M16/AR15 Function

[mnotx]

I threw this into another thread which touched on the subject, but the other thread sort of digressed from bolt lubrication. Thought I would post it alone since there are quite a few experts here and I was curious if I understood the function correctly. So here goes the previous post:

I thought a properly working AR15/M16 was designed to have the pressure equalize momentarily at the time the bolt head rotates to unlock. That is why the gas ring seal, firing pin flange, and timing are so important. The bolt lugs should be under little frictional pressure as they rotate due to the equalized pressure behind the bolt and in the chamber. As the pressure pushes the bolt forward, it also pushes the bolt carrier to the rear, rotating the lugs out of lock. The bullet should exit the barrel just before the lugs disengage and allow the pressures to drop, leaving the momentum imparted to the bolt carrier to finish the cycle.

 

[mnotx]

Re: M16/AR15 Function

Wow, 56 looks and not a single response. I was hoping one of the armorers on here would chime in and confirm or correct what I was taught.

 

[maladat]

Re: M16/AR15 Function

I don't buy it. The receiver has too many holes in it to pressurize to the same level as the bore behind the bullet and if it did there would be major damage. Maybe I am misunderstanding what you are asking. Are you asking if it pressurizes inside the BCG? I don't really see how that would happen.

 

[mnotx]

Re: M16/AR15 Function

Yes, strictly inside the bolt carrier. The gas is directed into a space inside the bolt carrier between bolt gas ring seal and the back of the bolt carrier. The firing pin flange should form a seal in the back of the bolt carrier, assisted by the hammer to resist the pressure. The gas pressure should push the bolt forward and the bolt carrier rearward. That is why the gas rings seal the bolt like a piston. The theory being that the pressure behind the bolt and inside the chamber are equalized for a moment, thus allowing the cam pin and bolt lugs to rotate without the extreme pressures and resulting friction. The bullet should exit the barrel just before the bolt lugs disengage, lowering the pressure before the bolt unlocks, allowing the cartridge brass to contract before extraction. The carrier group continues rearward through the cycle due to the initial momentum imparted to it.

 

[mnotx]

Re: M16/AR15 Function

It is difficult to visualize, but if you have a complete bolt carrier group (carrier, bolt, cam pin, firing pin, etc.) look at it closely. When the bolt is pushed back to it's "locked" position, the large flange on the firing pin completely covers the back of the bolt and the hole in the bolt carrier. This is the back seal, assisted by the fallen hammer to resist the pressure. The gas rings are the seal on the bolt. The area between these two is where the gas is directed from the gas key. When the bullet passes the gas bleed port and travels back down the gas tube to the gas key and inside the bolt carrier, the pressure is the same momentarily inside the barrel chamber as well as behind the bolt. The unlocking of the bolt should occur under very little friction at this point. The key is the timing of the bullet exit. Pressure should drop just before the bolt unlocks completely, but not earlier. Adjusting timing problems due to various barrel lengths can be accomplished (within reason) by different weighted recoil buffers. Heavier buffers resist rearward momentum longer, giving the bullet more time to exit and vice versa.

It makes logical sense. Why would the gas be directed through a hollow gas key to an area inside the bolt carrier just to make the bolt carrier move rearward? That could be accomplished by simply blowing the gas against a solid gas key which covered the gas tube a little like an outside piston. The momentum would be imparted to the carrier without entering the carrier and gumming up the inside and the back of the bolt. That would also negate the need for piston rings on the bolt, for that matter.

 

[maladat]

Re: M16/AR15 Function

My bad. Not much sleep lately and I wasn't thinking about it. I am sure that in a broad sense you are correct but I don't know enough about it to argue the details.

 

[LFOD1776]

Re: M16/AR15 Function

Here's what's going on inside the bolt/carrier...

Gas enters the space between the gas ring on the bolt, and the hole in the BCG through which the firing pin extends. That space is called the cylinder. It's a lot like the cylinder in a car engine.

The gas is under pressure, and hence it exerts a force across the entire inner surface area of that piston. Outside of the receiver, there are only two things that make up that cylinder that can move relative to one-another -- the carrier and the bolt. Inside the receiver, when a shot has been fired, only the carrier can move. That is because the bolt is locked up with the barrel extension.

