Today was really frustrating, as I put very few rounds through my new-to-me dedicated .22 LR upper.  I don't want to list all the problems it looks like I need to solve in one thread, so I will cover the most important one here, and make new threads as needed.  The problem - Click, but no Bang.

Data: I built the lower, this was the first time it has been fired.  Aero M4E1 Lower, CMMG LPK, Larue MBT-2S trigger.  The upper I bought used: CMMG bolt and collar, collar attached to a CMMG barrel (16" 1:16 twist), CMMG charging handle for .22LR. Mags were new and used BDM, various sizes and feed lip material.  3-types of ammo, Federal bulk pack, 36 grain 1280 fps CPHP; Winchester Super X 40 gr CPRN 1300 fps; and CCI "standard" velocity, forget the projectile weight, but 1070 fps.  This is way more info than needed I think, because I'm pretty certain the issue is firing pin related.

The very first round chambered, fired and ejected. Bolt locked back on the empty mag.  Same mag, one round, which chambered and fired and ejected, bolt locked back again.  I then put two rounds in the mag, and the next round chambered but did not fire. Racked the CH, the unfired round ejected, next round chambered but failed to fire.  This happened several more times, loading between 3 and 9 rounds in the 10-round mag.  The ammo is old, so I changed ammo to something bought last month, still no bang on several different rounds. I changed ammo again to something else bought last month, no change.  So I checked the unfired rounds. TA DA! No strike marks.  None.

This is my very first experience with .22LR in an AR, so all I knew to do was pull out the bolt and look at the firing pin. (Remember this is a used upper, so I have cannot be certain of the round count, which was said to be <1000, IIRC). I oiled the firing pin a bit, from both ends, working the oil in by pushing on the hammer end.  The business end moved in and out, but it didn't seem consistent in its range of travel.  I don't know how to take apart a CMMG bolt, and would not have done so there, as I didn't have the tools I suspect I would need..

The gun ultimately fired five more rounds, and those happened to be in a row, out of one mag, before the sixth round (of 9) failed to fire.  These five rounds that fired could not be checked for strike marks, as there was brass on the ground from my Marlin, which I was alternating in and out to have at least a little shooting time.

I am convinced the issue is related to the firing pin and/or spring (though I am open to possibilities). Here are my questions:

1) How do I diagnose the health and operation of the firing pin and/or spring? I have calipers, so specific measurements can be made to check against specs.
2) How do I take apart the CMMG bolt, and what tools will I need to do it correctly?
3) While I have it apart, what other parts do I need to examine and evaluate?
4) Will CMMG be willing to help?  Their website makes me think no, as I have no invoice for the parts, but threads here make me thinks yes, as they seem to be a company that stands behind their stuff.

As always, thanks for the help.  MODS - if this should have gone in Troubleshooting (which I considered), feel free to move it.

Is there a dent on the top of the barrel spud, just above the chamber ?
If it was dry fired empty, they will do this.

The firing pin could be peened.
Look closely, and get a new one if in doubt.

Make AR .22 run smoother link

I had a similar experience but mine was fairly easy to diagnose.  The bolt picked up the round from the magazine, chambered it, and then bounced out of battery.  if you push the bolt forward through the ejection port with your finger and it then moves into battery, there you have it.  I'm guessing that this is not your problem, but I thought I would mention it.

My fix was to find why the bolt was bouncing and not going back into battery. Found a rough spot on the carrier rails and smoothed that out.  Lubed the rails and the bolt well and that solved most of the issue, but the bolt would still occasionally bounce out of battery.  Bore Buddy weight kit solved the bounce problem.

Take it out of the bolt and look at it.  If it's in one piece, push it forward in the bolt to confirm that it emerges from the bolt far enough to strike the cartridge rim.



A cottering pin should be visible from the side of the bolt.  That's all that holds the pin in.



The firing pin rebound spring.  They can fracture.



I've found CMMG very responsive.  If you find that you needed a new firing pin, I'd buy two at a minimum.  Always have an extra at the range.

My guess would be broken or damaged FP

I had similar issues with mine and this was the cause.  I keep spare firing pins in the grip now.

No dent in the location you mentioned.

Don't know if the FP is peened, as I don't know what a new one should look like.  If I buy new, I will buy from Bore Buddy, as they seem to be better engineered from what I have read here.

Thanks for this link.  I have read most of this thread, which seemed to be focused on FTF and FTE.  I am having neither.  Feed ramp looks and feels smooth, though the steepness of the ramp surprised me. I have not polished anything, including the chamber, but I did rotate a .22LR brass brush in the chamber as part of my troubleshooting.

