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Old April 22, 2019, 02:58 PM   #26
FrankenMauser
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Barrel extension area, close-up: https://youtu.be/hk53e7lL8Io

Back to the bipod discussion:

I did not believe it to be a major factor, before the first round was fired; but did wonder if it would skew the result. After subsequent tests, like the first link posted - where we're right next to the rifle - we considered it to be a non-issue.

In this video, you can see the barrel assembly - with the additional mass of the bipod - recoil with the rest of the rifle, and the barrel only moves forward when the BCG comes forward and hits it. The bolt did achieve full lock-up - even though you can't see it in the video - due to the clearances in the upper receiver allowing the bolt carrier to travel farther forward that usual, with the recoil spring pushing the BCG (now locked to the barrel) and barrel assembly forward until the bolt carrier ran into the upper receiver.

The barrel, I believe, would have continued forward, and possibly fallen on the ground, had the bolt not been locked into the barrel extension. But, because it was locked up before the bolt carrier movement was arrested, the barrel comes to a hard stop.

Is the bipod a factor, in my opinion? Certainly.
Is it enough to skew the results? I don't think so.
But I would like to see your opinions.
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Old April 22, 2019, 03:02 PM   #27
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FrankenMauser you are my hero for doing this. So am I seeing correctly that the rifle was able to chamber another round and go into battery? So it still functioned as a semi-auto? I wouldn't have guessed that.

And FWIW interesting question on the bipod being a factor. I could see it limiting forward movement of the barrel. Time to support it by the lower receiver only for the ultimate test! (I don't think the results will be much different)

EDIT: Now I saw the video from the side showing the barrel coming out some due to chambering and bolt lock-up. Did it still fire without you pushing the barrel/extension back in? Wondering if it moved things far enough forward that the hammer wouldn't contact the firing pin...
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Old April 22, 2019, 03:08 PM   #28
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And, the complete playlist:

https://www.youtube.com/playlist?lis...Q1YVmmGl2pycIL
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Old April 22, 2019, 03:23 PM   #29
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I figured there'd be a little more discussion before I had to address the elephant in the room. But, 5whiskey, you went right for it...

We were unable to get it to feed a second round. Most of the testing was single-loading.
By the time we got around to trying another round in the magazine, we were low on ammunition and experienced three feeding failures in a row. The cartridges would get partially stripped, and then pinned against the front of the magazine - indicating to me that it was short-stroking. Since it wasn't feeding from that P-Mag, and I didn't want to risk damage to a 'good' magazine, we went back to single-loading.

There was no video of testing whether or not it would feed a second round, as satisfying our own curiosity was the order of the day - not, necessarily, filming to share with other people. If it had worked, we probably would have filmed it. But it didn't...

The bolt did lock open, however, on three or four of the test firings with a magazine in place (to assist loading). So, the short-stroking could have also been an ammunition issue.

I do believe that the second round would have fired, if it had been able to feed. But, of course, we were unable to test that theory without 'cheating' - and there was little point in trying to replicate a condition that the rifle could not achieve on its own.

I need to get that upper back out and test a couple other configurations, and test with a different magazine or two. That P-Mag caused issues with two other ARs that day, and I think it may have been dragging on the bolt carriers in all of them.

Far from scientific testing. But, I worked with what I had available.


(That's not a "beater" or spare parts lower, for the record. That's a good lower - but less valuable to me than my other [better] lowers.)
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Old April 22, 2019, 10:44 PM   #30
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Frank, please tell me you weren't the idiot, I mean brave soul holding the cell phone in the video?

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Old April 23, 2019, 01:08 AM   #31
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Very interesting test. Thanks for posting it!
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Old April 23, 2019, 02:40 AM   #32
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Once again,apparently someone does not understand the mechanics of Stoner's AR design.

There are no locking surfaces or lugs in an AR upper receiver. Almost always,when you buy an AR barrel,it comes with a BARREL EXTENSION permanently attached to the threaded,chambered barrel.

The BARREL EXTENSION has the locking lug surfaces machined into it.The Barrel has the chamber machined into it,usually to a nominal depth,and usually to tolerances that will provide proper headspace when matched with a bolt.

For military weapons,the index pin pressed into the bushing is driven in to the point t stakes the barrel to bushing thread in place.The uninformed may see the AR barrel as one piece,but it is an assembly.

Its a good idea to buy a barrel with a matched bolt.The headspace gets checked to that barrel. The receivers and barrel nut might be 1000 miles away,but the headspace is fixed and checked.

The bolt locks into the barrel extension.The relationship between the breech face and and the chamber datum has not one thing to do with the upper receiver or the barrel nut.The actual rifle bolt NEVER TOUCHES THE UPPER RECEIVER,It only touches the barrel extension and the bolt carrier.

To those who insist the barrel nut has something to do with the AR headspace,no offense intended,but you just don't understand the AR.

