I've been working on this a while now and just finished. I've added a separate troubleshooting guide to my Blowback9.com site.  I'd appreciate it if you could take a look and see if there's anything obvious I need to change or if there are other common problems I should consider adding (along with the resolution you used, please!)

9mm AR Troubleshooting Guide (simple blowback)

For those who don't know, the site is free, is not sponsored, and does not generate any revenue.  I put it together after nearly a year of researching and documenting everything I could find to fix a nightmare first build. I know I would have appreciated having all this information in one place, so I put it all on wordpress for anyone who needs it. I add "articles" to the site once in a while to share new learning/thoughts/insights.

Thanks in advance for the feedback.

I will do that............thank you!

Address broken firing pins. I have had this happen twice.
And the FCG was G.I. in both cases.

Wilson Combat is about twice the price, but I haven't broken one.

I did, under "Failure to fire – live cartridge in chamber."

Can you detail out the symptoms you encountered?

I broke a blue steel New Frontier Armory firing pin that came in their modular bolt at about 400 rounds (which they replaced)... I replaced it however, with an OEM Colt pin which I rounded the hammer strike end so that the hammer would strike closer to the center line of the pin... it completed about 3000 rounds of blow back, and another 2000 or so in a CMMG radial delayed bolt.... both bolts were using an Enidine rifle length hydraulic buffer...hasn't been shot during covid and primer shortage

failure to extract with CMMG radial delayed bolt... my extractor problems seem to be unlike other that trace their problems to the ejector spring... the polymer helper plug in the extractor spring had hardened at about 1000 rounds, and prohibited the extractor from snapping over the cartridge rim...

@lasnyder When the firing pin broke, did you just have the hammer fall on a live round in the chamber and nothing else happened (click, no bang)? Or were there other symptoms?

IIRC... I shot a USPSA stage and had a "click no bang" ...clear chamber... charge gun... bang.... than another click no bang...at that point I retired from the stage, and upon checking the bolt the pin was in two pieces

Excellent.  Thank you!

Awesome and thank you for your dedication. I have checked out site a few times now.

MAZDOG

OP..... I appreciate everything you do to help out others that have technical problems.
Your experience and knowledge on many components is extensive, and very helpful to those getting into 9mm builds.

BUT......What we really need are actual stickies at the top of the page with links to previous discussions on various topics.
Like bolt carrier/buffer components, triggers, barrels, mag types, etc.

Maybe even a new troubleshooting FAQ or update the current one with some basic procedures to those trying to decipher the mystery of the 9mm AR blow-back.  


Either way it happens....I'll be looking forward to having the information available to those of us that need it.

I agree. The mods have opted for 2 red "announcement" (largely ignored?) threads at the top with links to a few of those discussion threads.  It would be much more "user friendly" if the threads you mention were just pinned to the top of the forum.  Not my call, though.

OP, quite comprehensive.  Might have a squib page, and go into why a straight blowback gun won't explode from a squib, but you'll eventually get blown brass, and why a locked breech gun will explode from a squib.

Also might talk about the usual causes of squibs being someone chasing a powder puff load, and why that won't work in a blowback operated centerfire gun w/ a bolt weight of 22 oz.

Good suggestion. I'll dig into the topic.  Thanks!

A lot of good information here!  WOW!!  Having built a number of AR9s over the past 6 years I can agree with much of what you have written....but you asked for input so here is mine.  You skipped over the two basic types of bolts(that is unless I didn't see it).  The bolt with the 1911 style extractor...and the newer types like Taccom and others that are using the AR-15 type of extractor.....and along with this discussion....what's better when it comes to extractor tension?  Heavy tension or light tension on the extractor?  I have used both, but perfer the Taccom style bolt with an AR-15 extractor due to the fact that it snaps over the rim and the extractor is designed to do so.  I've seen a lot of the 1911 extractors breaks off at the tip so I stopped using them.  Your thoughts??  Also, going to try out you input on the heavier buffer with sliding weights.  I use my AR9 in USPSA competition and handload to a 131 power factor, so they are no where even close to factory loads.  Weighed my bolt and buffer at 19oz.  No problems....but I found the idea of 22-24 oz total interesting along with a sliding weight in the buffer, so it's off to purchase one to try.  Thanks for all the info.....got me to thinking!

@Alvin - thanks! I'll have to do some research into the whole extractor thing. I prefer the larger 5.56 extractors myself, but never really thought about it much beyond that.

There is a lot of blurring of the lines between what is right for competitors tuning their guns for shooting competition games, and someone building an AR9 for non-competition use. I believe we, as a community, need to start making the distinction between the two clear, and take it seriously when making parts recommendations.

Competitors seem to generally prefer lighter masses and stronger springs to reduce dot movement in a blowback 9mm. Custom handloads are part of the equation. Parts longevity and recoil are not as big of a concern. Speed and target transitions are. I would not recommend using a gun tweaked for competitions for any other purpose.

My site is oriented more toward non-competitors who may alternate their guns between fun range time and potentially more serious uses when they get back home. These folks would tend to use factory (and defensive) ammo, should expect excellent reliability, safety, and reasonable or reduced recoil. This use favors lighter springs, heavy masses, and I would stay away from competition-oriented parts.

In your case, the heavier mass will reduce felt recoil, but dot movement may increase so your splits may not be as fast. Keep me in the loop and let me know how it goes.

