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Posted: 1/24/2006 1:25:23 PM EDT
[Last Edit: Dano523]
Link Posted: 1/24/2006 4:31:55 PM EDT
nice write up.

A more detailed account of what happens in the FCG would be great also.
Link Posted: 1/24/2006 4:48:41 PM EDT
[Last Edit: Randall_Rausch] [#2]
Link Posted: 1/27/2006 4:54:18 PM EDT

Originally Posted By Randall_Rausch:
I wrote this the other night in response to another post, but I think it's good enough to have it's own post.
If I can get setup as the new Moderator to cover for Tweak, I'll probably tack this or add it to one of the other tacked threads.

Randall's description of AR operation and how everything works in harmony.
I have not written this up in a while and some day I really need to build a whole page dedicated to it.
This is just off the top of my head.
Much of it comes from Rick McDowel (competetion specialties) when I was first learning AR's & from Tweak along the way and my own experiences mixed in along the path to enlightenment.

Ok, starting with a cartridge in the chamber, hammer back.
Trigger lets the hammer fall.
Hammer hits the firing pin, driving it forward.
Firing pin drives the primer (and attached cartridge case) forwards in the chamber until the shoulder in the chamber stops the shoulder on the cartridge case.
This distance could also be called headspace which is the distance from the bolt face to the head of the cartridge when fully seated in the chamber.

Maybe I'm wrong, but it's my understanding that headspace is the distance from the chamber  shoulder to the bolt face;  that a properly headspaced rifle will have little or no "slop" of the cartridge in the chamber.  IOW, the cartdridge shouldn't move forward under the influence of the firing pin striking the primer because the cartridge is already fully seated, shoulder to bolt face, in the chamber.


Upon reaching the gas key bolted to the top of the carrier, it turns down into the bolt carrier where it is given a nice place to expand.
This is the area inside the bolt carrier where the bolt lives.
Gas expanding here forces the bolt carrier back AND the bolt forward.
Note that the bolt is also being forced BACK by the gas pressure expanding the cartridge case on the other side of the bolt.
For a short moment in time, these forces are about equal.
Ideally, this is while the bolt lugs are unlocking and before the extractor starts pulling on the case.
The bolt carrier starts to move backwards against the inertia of the carrier's weight, the buffer's weight and the operating spring.
My understanding here is that the gas escapes from the gas tube, through the carrier key, and impacts both the "back" of the bolt face and the rings on the tail of the bolt.  The only real gas effect on the carrier here appears to be radial, not longitudinal.  The bolt can't travel forward (locked against the barrel extension), so it travels backwards, operating the cam, unlocking the lugs, and pulling the carrier along with it.  The carrier really is just along for the ride on the extraction phase of the firing cycle;  it's only real contribution (aside from structural) is to provide a camming surface to alow the bolt to unlock.  Oh, and I guess it does keep the bolt travelling in a straight line :).  

Link Posted: 1/27/2006 6:52:27 PM EDT

Don’t take this personally, but the last place I would like to see any dealer or supplier is as Mod of the trouble-shooting forum.  Granted that you may have the experience to do the job, but the question comes up regarding best interest of the members/yourself.

Too many times we have seen favoritism in regards to one manufacture over another from dealers, or in regards to magazine writers or write ups, lack of calling a lemon a lemon when type/rifle/part crosses their desks (read posted more as a selling ad then a true review).

99.9% when there is a parts/production problem (not related to cleaning), you have to call it as it is. Not only is the option of sending the part back for replacement optioned, but the way to bring the part back into spec (as a system) is voiced as well. As a dealer-mod, there is the question of a generic responses in regards to fear of needing to sugar coat the response to prevent reprimanding from one of your suppliers, out of fear that it may cost you money if parts you do stock/sell would need to be replaced to customers (defective parts on a large scale at your own cost), or the question of motivation of further sales recommending a personally stocked part replacement when there may be options of simple smithing of existing to bring the system up to spec.

Tweak was the ideal Mod since he had worked for several shops on the smith level, had seen the problems in both the production area, and repairing the after math of less than idea kits that had been produced with out of spec parts.  Truthfully, if someone is to replace tweak, I think the integrity of the site would be better suited by another smith type (not sales type) that did not have a vested interest in one supplier over another, and would call the shots as they truly present themselves (not on a vested interest).