That's not a problem with us. You might take a deep breath,shrug,and say "Oops!" Its no big deal. That's how we learn.
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Old April 23, 2019, 07:18 AM   #33
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Since I've personally seen instances in which residual pressure in the gas tube was sufficient to at partially cycle the bolt a second time before it locked up on the next already chambered unfired round, I would say the barrel would be forced out of the receiver before a second round would chamber.
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Old April 23, 2019, 10:27 AM   #34
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Ed308. The idiot wasn’t frankenmauser it was me. This was after being fired several times, I felt comfortable, some may not have but I was fine with it.
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Old April 23, 2019, 10:50 AM   #35
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As cornbush mentioned, several test firings indicated, in our opinions, that the rifle was going to perform as theorized - at least during the initial firing sequence - and remain locked until chamber pressure dropped.

After 4-5 rounds, we weren't afraid to get closer. Cornbush and I were on opposite sides of the rifle for that shot.

Based on the lack of criticism, I'm guessing that no one clicked the playlist link and found the last video clip.


One note on something that I had considered when removing the barrel nut, but then forgotten about until yesterday when I noticed it:
Without a barrel nut in place, the dust cover hinge pin walked out about 3/4" during transport, firing, and subsequent transport. I was surprised that it was still functioning as intended, even though the pin was only supported at the front.

Why didn't I remove the dust cover all together? ...Because, for transportation, the dust cover was holding the BCG in the upper, and the BCG was holding the barrel in the upper. - A delicate dance, but one that worked.
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Old April 23, 2019, 07:38 PM   #36
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I'm surprised moderator hasn't locked this list yet.
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Old April 23, 2019, 08:41 PM   #37
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Why?
No one is advocating for anyone to perform the activities discussed here.
However, there are certainly plenty of people learning more about the design and operation of the AR-15.

Remember, usually more than 80% of the people viewing a thread never participate in it.

For every person that thought the rifle would instantly explode, there were at least four more that said nothing.
For every person that thought an AR's barrel nut affects head space, there were at least four more that didn't say anything.
For every person that thought this was a dumb idea, but still interesting to see tested, there were at least four more that just sat in the back of the class and didn't participate.

Overall, this thread - though covering a topic with inherent risk, and which should not be attempted by anyone other than professionals - has had primarily positive comments and discussion.

While the topic may be approaching the limit of what should be allowable on TFL, no one is suggesting that the new fad in AR building should be omission of the barrel nut, in order to save weight and make for quick take-down.
This was simply a test to prove or disprove the theoretical operation of an AR, and show that certain parts are 'unstressed' and merely exist to keep the important bits held together and in alignment.
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Old April 23, 2019, 09:16 PM   #38
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Have to say, the results have surprised me.
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Old April 23, 2019, 11:23 PM   #39
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Quote:
...residual pressure in the gas tube was sufficient to at partially cycle the bolt a second time before it locked up on the next already chambered unfired round...
Once the bullet leaves the bore, the gas system is completely open on the muzzle end. Once the bolt carrier moves back about an inch, the system is completely open on the back end. I can't understand how a tube about 18" long could retain any significant residual pressure for any significant amount of time once it is open at both ends--certainly not long enough for the bolt to cycle all the way back and then return forward.

Can you describe the situation in more detail? I can't get a picture in my mind of what's happening from the quoted statement.
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Old April 24, 2019, 06:04 AM   #40
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"Can you describe the situation in more detail? I can't get a picture in my mind of what's happening from the quoted statement"

I acquired an upper that some DUFUS had "built" using a 24" carbine gassed barrel. EVERY shot produced a double feed when the bolt cycled a second time after pushing the first round into the chamber.
An adjustable gas block turned down to nearly nothing sort of solved the problem but still not quite right.
I ASSume you've heard of gas system dwell time. Too much barrel in front of the gas port results in some odd functional issues.

Last edited by Mobuck; April 24, 2019 at 06:14 AM.
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Old April 24, 2019, 11:57 AM   #41
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Quote:
Based on the lack of criticism, I'm guessing that no one clicked the playlist link and found the last video clip.
I did. After watching everything else, I would've tried it as well. I bet it could have reliably functioned as a semi-auto handheld, given sufficient rearward pressure on the barrel with your off-hand. Get some asbestos gloves used by Machine Gunners and mortarmen and try it!

