Strangest Functional Diag ever...

[98Z5V]

Did the strangest Diag over last weekend that I've ever had to do on a gas gun. 

It would fire, and not cycle. The BCG would move a little less than an inch and a half.  16" midlength gas, .223 Wylde 1:8" twist barrel, 0.750" gas block journal, gas port diameter range of 0.078" to 0.083" - it was drilled 0.078", and should be enough.  NiB BCG, Carbine recoil system with a 7.000" internal depth, 2.750" AR15 Carbine H3 buffer, 5.4 oz.  Known good (new) carbine recoil spring. 

So, fire, no ejection, BCG movement less than 1.5" total travel.  First step, gas port goes to 0.082" (closest bit I had there to max in the range).  Fire again, exact same results, minimal BCG movement.  Have to use the charging handle to cycle the action, eject the case, feed new case, and fire again. 

Next logical solution, install the H1 buffer I had in the toolbox.  3.8 oz.  EXACT same conditions, same BCG movement.  Next, break out the tools, and take the buffer apart - go to the extreme end of the spectrum.  Empty aluminum buffer body, nothing inside, bumper reinstalled...   EXACT SAME firing condition, same BCG movement.  Okay, so this IS NOT a recoil system problem, not at all.  It's Gas System, and gas only, and the gas port diameter is right. 

So, gas tube is bad.  Strip the handguard, pound out the gas tube roll pin, remove the gas tube...   Measure the port size and port location.  Port size is 0.125" (correct), and it's location from the end of the gas tube is 0.600" (also correct).   WTF!!!

Remove the muzzle device, remove the gas block, which had red loc-tite on the screws.  That took a small portable propane torch.  Get those screws and that gas block body hot, remove screws, set the thing down to cool.  It took two full Coors Lights for it to cool down enough to remove, and off it came.

Looked at the barrel, and burn marks (carbon buildup), and it just looked weird.  Carbon blown all over the place.  No real distinct carbon ring around the gas port in the barrel, like you'd usually see when you remove a gas block - carbon was just everywhere under the gas block.  Hmmm...  Getting frustrated with this thing.  It was built from good (reputable) parts, but I didn't understand the functional (malfunction) description until I knew how far that BCG was traveling.  Or, NOT traveling. 

Looked into the barrel portion of that gas block, to look at the port in there, and it's the smallest fucking thing I've ever seen in a gas block before - and this is a YHM steel gas block.  Those dudes make some damn good parts, FFS! 

That port diameter in that gas block was 0.0525". !!!  52-thou !!!   It's supposed to be 0.125" !!!  

 

Went to drill it up to 0.125", and that is some hard metal to drill.  I broke the bit, ran the broken jagged piece through my left thumbnail, and bled everywhere.  Normally, fingers and toes gross me out, but this just made me more determined to get this block done.  Didn't want to go bigger on the bit, over 0.125", so I went down to the next size, and that was 0.119".  It's larger than the gas port diameter in the barrel, so it won't matter if it's smaller than 0.125" for this gas gun.  Owner had it perfectly centered up. 

Reinstalled everything, tightened it all back down.  Reinstalled the H3 buffer.  We'll find out next week sometime how it does, when it gets taken back out.  I'd like to be there, but don't know if I will be.  I'll report back on the results...   I already know it's gonna work flawlessly, and that was the only real issue going on.  The small-ass hole in the gas BLOCK was restricting flow so bad that no matter what was done, there was only enough gas available to move that BCG about 1.5"...  

That sucked.

Edited July 25, 2023 by 98Z5V

[Turd Furgeson]

Crazy...bet it works now.

[98Z5V]

Yep.  Nuttiest thing I've ever seen go wrong with building a gas gun.  I think they drilled the pilot hole in the block, and it was Friday at closing, and "get all your parts to the guy that does the Phosphate treatment on Monday mornings!" - and punch out for the weekend.  "Oopsie, forgot to finish that gas block before I left!  Oh well..."

I'm adding "inspect gas BLOCK gas port diameter to my own build checklist.  1/8" drill bit should fit, every single time.  It was frustrating.

[unforgiven]

Good troubleshooting brother 

[Sisco]

Nice diagnostic work. Probably a 1 in 10,000 case, but you found it!

[Armed Eye Doc]

Great job Tom! 👍

True diagnostic skills are not common anymore . Too many people are reliant on technology to tell them what is wrong.  I was always told in optometry school that 90% of problems can be diagnosed by listening to the patient and asking the right follow up questions.  Then you run specific tests to confirm your diagnosis.  Nowadays, too many doctors run tests first to figure out which diagnoses are left to consider.  Few diagnosticians are left out there.

[98Z5V]

It was pretty frustrating, I'll be honest.  I had to figure it out quick, either recoil system or gas system, because we were out there in this jacked up heat - time was definitely a factor.  It was all good parts, I checked everything over before it was put together.  First thing I checked was if a rifle recoil spring was installed accidentally - nope, it locked back manually, and a rifle spring would have coil-bound before that BCG will lock back in a carbine extension.  Looked at the spring, asked again what brand it was...   Good to go...