Many of us are saddened by Tweaks ordeal/absents, but the real travesty to him/ his legacy would be to allow the trouble-shooting forum to go in another direction that he instilled it maintained over the years. Having filled in for him from time to time over the past few years, I have always thought of the Tweaks/his direction maintaining and foreseeing the forum before posting any responses, and only wish that the new mod appointed by the God of AFCOM, does so to maintain the level of excellent as a correction type forum with smithing options (and not just as another sales base tool of the site).


Link Posted: 1/27/2006 9:56:30 PM EDT
Link Posted: 1/27/2006 10:04:54 PM EDT
Daniel, I am somewhat surprised by your post. First, Randall is not a moderator. Second, he gave one of the best write-ups about AR-15 functioning I have ever seen on this board. Hell, it's probably one of the five most useful things ever written on this board, much more valuble than slogging through page after page of EOTech vs. Aimpoint, Colt vs. Bushmaster, or 1/9 vs. 1/7 posts. It deserves a pin-up at the top of this forum. If you want to be a Mod, I advise you put that at the top of your agenda.
Second, I have never seen Randall express any favoratism for one brand or another. Let me repeat that again. Randall has NEVER expressed favoratism to one brand or another, even his own products.
If you want to be a mod, step up to the plate, help out on this forum, and do you part. I have no objections to you as mod in the absence of Tweak, but don't go after one of the most helpful people around.
Otherwise, in a manner of speaking, LAY OFF.
Link Posted: 1/27/2006 10:24:51 PM EDT
[Last Edit: j3_] [#7]
Getting ready to change into someone more comfortable. Name changes coming.
Link Posted: 1/27/2006 10:26:25 PM EDT
[Last Edit: Randall_Rausch] [#8]
Link Posted: 1/28/2006 1:02:29 AM EDT
[Last Edit: FredMan] [#9]
Randall, thanks for clearing up my headspace confusion.  I was introduced to the concept of headspace reading up on my M1A and I'm still not sure I 100% understand it.

As for the gas impingment,   it seems you are saying that carrier rearward movement is begun to be driven by the effect of gas on the carrier key, even before the effect of gas on the bolt rings.  I guess the question that remains for me is this:  does bolt unlocking occur primarily as a function of the carrier "pulling" the bolt rearward, or as a function of the bolt "pushing" the carrier rearward?  It seems to me that both forces are at work here;  can one be termed the "primary" function over the other?

[ETA that upon further thought it has to be the carrier that performs primary unlocking function;  as you said the bolt is locked fore and after by barrel extension and locking lugs.  Only way for bolt to unlock is to rotate via the cam pin, and the only way for that to happen is for the carrier to do the "primary" moving]

To carry this thought further, what would be the effect of gas being vented from the carrier key to a point outside the carrier as opposed to inside the carrier?

I find this fascinating and extremely helpful to my full(er) understaing of the functioning of the AR type rifle.
Link Posted: 1/28/2006 2:28:45 AM EDT
[Last Edit: Randall_Rausch] [#10]
Link Posted: 1/28/2006 5:39:37 PM EDT

Tweak and I talked about me taking over a few months ago, and I told him that I would take on the position.  At that time, the site was not taking on new personal.

My thoughts are that since I do tend to piss of the advertisers/other posters (read I pull no punches), the Gods of afcom are afraid that I will pick one day to declare total Jihad on them, and me being in the role of Mod would not be in the best interest of the site.  Granted that Tweak did not pull punches either, he simply did so in a manner more tack full than I ever attempted.  Hell, I think the best post that every comes to mind regarding my lack of tack was the one over in the shotgun forum where Mike informed the fellow posters that Afcom was signing me up for sensitivity training classes.

Either way, as a Mod or just a Ghost in the machine like now, I will still post on the site (time willing, and/or not pissed off at someone at the site for a few weeks) to help others out as much as possible.
Link Posted: 1/31/2006 11:56:40 PM EDT

Does the gas vent into the carrier forward or backward of the rings?  I've always thought the gas acts upon the "muzzle" side of the rings, but based on your last, Randall, it sounds like the gas acts upon the "back wall" of that portion of the carrier directly behind the bolt, and the rings just seal the carrier from venting gas towards the bolt face.

If this is the case then a lot of my misunderstanding about gas effects is explained!
Link Posted: 2/1/2006 12:54:01 AM EDT
[Last Edit: Dano523] [#13]
Gas flows down the tube, threw the key and into the gas chamber section.  

