View Full Version : Diagnosis check...bolt speed?
So a guy and his son bring their newly-assembled AR out of the range because it's not running. The weapon consists of a PSA 16" M4 style upper on a gunshow special lower. I field stripped it and checked the carrier gey and gas tube...all good. I went into the range and watched them fire it and observed a failure to feed where the bolt partially closed on a live round but did not chamber it. Also, 3 or 4 instances of the following sequence:
Insert loaded mag and chamber a round
Pull the trigger and fire after which the bolt cycles, the spent case ejects, and the next trigger pull is "click" on an empty chamber.
Carbine spring (good length) and carbine buffer.
I gave them an H buffer to try and it helped marginally...basically it began crushing cases as mentioned above.
My diagnosis is that the barrel is massively overgassed and I recommended they purchase an H3 buffer.
Thoughts?
Failure2Stop
08-17-13, 13:06
Is it consistently locking to the rear on an empty magazine?
Typos brought to you via Tapatalk and autocorrect.
Is it consistently locking to the rear on an empty magazine?
Typos brought to you via Tapatalk and autocorrect.
Yes, and I forgot to say I tried multiple mags that should have been good...Troy and USGI Magpul follower.
Where are the casings ejecting? 12 o'clock being the muzzle and 6 o'clock being the stock?
PSA + Gun Show = Junk
Why do you want to stamp your name on a woodshed build? I hope your getting paid handsomely...
Where are the casings ejecting? 12 o'clock being the muzzle and 6 o'clock being the stock?
Completely irrelevant...
Someone here pulled the FSB on a PSA carbine and it was around .093. My buddy locally grabbed a PSA upper out of curiosity and it was so overgassed it would run with an H3.
Was the bolt partially closed on the round like a bolt over base malfunction?
http://i.imgur.com/MCgCyX6.jpg
Bolt over base malfunctions can be attributed to bad magazines but you ruled that out. I think they can also happen by short-stroking. In extreme cases I believe over-gassing can cause the bolt to outrun the magazine because of the carrier/buffer bouncing off of the inside of the receiver extension - This would also explain why the bolt returned without picking up the next round those 3 or 4 times.
FWIW My BCM 16" middy will run anything with an H3 buffer. It does have over 5k through it though.
In extreme cases I believe over-gassing can cause the bolt to outrun the magazine because of the carrier/buffer bouncing off of the inside of the receiver extension - This would also explain why the bolt returned without picking up the next round those 3 or 4 times.
Given a serviceable magazine, the bolt group can't outrun it, period. This is one of those nagging errornet theories that just won't die.
Given a serviceable magazine, the bolt group can't outrun it, period. This is one of those nagging errornet theories that just won't die.
Really? How is that impossible?
I suppose another option is that the lower is out of spec such that the mag catch is machined too low or something.
Did you say what ammo they were using?
PSA + Gun Show = Junk
Why do you want to stamp your name on a woodshed build? I hope your getting paid handsomely...
Completely irrelevant...
My name isn't on the build...I was just trying to help them get it running.
Did you say what ammo they were using?
PMC Bronze 55gr
Given a serviceable magazine, the bolt group can't outrun it, period. This is one of those nagging errornet theories that just won't die.
A super worn out buffer spring (along with a CAR buffer) can cause bolt over run IMHO.
C4
What else would explain the bolt returning without picking up a new round? Unless someone can prove otherwise, I'm not convinced its out of the realm of possibility.
Given a serviceable magazine, the bolt group can't outrun it, period. This is one of those nagging errornet theories that just won't die.
A super worn out buffer spring (along with a CAR buffer) can cause bolt over run IMHO.
C4
A pretty smart AR guy once told me that "a malfunction that can't be intentionally induced can't happen".
I would understand this to mean that we should be able to take an otherwise reliable carbine, install bad parts in it, and in this case get it to consistently object to completing the cycle of operation by giving us a bolt over base malfunction. That being said, I've never seen anyone who could intentionally induce the malfunction.
Its been my experience that weapons that consistently exhibit bolt over base malfunctions are a result of unserviceable magazines, rifle barrel extensions in carbine upper receivers or an out of spec lower receiver. Every now and them the stars align and a BOB malfunction will occur. We all now the drill from there... take cover and perform a remedial action drill and get back in the fight.
A pretty smart AR guy once told me that "a malfunction that can't be intentionally induced can't happen".
I would understand this to mean that we should be able to take an otherwise reliable carbine, install bad parts in it, and in this case get it to consistently object to completing the cycle of operation by giving us a bolt over base malfunction. That being said, I've never seen anyone who could intentionally induce the malfunction.
Its been my experience that weapons that consistently exhibit bolt over base malfunctions are a result of unserviceable magazines, rifle barrel extensions in carbine upper receivers or an out of spec lower receiver. Every now and them the stars align and a BOB malfunction will occur. We all now the drill from there... take cover and perform a remedial action drill and get back in the fight.
Ever cut springs down on Mags or buffer springs??? I can make it happen and have seen it occur in the wild.
With that said, it HAS to be bad in order to get it to happen so it is not common.