Thus, the pressure from the gas pushes the carrier backwards. As it moves, the cam unlocks the bolt, and the bolt is yanked back with the carrier. Note that according to the patent (column 3 line 5), illustrated in figure 5, and also in reality, the instant that the carrier begins its rearward travel it is cut off from the gas supply in the barrel, so the pressure in the cylinder and the pressure in the barrel are actually decoupled. The cylinder is pressurized only enough to start the rearward travel of the carrier. So as a practical matter, no equalization can be guaranteed, nor should it be required.

You're right, that the gas key could be a cup instead. In some shape or form, that is essentially how all piston ARs work -- except they're self-cleaning instead of terminating in a little gunk trap.

Unlike the DI system, all piston systems impart torque on the carrier group. I think people call it "carrier tilt". In practice it doesn't seem to stop these piston systems from functioning.

So, why do you ask?

 

[mnotx]

Re: M16/AR15 Function

Curiosity and the pursuit of facts. My explanation was taught to me about 40 years ago. One of the points made back then was that the pressure equalized momentarily until such time as the bolt carrier traveled enough to disconnect the carrier key from the gas tube, or the bullet left the barrel. This was supposed to impart forward pressure on the bolt equal to the rearward pressure on the carrier, reducing the stress on the bolt as it is rotated. Looking at the design, it appears that for a brief moment the pressure would equalize (or nearly so) until the gas key left the gas tube, or the bullet left the barrel. (Not considering leakage) It makes sense since rotating the bolt while there is thousands of pounds of pressure pushing against it would tend to wear the lugs quickly.

 

[LFOD1776]

Re: M16/AR15 Function

It seems unlikely to me that there would be a near-perfect equilibrium of pressure, as it seems impossible to guarantee such a thing operationally. No mention of this benefit is made in the patent (or I missed it), of course that doesn't necessarily mean anything. There may indeed be such a "sweet spot", provided that the barrel length, propellant mixture, ambient atmospheric temperature and pressure, carrier mass and gas-block placement are all 'just so'.

In a piston-driven AR, there would be no back-pressure at all against the bolt. Yet they seem to work quite well. I'll add that the AR bolt design borrows heavily from the Johnson Model 1941, which is recoil-operated.

So I would conclude based on these facts that no back-pressure is necessary for the system to function reliably.

I hope at least my explanation satisfies your curiosity surrounding the function of the gas ring (to make the cylinder seal efficient) and the function of the cylinder (a gas expansion chamber which imparts rearward momentum to the carrier without torque).

Mr. Stoner's design is certainly elegant, in my opinion, but not magical.

 

[mnotx]

Re: M16/AR15 Function

I can't open the patent description for some reason, but I will try again once I get home. However, if it isn't mentioned in the patent description, then I would think it was not very relevant.

Thank you for the explanation. I was hoping someone with knowledge would chime in.

 

[mnotx]

Re: M16/AR15 Function

Well, now it gets controversial. I was wandering the AR15 forum and found this:

http://www.ar15.com/content/page.html?id=256

This is basically what I was told years ago.

If it isn't mentioned in the patent, then I would think it was either not an important feature (No way), or not a true function. But the way the Carrier group is designed, it would lend credence to the pressure equalization theory. Anyone have Eugene Stoner's email address?

 

[LFOD1776]

Re: M16/AR15 Function

For some reason QuickTime only displays the patent image in IE (not Firefox), at least on my computer. YMMV.

You may be able to email Gene's buddy C. Reed Knight!

 

[mnotx]

Re: M16/AR15 Function

Yeah, I got it open at home. The patent doesn't mention it anywhere, so apparently it wasn't an original design function. But does it make a difference? I think the only way to know that would be to test two identical rifles, except one being DI and the other piston, and see if there is a difference in wear on the locking lugs. That probably won't happen. But it was fun discussing it! Thanks for your input.

 

[PLeighton]

Re: M16/AR15 Function

Interesting conversation...

Here are a few thoughts to consider:
- If the OP's question was indeed fact, then piston ARs would be wearing out bolts and barrel extensions at an alarming rate.
Some manufactures have started to install springs to simulate what you are talking about though.
- As the article that you referenced correctly states, it is all about extracting the case after the chamber pressure is lowered to a sufficient level.
It is the timing of the operating sequence, not the matching of pressures / forces.
- Yes indeed there is some forward pressure exerted on the bolt and it does contribute to less stress on the locking lugs, but not to the extent that the operating principle is dependent on it.
- Think of the direct impingement system as an inline piston that is located on the same plane as the bore.
This allows for less impact on the barrel and is the main reason that precision AR platforms built by folks like APA & GAP are DI & not GP systems.