The back end of the bolt semi seals the rear of thee gas chamber, and the rings seal the front.  With pressure added between these two points, the expansion would cause  the bolt to move forward , but since it can't go anymore forward, the carrier is pushed rearward, and since the carrier/bolt is prevented from spinning by the upper track slot via the cam, the bolt is cam'd open as the carrier moves back (cam tracking down the carrier slot).

To add, since the gas system does leak in more than a few points along the way, the barrel gas port is sized to include this normal leakage.
Link Posted: 2/4/2006 5:42:03 AM EDT
Link Posted: 2/4/2006 1:45:24 PM EDT

Outstanding.  I think I may have solved a porblem with an upper based on your writeup.  Thanks!
Link Posted: 2/4/2006 3:25:18 PM EDT
Link Posted: 2/4/2006 6:00:44 PM EDT
By any chance you have information on gas port sizes of mid-length and pistol gas systems? Thanks.
Link Posted: 2/4/2006 7:34:29 PM EDT
Link Posted: 2/4/2006 8:09:54 PM EDT
Nice post Randall, very informative.

This definately deserves a sticky
Link Posted: 2/9/2006 2:33:09 AM EDT
Link Posted: 2/10/2006 1:44:37 PM EDT

Great write up!  This should be required reading for all AR owners...
Link Posted: 2/10/2006 2:20:14 PM EDT
[Last Edit: AR15barrels] [#22]
Link Posted: 2/10/2006 2:53:06 PM EDT
Just wanted to say thanks for putting that out there, it was an excellent post and a great graphic.
Link Posted: 2/11/2006 8:49:40 AM EDT
Awsome post, Randall.
I converted it to a PDF, for whoever is intersted in downloading it, Randall, with your permission I'll host it and post a link
Link Posted: 2/13/2006 2:26:19 AM EDT
[Last Edit: AR15barrels] [#25]
Link Posted: 2/13/2006 7:51:27 AM EDT

Originally Posted By AR15barrels:

Originally Posted By Punani:
I converted it to a PDF, for whoever is intersted in downloading it, Randall, with your permission I'll host it and post a link

No offense, but I would rather you not distribute a PDF copy of this.
I intend for it to be an ongoing topic about ALL the different systems at work in the AR action and your PDF would just be a snapshot of the post on a specific day.
As I add more to it, your PDF gets more and more out-of-date.
This topic is tacked now, so there's no worry it will fall off or be hard to find.
Just spread around the link to this topic.

No problem!
Link Posted: 2/27/2006 11:49:02 PM EDT
[Last Edit: CCW] [#27]
Good stuff Randall,

  Please go back to the first:

[Trigger lets the hammer fall.
Hammer hits the firing pin, driving it forward.
Firing pin drives the primer (and attached cartridge case) forwards in the chamber until the shoulder in the chamber stops the shoulder on the cartridge case.
The case will already be seated against the shoulder due to ejector tension, but the primer can sometimes move before the anvil legs on the primer stop against the primer pocket.

Can the trigger fall against the firing pin if the bolt is full forward in the chamber, against the brass, but not rotated and locked, or just partially rotated,  ie. bolt carrier slightly back off of the full forward position?  If not, why not?


I suppose another way to think of the head space clearance is that if you did not have clearance, you would have line to line contact between the bolt face and the brass or an interference fit between the same.  The clearance allows an imperfect fit to still work over temperature ranges, over machining and assembly tolerance stack-ups,  etc.

Link Posted: 2/28/2006 12:28:23 AM EDT
[Last Edit: AR15barrels] [#28]
Link Posted: 2/28/2006 2:10:17 AM EDT
Link Posted: 2/28/2006 2:12:02 AM EDT
[Last Edit: Dano523] [#30]
Link Posted: 3/1/2006 2:40:43 AM EDT
[Last Edit: CCW] [#31]

Originally Posted By Dano523:

The safety feature of the AR/M-16 system is that when the bolt is unlocked, the back of the bolt (FP stop collar section on the bolt) is retactedreacted into the carrier.  As the bolt locks home via rotation of the bolt by the carier cam as the carrier drives forward, the back of the bolt then protudes beyond the back of the carrier to allow the firing pin to protude the needed amount of.032-.037* for correct ignition into the bolt it's self.
*Correct FP protusion threw through the face of the bolt with the FP bottom out on the back of the bolt (stop collar section) is .032 to .037 for 223/5.56 nato, and up to .039 in the 7.62X39.