@zukiphile  I sent you an IM.

I agree with the comments about the firing pin.  Definitely start there.  I had a similar issue with a lower I purchased used.  It was dirty when I bought it so I know it had been fired.  He told me he assembled it with a Daniel Defense LPK with their "improved battle trigger".  I was getting light strikes.  Had one very frustrating range outing.  I can't remember what prompted me to check this but I was going to install a heavier hammer spring.  When I started to disassemble the trigger I noticed the hammer spring was in upside down and didn't have the force it normally would.  Duh!  I should have checked it thoroughly myself before firing.

I'm not saying this is what your issue is but it's worth checking.

Tell me if it isn't OK to respond here.  Yes, I'm the same one as at RFC.

Sounds as if you have a CMMG bolt before they used cotter pins.  Or perhaps not a CMMG bolt.  Mine look like this.



It sounds like a PITA to pound out a roll pin every time you check the firing pin.  I don't know how practical it would be to open the current roll pin hole to accept an AR cotter pin.

It is also a possibility that your collar is longer than the barrel stub and the bolt isn't getting close enough to the actual barrel breach when the bolt closes.  If that's the problem, you'll be able to see it readily by inspecting the breach with the collar in place.

While that is a common phenomenon with many .22LR firearms, the .22LR assembly under discussion has been designed to avoid firing pin contact to the chamber lip, and the assembly can be dry fired with impunity:
https://www.ar15.com/forums/Industry/22LR-dedicated-upper-dry-firing/145-291822/

MHO, YMMV, etc.  Be powerful; be well.

The guy at CMMG did say that I needed at least a 4-lb fire control group to adequately move the firing pin.  I have emailed them to confirm that they mean a hammer spring requirement, rather than a trigger spring (pull weight) requirement.  I can't see that the trigger spring would affect how much force the hammer exerts on the firing pin.

The Larue MBT comes with two hammer springs.  Use the heavier spring.

The MBT-2s comes with two TRIGGER springs, not hammer springs. I am not interested in changing the trigger pull weight.

Just got off the phone with LaRue.  They told me trigger springs would not affect the force with which the hammer falls.  That is controlled by the hammer spring, which they have not measured the force on.

With a standard mil spec type fire control group, the hammer spring force is a major determining factor in trigger pull effort.  That is, a lighter hammer spring will produce a lighter trigger pull as well as lower total spring force to the bolt (unless a higher than typical recoil spring is used to compensate for the reduced hammer spring force).
From what I can eyeball by comparing a 2010 CMMG unit with the roll/tension pin retainer and a 2018 CMMG unit with a cotter pin retainer, the older unit bolt probably can not be easily modified.  The through hole for the respective retainer pins are the same diameter, but at different heights relative to the frame slots.  When I clean the older unit, I only drive the tension pin out far enough to free the firing pin and spring.  It really doesn't take that long.
MHO, YMMV, etc.  Be powerful; be well.

If breach and collar are flush, that's at least one problem you don't have.  



Excellent observation.  Could someone forgo the roll pin and use a solid steel pin rounded at the ends so it doesn't scratch the receiver interior?

CMMG does sell complete bolts with all the updates that make it easy to live with.



Emphasis added.  Correct.



Also correct.  I attribute a couple of broken firing pins to the Larue hammer spring.  I switched to a lower weight hammer spring from Taylor Tactical or Kaw Valley and still had reliable function.

The trigger spring doesn't have as much effect on trigger pull as the hammer spring.  The tension of the sear engagement/hammer notch is what affects trigger pull the most.  But, yes, there are two trigger springs and one hammer spring.

On a trigger with a two stage design, a Rock River, a Geissele or a Larue two stage, the hammer spring has little to do with pull weight; a stronger hammer spring may add a bit of friction.  The dominant influence on pull weight is the trigger spring (first stage) and disconnector spring (second stage) because there isn't the same kind of camming of the hammer you see in a single stage design.

In the 2010 unit that I have, the roll/tension pin that retains the firing pin fits very snugly in the through bore and does not move unless tapped out with a punch and hammer.  There is no contact of the retainer pin with the upper receiver interior unless the pin is not seated properly.
For me, the cost of the new bolt isn't worth the difference in time and effort; but others, including the OP, may well feel otherwise.

@secondamendment, thanks for pointing this out.  It seems obvious, but I don't know that I would have thought that far ahead.