As strange as it sounds, instead of counting sheep when I can't sleep, I visualize the firing sequence and subsequent actuation of all the components in a DI AR type platform.
Looks a little like this: M16 Bolt Animation
Puts me to sleep every time...

 

[LFOD1776]

Re: M16/AR15 Function

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Dr. Phil</div><div class="ubbcode-body">Some manufactures have started to install springs to simulate what you are talking about though.</div></div>

My Adams Arms PDW upper has a spring to put some backwards force on the carrier against the buffer (you have to press the carrier in when closing the receiver). They say it's to prevent (or counter) carrier tilt.

 

[Rovieairto]

Re: M16/AR15 Function

Yes the forces acting on the bolt come from two sides.
The bolt face is pushed rearward under gas pressure in the chamber, that pressure is conducted back into the carrier and the two forces acting on the bolt fight each other becoming subtractive, thus lessening the force of the bolt on the lugs.

The carrier does not cut off the gas immediately upon movement.
It has less than 1/4" of rearward travel before gas pressure is vented (free travel + camming travel), and the lugs are rotating then so the stresses, at that time, would be reduced.

This is not done on purpose to lessen the stresses on the bolt while it rotates (it's nice that it helps so the designer can forget about it), that is the job of the dwell time designed into the "free travel" portion of the camming slot in the carrier to abate lug rotation pressure.
The peak stresses occur before the gas system can relieve any of it, and the "timing" needed to be a significant design quality as it relates to lessening the lug forces during rotation, would be spotty at best because propellants do not behave with enough precision to do so.
Basically, it usually helps a little, but it was designed to operate without any benefit.

I've reduced the entire AR15 down to numbers, and the physical application of.
I do stuff like that for fun like some folks like to work crosswords.
It's been a while since all those numbers were fresh in my head but I love talking about crap like this.

The D.I. AR really is a (annular) piston operated rifle for those who didn't know.
The AR-15's gas system physics is very similar to the AR-18's.

mnotx, the best way to learn is to teach yourself IMHO.
The internet should not be listened to, including me.

I have a tremendous amount of technical writings concerning firearms, and I'd be willing to share with you if you are interested.

 

[PLeighton]

Re: M16/AR15 Function

A really need variation on some of the specifics that we are discussing here is the LMT Enhanced Bolt Carrier.

LMT made several modifications to change the bolt lock timing and to as well as to slow down bolt velocity.
I've been using them for years and have had nothing but positive results.

Here is the best description of the product that I've ever come across.
http://www.m4carbine.net/showthread.php?t=3379

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bill ALexander</div><div class="ubbcode-body">Let us perhaps be a little more precise with regard to the details of the LMT enhanced bolt and carrier system. As shown the details are very often misinterpreted.

In the first instance one must understand why the system exists, more particulaly in the context that the design is brilliant in its execution, but as reported frequently on the errornet is not always reliable. The design basis for the assembly is specifically to enhance the durability of the M4 configuration as currently issued. This gun is to some extent unbalanced. Port pressures running military grade ammunition are well in excess of the design levels which leads to very high carrier acceleration rates (+22 f/s/s). this combined with chamber pressure drop often creates the undesirable condition that the bolt commences to unlock while still subject to a degree of head thrust from the cartridge case. The result is that the lugs are subject to a biaxial loading of both shear and bending.

The LMT system being both the carrier and the bolt seeks to operate directly in this gun and elleviate symptoms. The carrier is set up with a longer delay during the initial portion of its movement. To facilitate this longer cam path and movement of the bolt forwards in the carrier, the front edge of the carrier is extended such that it still continues to retain the extractor pivot pin. Additional exhaust vents act to drop the piston pressure faster and to relieve any blowby at the tail of the bolt.