I will keep you in the loop....never considered heavier mass behind the bolt, nor the idea of eliminating "bolt bounce" but I see where you're going with that.  My buffer is on the way....good to keep an open mind of these sort of things. I have four AR9s to play around with so I'll see what I can do.  I agree with you on the difference between "competition guns" and basic   firearms to use at the range or in home defense.  Two different animals....but some cross over is possible also.

Alvin, if you're competing w/ AR9s, have you considered the CMMG delayed blowback?  Amphibian has his tuned to a T.

Hadn't really thought about it....I have four AR9s now....and have shot all of them in USPSA matches over the past 6 years.  Having another one, even with a delayed blowback style, really isn't in the cards at this time.  Not real sure of any advantage....perhaps you can explain the advantage of delayed blowback....I would appreciate any information on this.  I've heard about it, but was curious how it would help.  Your thoughts?

Lower reciprocating mass, so less dot bounce.

Interesting....I already have lower mass weight by handloading "down" and not using factory loads.  Not much dot bounce at the present time...but I see your point.  Thanks!

I believe that the approximately $400 or more premium over the cost of a direct blowback set up (which advocates of the Amphibian set up always seem to fail to mention) is what discourages many would be purchasers.
And the $400 comes from (numbers I looked up about a year ago):  Cost premium from CMMG (their MSRPs) of an RDB 9mm carbine vs. a direct blowback, $200; cost of the A5 buffer tube, flat wire recoil spring, buffer tube spacer, and 9mm hydraulic buffer, another $200 or so.  And if someone does not have the tools and facilities to change the buffer tube, $25-$75 for gunsmith to make the change and test fire the carbine.  I'm calling the magazines a wash, but I'm not sure about  that.

Disclaimer:  I have no affiliation with or financial interest in any vendor or manufacturer; and I am not a competitive shooter or a blogger who receives any form of compensation for endorsements or favorable public or private comments.  Use of vendor and/or product brand names, if any, is for informational purposes only.

Best of luck.
MHO, YMMV, etc.  Be well.

Oh, it's absolutely more expensive and potentially less reliable.  It adds complexity to reduce weight & reciprocating mass.  It's intriguing to me, I don't have one.

Less reliable?

Have you followed the woes CMMG has had w/ their ejector springs on their delayed blowback AR-9s?

Well, after looking this up I have now...wow!  Think I'll stick with what I have.  Still waiting on my heavier buffer and I'll try that out next week when it gets here.  Sorry if it seems that this thread got hi-jacked! Thanks for all the good informaion!!

As far as expensive goes, I only use hydraulic buffers (several manufacturers/models) for all AR platform applications, except for the AR 'true' pistol which doesn't use a buffer.

MHO, YMMV, etc.  Be well.

Droppoint......received my KAK 8.5 buffer this week and went out two afternoons and shot up a storm to try it out.  I used two AR9s...identical even down to the same JP Comp.  I put the KAK buffer in one.....I had to remove my 6 quarters from the buffer tube since the KAK buffer is much longer and I don't need to shorten up the stroke.  With the new buffer the bolt was about 1/4-3/8" behind the bolt lock.  Bolt and KAK buffer weighs 22 3/4 oz.  The other one one used a 5 1/2 oz buffer with movable weights and bolt and buffer weighs 19 oz.  I use 6 quarters in the tube to shorten up the stroke.  Standard carbine spring in both.
Now, comes what you and I discussed about loads.  My load is only 131 Power factor.  135 grain bullet running 980-990fps.  This from a 16" barrel.
Worked up this load many years ago and it works great in my AR9s BUT it will NOT function a standard 9mm handgun.  SO....my handloads are tuned to my ARs....and nowhere even close to factory ammo.  Which is where I think the difference was.
SO.....using a timer and my targets, I switched back and forth on guns.....tried double taps...Bill Drills, just about everything that you can think of.  My splits were between .15 and .20 on both guns.  Both guns hit on target the same....two close hits on every target....small clusters on the Bill Drills(as long as I did my part).  Was really thinking that there would be a difference between guns, but they both acted the same.  Dot bounce was the same, actually very little dot bounce at all for both of them.  I've been told at the matches that my AR9 doesn't move much between shots...stays very flat.
So...what did we learn?  Shooting 500 rounds with two guns is still a lot of fun, for one thing!  I don't think I proved or disproved anything concerning buffer weight.  I believe that using a light load that is tuned for a particular gun eliminates a lot of dot bounce.  In theory, I can see where the heavier buffer would be better with standard factory loads perhaps. But with my loads it just didn't seem to make any difference at all.  Since I don't have any factory loads, nor have any intention to buy any, what is working for me is the way I'm staying with.  I do like the longer, heavier buffer and will leave it in one of the AR9s for now, but I feel that the way the cartridge is loaded may have more to do with dot bounce.  
It was an interesting experiment...glad I did it.  Thanks for bringing this up....
One last thing before I forget....checked the empties....no bulges or deformed brass, so everything fired from within the chamber...no "out of chamber" firings.
No...I don't have full auto...so I can't really "dump" a whole mag that fast.  Perhaps others could chime in here on that portion.

Thanks for articles DropPoint!!  Really got me to thinking!

Thank you for the experimentation and the write-up! Very interesting results.

I think we may have learned one additional thing: it's more the shooter than the gun.  You have lots of practice and know how to handle the gun to make it perform the way you need it to!

19 oz mass isn't terribly low, especially if you're using lighter loads. Changing the powder load changes all the equations.

Thanks again!