C4
Ever cut springs down on Mags or buffer springs??? I can make it happen and have seen it occur in the wild.
With that said, it HAS to be bad in order to get it to happen so it is not common.
C4
Cats got my tongue...
Well, if the gent and his son return next week, I'll throw their PSA upper on my DD lower and see if it runs. That'll rule out an out of spec lower if I still have malfunctions and either prove or dis-prove the bolt-outrun issue.
I did manage to get it to fire three rounds in a row, though, so if it's an out of spec lower issue, the stars must have somehow aligned with the angle of the magazine, etc.
Personally, it sounds under or severely over gassed. I have had overrides. It came from running dirty and really hard, short stroking.
The reason I asked earlier about brass is because where that first brass lands before jamming up could lead to your culprit. Under-gassed with too much bcg weight or too heavy buffer, or too weak spring (short stroking). Or over gassed, too light buffer, or wrong spring. Most guns can be tuned properly by changing out components to create a balance.
Where are the casings ejecting? 12 o'clock being the muzzle and 6 o'clock being the stock?
Completely irrelevant...
IMO, Ejection angle is totally relevant.
That in combination with a lockback test, helps narrow down an over gassed/under gassed condition.
If the gas port turns out to be rifle sized, a CustomTune Gas Port can bring it back down to the .060" that it should be.
A friend of mine had these exact malfunctions on a PSA upper while running PMC bronze. We ruled out mags, lowers, BCG's, everything but the ammo.
It would run on full pressured ammo just fine and even cheap Herters/Tula, but not PMC.
He sent it back to PSA and they replaced the gas tube, even though we could find no excessive gas leakage and they polished the feed ramps a bit, but it still doesn't like the PMC. It will bolt override about 2 times per magazine.
Strangely enough, the same lot of PMC will run in all 3 of mine. Two BCM middies AND a PSA middie.
Kind of bizarre.
Someone here pulled the FSB on a PSA carbine and it was around .093. My buddy locally grabbed a PSA upper out of curiosity and it was so overgassed it would run with an H3.
If a 16 carbine has the correct gas port, what should you be able to run?? Just wondering.
I was running some tests yesterday on my wifes PSA I put together for her ( lucky for me it has functioned flawlessly the couple years she has had it ).
I was able to run a H2 with a Red spring, ran no problems, I didn't have a H3 to try. But she settled on a H /White spring combo that she said felt best.
I have not meassure the port, so I don't know what it is.
Mall_Ninja
08-19-13, 17:13
Given a serviceable magazine, the bolt group can't outrun it, period. This is one of those nagging errornet theories that just won't die.
Add me to the believers list. Remember that the BCG puts downward pressure on top of the ammo stacked in the mag. If the BCG velocity is high enough, I believe the sudden downward pressure as the BCG moves rearward can push the ammo stack down into the mag enough to cause an air gap. Then the BCG/buffer bounce back off the RE and start forward before the mass of the ammo stack can reverse direction. Obviously the more ammo in the mag, the weight of said ammo and the particular amount of drag present all affect this. Someone with a transparent mag or Magpul window mag and a high speed camera could confirm or lay to rest this theory...
FWIW My BCM 16" middy will run anything with an H3 buffer. It does have over 5k through it though.
After reading a bunch of threads on here I decided to buy an H3 and take it with a standard carbine buffer to the range with some Tula to do a short stroke test:
Both my BCM 18" SPR rifle gas and DD 16" middy, will run Tula on top of a Rifle lower or carbine with H3...
FYI, brass (steel LOL) ejected straight to 15:00 and dropped about 2 feet away (pretty weak ejection, but 100% bolt lock back none the less)...
It's my opinion that a 16 barrel with carbine or midlength gas system should be able to run with any buffer weight and a USGI spring using NATO pressure ammo. Using heavier buffers should mostly be for fine tuning. If a 16" gun with mid or CAR gas can't run with a heavier buffer I think it would be too close to the threshold of failure to consider reliable.
If a 16 carbine has the correct gas port, what should you be able to run??
It's my opinion that a 16 barrel with carbine or midlength gas system should be able to run with any buffer weight and a USGI spring using NATO pressure ammo. Using heavier buffers should mostly be for fine tuning. If a 16" gun with mid or CAR gas can't run with a heavier buffer I think it would be too close to the threshold of failure to consider reliable.
By "heavier buffer" do you mean H2, H3?
Correct.
By "heavier buffer" do you mean H2, H3?
My 16" carbine has run well with an H2 using NATO pressure ammo. Consistent ejection and always locks back. I'm going to shoot some 55gr. reloads next so I'll take my H buffer along in case the H2 is too much.
uncle money bags
08-19-13, 20:49
Add me to the believers list. Remember that the BCG puts downward pressure on top of the ammo stacked in the mag. If the BCG velocity is high enough, I believe the sudden downward pressure as the BCG moves rearward can push the ammo stack down into the mag enough to cause an air gap. Then the BCG/buffer bounce back off the RE and start forward before the mass of the ammo stack can reverse direction. Obviously the more ammo in the mag, the weight of said ammo and the particular amount of drag present all affect this. Someone with a transparent mag or Magpul window mag and a high speed camera could confirm or lay to rest this theory...