The bolt itself is of particular interest. The function of the dual spring extractor is frequently misinterpreted as an attempt to add spring force to the extractor claw. Rather it reduces the fatigue that the extractor spring(s) undergo by allowing the use of longer springs with lower K values; the % relative compression during the movement of the extractor is reduced. Remember that additional extractor force is not required now that the carrier is slowing the extraction cycle. The mitigation of stress in the bolt is accomplished in several ways. Material is the least visible change but is important to the design. The traditional Carpenter 158 is abandoned, being replaced by a significantly tougher grade from a different manufacturer. The lugs themselves are generously radiused between lugs and at the rear the diameter is actually reduced to allow a larger transition radius to be machined. The incorrectly identified sand cuts on the lugs are stress relief cuts. These allow any individual lug to elastically deform and give a smoother load over the contact patch. While this type of feature is very difficult to calculate and even more difficult to implement it helps to place the lug in a true shear load rather than amplify the bending moment. As noted the lug opposite the extractor is relieved. This feature prevents the unequal transfer of load to the two opposite lugs but I would argue that the stress relief groove already in place largely accomplishes this purpose. This is a academic quibble so I will bow to LMT in this respect. There is one additional feature that can be found in the bolt, but I am not at liberty to disclose the detail.

When considering wether to use the LMT parts one must consider the weapon. Correctly ported guns will derive little or no benefit from a carrier that is specifically set up to absorb excessive port pressures and some degree of residual case pressure. If not sufficiently gassed the reliability will suffer. This is not a fault of the carrier rather a mistake made in the application so be careful with simple substitutions. The bolt itself is exemplary. If not constrained by a $ value the bolt is a worthy addition to any rifle and will do nothing but enhance the durability of this part of the system.

Bill Alexander
</div></div>

 

[mnotx]

Re: M16/AR15 Function

Hamilton

I received my education at Aberdeen Proving Ground many, many years ago. (I love the Internet, but don't take it as gospel) My original post was in a thread about the "best" heavy grease or oil for the bolt lugs. From what I remembered, the bolts lugs didn't need such special lubricants...they weren't under as much stress as some thought. In the other thread, I was relating what I had been taught, but cautiously since it was 40 years ago. I re-posted here in hopes that an expert would confirm or correct what I had been taught.

As for teaching myself, I still own 6 AR15s of various makes, 4 of which I built from scratch (every pin except the barrel extension pin). I have worked on many and built a few others. My knowledge of them is fairly good. They are fascinating weapons. (On a sad note, I just recently sold my class 3 Colt Model 727, suppressed by AWC System Technologies.)

I would love to see a wear comparison between the DI and piston AR15 bolt and barrel extension lugs. I lean towards less wear on the DI system. However, I simply don't have the equipment to test such theories.

My recollection appears to be fairly accurate. I am happy for that. If you have data regarding the M16/AR15 function, I would <span style="font-weight: bold">love</span> to read it. I mean that. I bet there are many others here that would, also. This is an interesting discussion. I would hate to see it end so soon.

 

[Rovieairto]

Re: M16/AR15 Function

I would love to start a section where people upload files that are of interest to the shooting community.

With rapidshare I believe one can upload up to 2 GB in a single file, and post a link that others can download from, for free.

I have many files that I know others would love to have, especially those serious about firearms design.

You would need winrar to unpack the single file.

I will upload a few files into a rar and post them here.

I will start with:
The Machine Gun (George Chinn), vols. 1-5.
The fifth (final) volume was really hard to find.

This is a complete firearms design reference that was paid for by our tax dollars, for our military.
I referenced it so much, I eventually printed it.

The file will take awhile to upload.

 

[Rovieairto]

Re: M16/AR15 Function


http://hotfile.com/dl/111399406/6d6aafa/The_Machine_Gun_(George_Chinn)_V_1-3.html
http://hotfile.com/dl/111398730/795084b/The_Machine_Gun_(George_Chinn)_V_4-5.html

Add .rar to the end of the file.
Use winrar to unpack.

I had to do it in two parts as the limit is 400 MB.

This is a compendium of autoloading developments and data.
The title is misleading.

 

[mnotx]

Re: M16/AR15 Function

This is going to take a while to read!

 

[Rovieairto]

Re: M16/AR15 Function

I seriously printed it.
The books would've been around $800 used, and that's if I could have found them.
I consider it a "must have" for the gun design enthusiast.
I currently have 109GB of data pertaining to firearms design.

Anyone out there have anything interesting?