I've never had an issue running 16" midlength 5.56 with a carbine H3 buffer - they shoot soft like that.  It's not too much buffer...  Out with the H3, in with the H1 - and it was identical in BCG movement.  Okay, that was weird, but one more test, GUT the H1 buffer and make that fuqr super-light.  SAME BCG movement.  Alright, this isn't recoil related, then.  It's gas.  Let's figure it out, because something is fukked up in here, and I know for a fact it's not the gas port in the barrel...

It was frustrating, but crystal clear when I looked into that gas block...  Just a machining mistake.  Probably never see that again from a good company, not in this lifetime.  But, I'll check for it every time from now on...  

[Armed Eye Doc]

You followed a procedure similar to what we were taught in school.  If you are in the US and hear a horse running in a field, you assume it is a horse before you look and  expect to see a horse when you do look.  It is not until you look and don't see a horse that you start looking for a zebra.  Rule out the most likely causes before you look for the obscure.

[98Z5V]

JMPI, brother.  JumpMaster Personnel Inspection.  It's very time-restrictive, stress filled (on purpose), and it's GO or NO-GO.  Miss a Major, you're done.  Miss 3 Minors, you're done.  You cannot inspect Jumpers and looks for mistakes...   You have to "know what right looks like," and the mistakes will stand out like a sore thumb. 

Which I have right now. 

[Cunuckgaucho]

3 hours ago, Armed Eye Doc said:

Great job Tom! 👍

True diagnostic skills are not common anymore . Too many people are reliant on technology to tell them what is wrong.  I was always told in optometry school that 90% of problems can be diagnosed by listening to the patient and asking the right follow up questions.  Then you run specific tests to confirm your diagnosis.  Nowadays, too many doctors run tests first to figure out which diagnoses are left to consider.  Few diagnosticians are left out there.

Learning to ask the right questions is a skill in itself. For me diagnosis is putting together a jigsaw puzzel. I have a 'picture' of what I should have. I then start filling in the easy/key pieces by testing ang checking. Unlike a jigsaw puzzel once I correct the wrong piece the rest of the picture auto fills itself.

57 minutes ago, 98Z5V said:

JMPI, brother.  JumpMaster Personnel Inspection.  It's very time-restrictive, stress filled (on purpose), and it's GO or NO-GO.  Miss a Major, you're done.  Miss 3 Minors, you're done.  You cannot inspect Jumpers and looks for mistakes...   You have to "know what right looks like," and the mistakes will stand out like a sore thumb. 

Which I have right now. 

Once I learned to scan looking for what is right, what is wrong is way more likely to jump out at you. 

[98Z5V]

I diag shiit everyday, on EFI vehicles, and I'm getting (not my choice) deeper and deeper into electric vehicles.  It's not simple anymore, but it used to be, even fuel injection system, in the past days.  Now, there's CAN/BUS comminucations in these things.  That's fun.  The rig comes in, the "customer's comments" are always bullshiit, because they don't know what the fuk they're talking about...  so it's just you and the machine, the computer that reads the ECU to find fault codes, shiit electrical numbers that are out of place for specific sensors, stuff like that.  It would be easier to be a medical doctor - because the patient can fukkin talk back when you ask them something.  These days, with rigs - it's like being a Veterinarian, and we don't speak animal.  These things won't tell you what's going on, you have to figure it out, with the bullshiit "clues" from the customer that doesn't know shiit in the first place.  It's even worse, when they think they know everything...  

"I was a top-rate diesel mechanic for 39 years!"   Yeah, cool - this thing isn't a diesel.  Congrats, though.  Glad you're retired now.

"I've been an electrician for 53 years!"  Yeah, awesome.  That's house shiit.  Your grounds are different from my grounds, and this is a DC system - not the AC shiit that shocked you and fried your brain for 53 years.  Thanks for the "help" though - 'preciate ya.

Gas guns are different.  They are 100% mechanical in nature, not complicated (like women), and they have one job to do.  It works, or it doesn't work.  If it doesn't work, apply this and see what happens.  It's flow-chart shiit, when you get the whole system - as a whole, and it's independent systems (gas and recoil).

It's like the old days, working on shiit with a carb, points, etc.  All mechanical.  Digital Ignition kinda changed things, but that was easy, now, compared to EFI.  Which is easy, compared to the modern shiit with complicated electronics.

Which brings me full-circle.  You don't complicate shiit with complicated shiit.   Thus endeth the sermon...  

[98Z5V]

Full AAR - first test after drilling the gas block port up was a single round in the mag - test function with single-round lockback.  It was immediate, worked flawlessly.  Approx. 60~75 rounds sent through it after that with absolutely zero functional issues at all, recoil impulse was soft with the H3 buffer, everything is good with that 5.56 gas gun.     I'm relieved...