..
I thought the bcg at rest was already in its lowest position, and that as it travels to the rear and the skid clears the round stack the next round will actually move up a fraction of an inch, well in advance of the bcg as it returns to battery.
Are you claiming the bcg pushes the rounds down when it travels to the rear?
steyrman13
08-19-13, 21:35
FYI, brass (steel LOL) ejected straight to 15:00 and dropped about 2 feet away (pretty weak ejection, but 100% bolt lock back none the less)...
Is it a Stag left-handed model? 1500 would be over your left shoulder around the 7-8 o'clock position....or the brass circled 360 deg around you back to the 3 o'clock position! That's wild!!:D
Maybe the latter and that's why it was only two feet away, but that would mean really strong ejection to create a whirlwind!
We tested a S&W M&P15 at the Sandy Springs, GA class I did and the carbine ran with 4 different types of ammo with an H3 buffer. We tested the carbine with all 4 buffers and the Vltor A5.
100% functioning and cycling.
It's my opinion that a 16 barrel with carbine or midlength gas system should be able to run with any buffer weight and a USGI spring using NATO pressure ammo. Using heavier buffers should mostly be for fine tuning. If a 16" gun with mid or CAR gas can't run with a heavier buffer I think it would be too close to the threshold of failure to consider reliable.
Mall_Ninja
08-19-13, 23:59
I thought the bcg at rest was already in its lowest position, and that as it travels to the rear and the skid clears the round stack the next round will actually move up a fraction of an inch, well in advance of the bcg as it returns to battery.
Are you claiming the bcg pushes the rounds down when it travels to the rear?
We are on the same page, the downward pressure (from the BCG) is already there, essentially the magazine spring is the only thing resisting this. But since there is a "stack" of ammunition above it there is a "moment of inertia" (column inertia) that takes a bit of time to "get moving". Hence why I said that a "just under full" magazine should be the most prone to this. Essentially the same effect as a mag spring that has lost some tension and is beyond service limits. It cant push the "column" of rounds up fast enough. I believe that a BCG velocity that is extremely high could duplicate the same effect. After all Mr. Stoner had to have had a "spec" for minimum magazine spring tension. He would have based this on a 20" barrel and rifle buffer. Compare the BCG velocity of:
16" (carbine gassed) barreled Semi auto carrier with a carbine buffer.
20" (Rifle gassed) barreled M-16 carrier with rifle buffer system.
I dont know the "numbers" but it wouldn't be hard to figure the carbine velocity much higher...
In the simplest terms, if anyone can believe an old worn out mag spring can cause the same malfunction, then you must also believe an "in spec" mag spring can fail to be sufficient if the spec it is "up to spec" for was written for a very different configuration (20/rifle/auto carrier, etc. vise 16"/carbine/semi-auto carrier)
Is it a Stag left-handed model? 1500 would be over your left shoulder around the 7-8 o'clock position....or the brass circled 360 deg around you back to the 3 o'clock position! That's wild!!:D
Maybe the latter and that's why it was only two feet away, but that would mean really strong ejection to create a whirlwind!
You bump your head? 15:00 is "3 o'clock" which would be straight out the ejection port in a straight line to the right of a right handed shooter.
Or are you messing with me and im too tired to comprehendo? :D
steyrman13
08-20-13, 00:07
You bump your head? 15:00 is "3 o'clock" which would be straight out the ejection port in a straight line to the right of a right handed shooter.
Or are you messing with me and im too tired to comprehendo? :D
Messing with you. You used 24 hour time. If you look at a 24 hour clock(mil clock) there is two versions. 24 hour marks where 1500 sits at around the 7-8 o'clock position of a normal clock, or where the hour hand travels a full circle and then lands back at the 3 o'clock position on second time around. I was just joking saying no reason to say 1500 instead of 3..... ;)
Even though the spring was the correct length, is it possible it is a cheap brand that isn't the correct spec...metal composition and spring rate too weak?
If it is a PSA hammer forged upper, I would think the upper is fine but not guaranteed. Hopefully you can test it on your DD lower if they come back.
I just noticed it was PMC bronze. Isn't that known to be weak ammo? Try some quality ammo or even xm193 or something to see if it still happens. That would alleviate if it is short stroking or bolt out running. Also try the last round lock test.
Mall_Ninja
08-20-13, 00:22
Messing with you. You used 24 hour time. If you look at a 24 hour clock(mil clock) there is two versions. 24 hour marks where 1500 sits at around the 7-8 o'clock position of a normal clock, or where the hour hand travels a full circle and then lands back at the 3 o'clock position on second time around. I was just joking saying no reason to say 1500 instead of 3..... ;)
HA! Thats funny, actually at the time it was faster to type 1500 rather than 3 o'clock, to which I ended up going back and putting 15:00 so it wouldn't get confused, then realizing how going back to add the "colon" wasted more time than typing 3 o'clock to begin with! Derrr... :rolleyes:
If the BCG velocity is high enough, I believe the sudden downward pressure as the BCG moves rearward can push the ammo stack down into the mag enough to cause an air gap.
How exactly is this sudden downward pressure generated?
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