On in spec parts, the bolt lugs will need to engaugeengage the barrel extension lugs at least 70%(+/-) before the back of the bolt clears the the back of carrier section. If in fact the bolt has not locked at least this amount, then as the hammer strikes the FP, the FP will bottom out on the back of the carrier, and then the hammer needed igniton force would be lost driving the carrier forward/home before the FP could even begin to bottom out in the bolt.

As for carrier face to barrel extension face gap of a M-16 for hammer release from the auto sear, the normal gap is .082 +/_.  If the release is advanced past the .110 range/gap, then the hammer comes down on a blocked FP (FP strikes the back of the carrier, with the back of the bolt not yet clearing the back of the carrier threwthrough lockup).

Excellent!  Thanks Randall and Dano523.  

Don't want to run a good thing into the ground, but one last question about the gas works, please.  When the carrier is accelerated to the rear by the expanding gas and the cam pin on the piston bottoms in the carrier slot (bolt unlocked and starting to travel with the carrier now), does the residual gas in the gas key and cavity between piston rings and carrier do any more work in accelerating the bolt carrier group (BCG) rearward, or is it all spent by now?  In other words, when the gas key clears the gas tube, is residual gas in the BCG jetting back out the gas key in any sufficient amount to add thrust?
Link Posted: 3/1/2006 9:28:02 AM EDT
Link Posted: 3/13/2006 2:26:22 PM EDT
Wow! Just found and read this thread, and can't quite remember learning so much at one sitting!

One quick, minor question: In the paragraph below, instead of "retacted" or "reacted", shouldn't the word be retracted?

The safety feature of the AR/M-16 system is that when the bolt is unlocked, the back of the bolt (FP stop collar section on the bolt) is retacted reacted retracted into the carrier. As the bolt locks home via rotation of the bolt by the carier carrier cam as the carrier drives forward, the back of the bolt then protudesprotrudes beyond the back of the carrier to allow the firing pin to protudeprotrude the needed amount of.032-.037* for correct ignition into the bolt it's self.

Thanks! (Not for the answer to my question; but for the thread!)

Link Posted: 3/13/2006 3:06:17 PM EDT
Link Posted: 3/14/2006 9:16:54 PM EDT
I have a gas port question that's been bugging me.

If the chamber pressure is high at beginning of the barrel and drops off as it travels down the barrel, then when the gas port is closer to the chamber, why do we need larger port size?  Wouldn't the higher pressure provide more cycling force?  If we need to match the pressure to longer barrel, won't we be using smaller hole?

It is also demostrated that short barrel can cause pre-mature extraction (high chamber pressure and with larger port size), people then use heavier buffer, longer gas tube to retard the timing.  Why not just reduce the port size and reduce the pressure?  (of course smaller port probably won't cycle)

Could it be that we need not only pressure but volumn (mass) for the gas as well?
Link Posted: 3/14/2006 10:10:01 PM EDT
Link Posted: 3/16/2006 8:35:32 PM EDT
ahh... It is all clear now

So theortically, we should be able to calc the size of the port based on distance from muzzle to gas port and distance from gas port to chamber - to establish the min gas volume required.

Now the tricky part is to figure out the time it takes to unlock the bolt and how long it takes the bullet to leave the muzzle to drop chamber pressure.  Thanks again.
Link Posted: 3/31/2006 11:59:38 AM EDT

Great writeup.  One question though, is that a notional trace or is that an actual trace using a strain gauge?  Reason I ask is you are showing a 60,000 psi MAP but that's above SAAMI max.  Is that laod from a manual or is it something you worked up?
Link Posted: 3/31/2006 1:48:40 PM EDT
Link Posted: 3/31/2006 2:18:36 PM EDT
[Last Edit: jmart] [#40]

Originally Posted By AR15barrels:

Originally Posted By jmart:

Great writeup.  One question though, is that a notional trace or is that an actual trace using a strain gauge?  Reason I ask is you are showing a 60,000 psi MAP but that's above SAAMI max.  Is that laod from a manual or is it something you worked up?

The trace is purely an estimation, generated by Quickload.
The test conditions are in the bottom of the graphic.
I can run a trace like that with ANY combination of projectile, powder and pressure.

The trace is "close enough" as it's purpose is just as an educational aid.
SAAMI and NATO specs are different on the pressure by the way and most of us shoot NATO spec ammo at one time or another.

TM 43-0001-27

Thanks for clarifying.

