pin hole isn't gunked up with stuff, which I would have caught when assembling the BCG after cleaning.

Haven't changed anything since the gun was built.  I didn't get to clean her yet because we got home late, but I will in the next few days.  I will double check to make sure nothing came loose.

1 post mentioned make sure the firing pin hole isn't gunked up with stuff, which I would have caught when assembling the BCG after cleaning.

Check the firing pin channel when cleaning, along with the firing pin hole in the bolt. 

How long since you cleaned the chamber?  Do you shoot combination of steel cased and brass? 

One cheap check along the way is to try a new firing pin.  That's not common, but cheap to take that off the list. 

Haven't changed anything since the gun was built.  I didn't get to clean her yet because we got home late, but I will in the next few days.  I will double check to make sure nothing came loose.

1 post mentioned make sure the firing pin hole isn't gunked up with stuff, which I would have caught when assembling the BCG after cleaning.

if it only started this range trip after cleaning, it's most likely a reassembly error...

Check the firing pin channel when cleaning, along with the firing pin hole in the bolt. 

How long since you cleaned the chamber?  Do you shoot combination of steel cased and brass? 

One cheap check along the way is to try a new firing pin.  That's not common, but cheap to take that off the list.

I clean the chamber with the brush after every shoot. So 3 weeks ago would be the last time I did it with the chamber cleaner brush.

No steel ammo used.

I do have a spare firing pin somewhere in the home.  I gotta look.

if it only started this range trip after cleaning, it's most likely a reassembly error...

Wouldn't that cause malfunctions like all the time, not just 4 of 200rds?

if it only started this range trip after cleaning, it's most likely a reassembly error...

Wow. . . #harshinghisvibe 

Intermittent light primer strikes are difficult to figure out.  I can't think of a reassembly error that could lead to that though.  Can only think of some rounds not going back into battery fully for intermittent.  Castle nut backing out is common for builds. 

I clean the chamber with the brush after every shoot. So 3 weeks ago would be the last time I did it with the chamber cleaner brush.

SNIP

 

 

Is this not supposed to be done this often? I do notice when inserting and pulling the chamber brush out, it's much more difficult to do than on my M&P15.  Mrs. CMO has a Faxon pencil barrel.

Wouldn't that cause malfunctions like all the time, not just 4 of 200rds?

true...

Weak hammer strike most likely.  Oiling the firing pin sounds like a good way to attract fouling and screw things up.   Gunk around the firing pin could cause slam fires.

Keep it dry and make sure the pin slides freely.

Examine fired cases to see if the pin indentation is consistent.

Only time I had this was trying to use the light hammer spring in a JP trigger.  Maybe 1 0ut or 6-7 would not fire first strike but would if run through again.  Would not fire mil-surp 7.62mm ammo reliably, but .308 Win just fine.    CMC triggers do so with authority and a lighter trigger pull..

Might have been a batch of ammo with hard primers?

I'm pretty sure elftmann triggers are cassette style so cant really install wrong.

As for measuring the protrusion... Take the bolt out of the carrier and put it in a vise. Grab a caliper and rest it against the firing pin hole. (You gotta use the tail end of the caliper.. Meaning when you slide it to take measurements you'll see a rod on the opposite end of it stick out.)

So you rest the non moving part of the caliper (tail end side) against the lugs then push the part that moves against the firing pin hole. Then memorize or copy that number. With this in place...
push the firing pin in and you'll get your amount of protrusion by subtracting the first number by whatever the caliper says afterwards.

Id do it a couple of times to be sure.

I forgot what the exact amount is but Google it and you'll see.

As for oiling firing pin you can put a light film.

Thanks for all the input. Due to the lack of tools, I'm gonna do the following:

1) Double check the BCG prior to cleaning to see if anything is blocking the firing pin
2) Examine the firing pin, compare it to mine, as I don't know what I'm really looking at without a side by side.
3) Use diff ammo for the next skill builder. Since this isn't as bad as every round getting a light strike, it would be very expensive to mag dump 200 more rounds.  And it wasn't often enough to cause a huge problem or a huge concern if used for HD.

Also could it be the bolt not seating all the way in the chamber end?  Like I mentioned when cleaning with the chamber brush, hers is very tight compared to mine. So maybe some kind of deformity/wear where the 2 meet that's preventing the perfect fit?