On your copy of the older unit, does the retaining pin for the extractor drive downward to change the extractor or spring?  By downward, I mean the punch would enter from the same side of the hole as the recoil spring, and be driven away from the recoil spring.  What size punches do you use to drive these two retaining pins?

@zukiphile - Do you point this out because you have successfully used an MBT-2S with a CMMG dedicated bolt, and eventually had to move to a lower weight hammer spring? CMMG today warned me that a possible cause of light/no strikes could be the hammer falling with too little force.  If you actually had to reduce your hammer force, I will move this variable MUCH farther down the list as I troubleshoot.  

I ask because LaRue was very nice on the phone today, but they have never measured the force of their hammer springs (!).  The gentleman I spoke with also said I was the first caller in his 5 or 6 years with the company that was using an MBT-2S with a dedicated CMMG bolt (!?!?).  He could discuss the idiosyncrasies of that trigger in a 9mm setup (including his own personal PCCs), but wondered if my issue was simply a case of aftermarket parts not playing well with each other.

IDK, I have used both springs in the Larue and noticed very little difference after hitting the wall.  Reduce the weight of the hammer spring and there is a noticeable difference.  Not as  much as a mil-spec trigger, but a difference nonetheless.

I have a Larue MBT-2 in a lower I use with a dedicated 22LR upper with the CMMG bolt/collar and it runs fine.

@s4s4u - Thanks for this confirmation.  Just out of curiosity which trigger spring are you using, the factory installed (lighter) one, or the heavier one that came with the trigger?

1) Need to have some photos.
2) buy a Wolff xtra power hammer spring from Brownells.
3) culprit could be the collar barrel interface.  Q?- Is the leading edge of the ramp touching the barrel nub?  Many dedicated uppers have a nub that is the chamber that sticks out about an inch or so.  If not, you may need to sand the collar down so that the ramp rests against the face of the chamber.  If there is a gap, the firing pin isn’t reaching the rim.  If it doesn’t engage the rim, no ignition.  How do I know this?  I recently built a dedicated upper using a Lothar-Walther barrel and CMMG dedicated bolt with collar.  CMMG changed the length of their barrels and collars.  They didn’t tell anyone until, OOPS, you barrel nub is too short? Shoulda used ours.  Mine was out of spec but sandpaper and calipers were the key.  If your collar ramp isn’t touching the chamber, then start sanding some material off the leading edge of the collar, the edge that actually contacts the barrel.  Start small and you will be good.

The trigger weight after hitting the wall is primarily governed by the disconnector spring in a stock MBT2.



Yes.  The MBT2 functioned very well.  I could stack a bunch of Taccom FP shims so the FP would only make a shallow mark on the cartridge rim, but it would light off every single time.  Broken pins were the only problem the Larue hammer spring caused me.

Bryan at CMMG was mystified by this result, but he never complained about sending me replacements.



If your bolt isn't going all the way into battery, apparently a common problem with conversion units, the movement of the bolt caused by the hammer fall can soak up a bunch of energy, so a monster hammer spring can be a bandaid.  I've found the CMMG dedicated barrel set-up to be amongst the most reliable 22lrs I've ever had, even with reduced power hammer springs.



That isn't true.



I do not believe that your Larue is the source of a failure to fire problem.


A great rule of thumb for any problem with the CMMG style bolt is to check the FP first.

I've worked on a lot of older .22's.  If the cartridge doesn't go fully into the chamber, until the rim touches, you have a higher chance of light strikes.  What happens is a lot of the hit from the firing pin is absorbed pushing the cartridge the rest of the way in before denting it.  A dinged, dirty, or otherwise tight chamber can cause this problem.

I tried both.  Didn't see any advantage to the heavier one so went back to the lighter one.  That lower also runs with my 20" DMR in 5.56 that I shoot in an informal 300 league.

UPDATE 2/27/21:  I contacted CMMG for tips on what to do.  The guy that answered the phone was helpful and gave me some ideas. He was not concerned that I had bought the unit used, and had no paperwork. In thinking about his troubleshooting ideas, I had additional questions, so I emailed their Customer Service department for clarification.  That led to a request from CMMG for a picture of my bolt.  The picture led to an offer to replace the BCG and collar with the current version.  The guy that I had talked to on the phone took the initiative to talk to a supervisor to make that happen!  I took them up on their offer and within a couple of weeks of them getting mine they sent me another.  I was able to test it yesterday at the range, and all was well.  I am really impressed with CMMG's customer service.  I will never know what the original issue was, but the rifle runs now.