Quote:
no one is suggesting that the new fad in AR building should be omission of the barrel nut, in order to save weight and make for quick take-down.
Ah, but I'm now thinking that a quick take-down system could be designed easily enough with a torque slip-nut. And I mean by a moderately skilled yet innovative home machinist with a decent home shop. May not be sub-moa accurate, but I bet it would still be much more accurate than your standard Century Arms AK. While I think the armorer manuals for the M4/AR15 weren't written haphazardly, I also think that the torque specs for the barrel nut are quite possibly forgiving.
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Old April 24, 2019, 12:23 PM   #42
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Interesting experiment that has shown me how unnecessary it is too be overly concerned about specific barrel nut torque settings. Looks like "good and tight" is good enough.
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Old April 24, 2019, 02:23 PM   #43
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The barrel nut torque specs are a pretty broad range. It’s to get in the range of “it won’t come loose, but not to tight to damage the receiver”

An experienced technician that turns wrenches every day could hit the mark without a torque wrench.
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Old April 24, 2019, 04:39 PM   #44
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What nobody has mentioned is that when the bullet exits it leaves behind a barrel acting as a rocket motor pushing the barrel back into the receiver hard enough to keep it in place and overcome any forward propulsion effort by gas exiting the back end of the gas tube. This continues until the pressure in the barrel drops substantially. After all, not much rearward force is needed. At .224 bore diameter, a residual pressure of just 25 psi would still apply a pound of reward push. It takes a while to drop that low. Over 4 milliseconds.

Disassembly could be expected if you tried to sling it over your shoulder.
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Old April 24, 2019, 08:24 PM   #45
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I hadn't really considered the "rocket nozzle" effect, but it does seem reasonable. It does, after all, contribute to recoil; and the chamber is 'plugged' by a case.

Quote:
I did. After watching everything else, I would've tried it as well. I bet it could have reliably functioned as a semi-auto handheld, given sufficient rearward pressure on the barrel with your off-hand. Get some asbestos gloves used by Machine Gunners and mortarmen and try it!
It was fun.
I do not advocate trying such, nor will I say that I intend to do such again.

However, the list for another round of testing includes the following possibilities:
Mandatory: More consistent ammunition.
1. Barrel totally unsupported.
2. Multiple rounds in a better magazine.
3. Bolt locking open if properly supported (not recoiling against a rotting, unstable log).
4. Will it fire after the BCG pushes the barrel forward?
5. Yank on barrel after firing, to show that it's solidly locked.
6. Hand-held semi-auto.
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Old April 24, 2019, 09:06 PM   #46
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Next time I’ll bring a lancer or GI mag and a supply of known ammo
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Old April 24, 2019, 10:13 PM   #47
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Quote:
Ah, but I'm now thinking that a quick take-down system could be designed easily enough with a torque slip-nut.
The AR already has a quick take down feature, its the TAKEDOWN PINS...

You might come up with a "quicker" barrel change with a slip nut type thing, but it won't be a true "quick change barrel" as found in most belt fed machineguns.


Quote:
While I think the armorer manuals for the M4/AR15 weren't written haphazardly, I also think that the torque specs for the barrel nut are quite possibly forgiving.
I don't know what the current manuals say, but I can tell you that for Army Small Arms repairmen in the 70s, there were no "exact" specs, The manual said to tighten the nut to 35-40 ft/lbs and then tighten to align the nut to allow the gas tube to pass through,
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Old April 24, 2019, 11:10 PM   #48
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Quote:
I ASSume you've heard of gas system dwell time.
Gas dwell time is usually defined as the time it takes the bullet to travel from the gas port to the end of the barrel and is the time during which the gas is at high enough pressure to cycle the action.

In the case of a carbine gas system on a 24" barrel using relatively normal ammunition, gas dwell time would be well under 0.001 seconds while it should normally take around 0.07 seconds for the bolt to cycle.

The gas gives the bolt carrier a "kick" that gets it moving, but then very rapidly after that point, the pressure will have dropped. In contrast, it will take around 100 times longer for the bolt to cycle than it will for the gas pressure to drop off.

I don't know for certain why the gun was double-feeding, but I don't see how it could have been residual pressure in the gas system. The pressure in the gas system drops off much too rapidly for there to be any useful amount of pressure in the system by the time the bolt has cycled back forward.

If I had to guess what was going on there, I would say that due to the carbine length gas system on the longer barrel, there was a lot more gas pressurizing the system and that increased the force on the bolt carrier a lot. So the bolt was being flung backwards so violently that it came forward with sufficient force to bounce back off the breechface far enough back to pick up another round. It was probably very hard on the rifle.

This video shows that the bolt can bounce back with defective ammo (this ammo was causing case separations). It stands to reason that if it came back forward a lot harder due to being flung back with a lot more force than usual, it would bounce back a lot more than normal.

https://www.youtube.com/watch?v=aqt9O74nlbA
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Old April 25, 2019, 12:07 AM   #49
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I have not looked at the book in several years,but in "Black Rifle 2" they discuss buffer development,and "bolt bounce" was a factor
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Old April 25, 2019, 09:27 AM   #50
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As I predicted, someone actually tried it. So now that the "Hold my beer" episode is over and thankfully, no one was hurt, trying to figure out why it turned out the way it did is in fact, very interesting.

For those that feel the basic barrel nut torque specs no longer apply I would agree but only to the extent that the barrel will probably not fall off. I still feel they apply to the point of controlling barrel harmonics. I think most of us understand that repeatable harmonics are the key to improving accuracy.
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