Thanks for all the input. Due to the lack of tools, I'm gonna do the following:

1) Double check the BCG prior to cleaning to see if anything is blocking the firing pin
2) Examine the firing pin, compare it to mine, as I don't know what I'm really looking at without a side by side.
3) Use diff ammo for the next skill builder. Since this isn't as bad as every round getting a light strike, it would be very expensive to mag dump 200 more rounds.  And it wasn't often enough to cause a huge problem or a huge concern if used for HD.

Also could it be the bolt not seating all the way in the chamber end?  Like I mentioned when cleaning with the chamber brush, hers is very tight compared to mine. So maybe some kind of deformity/wear where the 2 meet that's preventing the perfect fit?

Yes, it could be, which is why I asked about cleaning the chamber.  But you said you clean it all the time.  To clarify, how do you brush the chamber?  In a twist in/out motion?  Or like a bore brush?  Maybe all my ARs have same chamber, but I can't tell a difference.  But yes, something causing the bolt to be slightly out of battery could lead to light primer strikes.  I have just never had it happen on any of my guns.  I've seen it on some guns where they were shooting a mix of steel and brass. 

I have an extra caliper.  LMK if you want to try it.

To clarify on firing pin.  Yeah, slight coating of oil, or at least that's what I do.  I've never heard of no oil causing problems, nor excessive lube.  I prefer my BCG wet over dry.  Cue sukebe comment from macsak 

Yes, it could be, which is why I asked about cleaning the chamber.  But you said you clean it all the time.  To clarify, how do you brush the chamber?  In a twist in/out motion?  Or like a bore brush?  Maybe all my ARs have same chamber, but I can't tell a difference.  But yes, something causing the bolt to be slightly out of battery could lead to light primer strikes.  I have just never had it happen on any of my guns.  I've seen it on some guns where they were shooting a mix of steel and brass. 

I have an extra caliper.  LMK if you want to try it.

To clarify on firing pin.  Yeah, slight coating of oil, or at least that's what I do.  I've never heard of no oil causing problems, nor excessive lube.  I prefer my BCG wet over dry.  Cue sukebe comment from macsak 

I'm unable to do any twisting.  So repeated in and out motion. Then use a q-tip with solvent on it until it's not dark black anymore. This takes about 5 Q-tips for each rifle.  I'm pretty sure that there is no carbon in the Chamber because compared to a carbon build up on the side of the bolt that's just above the gas rings and not the star shape side, the Q-tip used on hard carbon still comes out black.  For this end of the bolt, I use my dental pic.  So this leads me to believe that there is no hard carbon stuck inside the chamber end.  LMK if I'm wrong.

I'm unable to do any twisting.  So repeated in and out motion. Then use a q-tip with solvent on it until it's not dark black anymore. This takes about 5 Q-tips for each rifle.  I'm pretty sure that there is no carbon in the Chamber because compared to a carbon build up on the side of the bolt that's just above the gas rings and not the star shape side, the Q-tip used on hard carbon still comes out black.  For this end of the bolt, I use my dental pic.  So this leads me to believe that there is no hard carbon stuck inside the chamber end.  LMK if I'm wrong.

Chamber and bolt head are different zones.

Is this the chamber brush you are using? 

https://www.amazon.com/Otis-Technology-FG-367-TP-Chamber-Brush/dp/B000E59A8E

Q-tip and dental picks?  Bruh. . . 

picks

Chamber and bolt head are different zones.

Is this the chamber brush you are using? 

https://www.amazon.com/Otis-Technology-FG-367-TP-Chamber-Brush/dp/B000E59A8E

Q-tip and dental picks?  Bruh. . . 

Idk bout the chamber. I mean if its dirty then the round would not seat fully therefore it would be more of a peirced rimer than a light primer strike. Cause the round would not seat fully into the chamber.. Pushed more back towards the rear of the rifle.

If the bolt locks in the barrel extension id say it's good to go (cleanliness wise). Did ya headspace this build? It couldve been on the high end from the beginning. And now thousands of rounds later it's getting more sloppy?

Check the firing pin (circular part) where it hits the tail of the bolt.
As well as check the tail of the bolt itself. Look for any uniform abnormalities.  Use a straight edge or a known flat surface and stand the bolt up by the end.

Got a question CMO..

Was the bcg all the way home when you had light primer strikes?

Like if you pulled the trigger, then click... Did you look to see if the bcg was locked into the extension? Did ya tap the forward assist then rack it?

Were you able to pick up spent brass and compare the primers (the ones that went bang vs the LPS)?

Do ya have a set of calipers? Can do a OAL of firing pins.
.029-.036 is the range for a good firing pin.