About your comment regarding NATO pressure vs SAAMI, the link above (Chapter 10) lists pressure as 52,000 psi for M193 ball and 55,000 psi for M855.  This falls right in line with SAAMI (actually, a tad lower for the M193).  

I've heard over and over that NATO pressure runs higher so I was pretty surprised when I saw these specs.  Do you have any idea if the TM is in error?  Or are we talking that NATO pressure loads, when fired in a NATO chamber, comply with the 52K psi limit, but when fired in .223 SAAMI chambers the perssures are way higher.  I'm wondering if your QL model is basing it's pressure calculation on a .223 SAAMI chamber vs a NATO chamber.  Any comments?
Link Posted: 3/31/2006 7:28:43 PM EDT
Link Posted: 3/31/2006 8:38:10 PM EDT
Roger that.  I'm well aware of temp sensitivity of different powders, especially ball powders.  Still I was surprised to see pressures in the low-mid 50,000 psi specified in the TM.  Also had never heard of it, but there's a proof load cartridge designated, something like 70,000 psi, but again that's a proof load.

Just out of curiosity, based on your QL plot, I went to Hogdon's website to see if they listed the Sierra 63 and H-335.  They did but it was 25.0 grains and 50,000 CUP.  I realize CUP and PSI are two different measurement systems, but after seeing their load data, it doesn't surprise me that QL spit out a charge that was more than 2 grains over Hogdon's listed max.

I dig internal ballistics.  Call me weird ........
Link Posted: 3/31/2006 9:37:00 PM EDT
Link Posted: 3/31/2006 11:06:37 PM EDT
Link Posted: 5/24/2006 10:34:36 AM EDT

First, thank you for helping an old retired bolt-action sniper to better understand the operation of his AR-15.  It helps more than you could ever now.

I now own a Colt Sporter AR-15 7.62x39 and in love with its performance and round used.  I have learned that the pressures from the 7.62x39 are less than that of the .223/5.56.

My question revolves around this caliber and is their any areas of concern that I should be aware of since I am using the 7.62x39 over the .223/5.56?  This AR eats Wolf Russian ammo like a starved child.  Is their any thing I should as well be concerned about with the steel cases used?

BTW, since the news of the new mags coming out for my AR, two of my friends have now asked me if I would consider selling it.  My reply was only if someone offered me between $7,000 to $8,000 for it.  That would allow me to obtain another to replace it and some extra toys to go with it.

Thanks Again,

Link Posted: 5/24/2006 10:55:41 AM EDT
[Last Edit: Dano523] [#46]
Link Posted: 5/24/2006 11:14:44 AM EDT

Originally Posted By Dano523:

As you stated, and have figured out, the mags for the rifle have been the weak link with the system since some of the cheap clones were just a waste of time (read jam city, and a big reason that receivers like the AK-15 that used AK mags where designed).


I must have the luck of the Irish on St Patty’s day Dano.  When I got the rifle, it came with 4 (20 round) after market mags and 1 (10 round) quality mag.  While I had to rework only one of the mags, I have 5 dependable mags that I would go into combat with.

The reason that I’m alive today is based in the fact that I had good instructions that taught me that rule #1 is “KNOW YOUR WEAPON”!  Having additional information such as posted in this thread only helps to obtain that goal.

I have held off for over 25 years in buying the AR-15 because of the limits placed on the .223/5.56.  When I found on in the 7.62x39 I pulled the strings necessary to buy it and have NEVER regretted (other than the offers to sell) it.

Thanks for the additional info Dano.


Link Posted: 5/24/2006 2:26:01 PM EDT
[Last Edit: AR15barrels] [#48]
Link Posted: 5/24/2006 3:12:31 PM EDT
thanks Randal,
this was a huge help to those of us who have relativaly no knowlage of the inner workings of our equipment! Thanks again!!
Link Posted: 5/24/2006 3:59:40 PM EDT

Thanks for the extra information.  I find it very helpful to better understand the parts at play in this system.  I have always had some concern over the ‘backtrust’ issue in this weapon system using the 7.62x39.

I am looking to have a good parts supply for this weapon.  However, finding quality parts is another mater when it comes to this combination as I have found.

With your experience what do you think the rounds/bolt rate would be?  Do you have any suggestions as a good place to start looking for a good quality replacement bolt for my AR?

I for one am very glad that you made your post as this has been the best source of information beyond load, point, shoot that I have come across.

Thanks Again,

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