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 As I add more to it, your PDF gets more and more out-of-date.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!

Originally Posted By CCW: 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?

The carrier blocks forward firing pin travel.
If the bolt is partially locked, the bolt carrier is still at least 0.075" from the barrel extension.
As the firing pin protrusion is somewhere around 0.040" when seated, the firing pin would not be able to reach the primer when being held back 0.075" by the carrier.
What would happen is that the extra energy of the hammer would tend to drive the carrier closed.
Maybe, BIG MAYBE, there is enough momentum to fire the primer, but not likely.

If the carrier is in motion, it will have finished locking before the hammer gets there if the hammer is released while the carrier is still back 0.0.075".
When you check the autosear timing on an M16, you are looking at the distance the carrier is from the bolt carrier when the hammer is released.
Off the top of my head, this is about 0.100"or less.
Any more and the hammer will beat the carrier and you get misfires.

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.

Exactly right, that's why we NEED some headspace, to allow for manufacturing tolerances.
Standards and Gauges are just an implementation method so that multiple makers in different areas can make products that are all compatible.

Originally Posted By CCW: Can the trigger hammer 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?

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 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 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 engauge 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 threw lockup).

Originally Posted By CCW: 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?

By the time the bolt is completely unlocked, the gas rings have cleared the exhaust ports in the carrier and vented all excess pressure out the ejection port, so NO, there is no residual gas pressure doing any work.

Originally Posted By Ingeniero1: One quick, minor question: In the paragraph below, instead of "retacted" or "reacted", shouldn't the word be retracted?

Yeah, you have it right, it's retracted and protrude, Dano probably types fast and posts before proofreading just like I do.
Maybe his "R" key is not working good on his keyboard.
Ignore minor spelling issues, you probably understand what's happening if you caught the spelling errors.

Originally Posted By Walmart_Special: 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?

HaHa, he gets it...

You need a certain VOLUME of gas to function the action, not a certain pressure.
You can get this volume with a large port and short duration or low pressure or with a small port and long duration or high pressure.

With the SAME barrel length, you make the port smaller as it gets closer to the chamber.
What you are doing here is two different things, first tapping into a higher PRESSURE gas supply, but ALSO increasing the DWELL TIME.
Now, when you shorten a barrel, while keeping the same gas system, you are simply reducing the dwell time and it's appropriate to enlarge the gas port accordingly.
If you are COMPARING two different barrels and both have the same amount of barrel length past the gas port, then the port will likely be smaller on the shorter barrel as that port is nearer to the chamber and therefore gets a higher pressure gas supply.

Running a proper size gas port or adjustable gas system is FAR BETTER than resorting to a heavy buffer (more reciprocating mass means more muzzle rise), but it's easier to just buy a heavy buffer and swap out parts than properly correct the gas flow.
Adjustable gas tubes are $60 and offer one more thing to go wrong and heavy buffers are like $15, what would you choose?
MOST AR's come over-gassed from the factory so that they will run correctly right out of the box without waiting for the gas rings to seat into the carrier.
Once the gas rings seat, the rifles are more likely to show signs of over gassing.
That's one more reason heavy buffers and o-rings on extractors are so commonplace.
For these very same reasons, you can take a 16" (carbine length gas system) barrel, cut it to 14.5" and they run perfect.
This is not always the case though, I recently cut an LMT MRP 16" (mid-length gas system) down to 14.5" which DID require being opened up some.
I was pleased that the MRP barrel gas port was more towards the small side of the range.

Randall,

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?

Originally Posted By AR15barrels: Originally Posted By jmart: Randall, 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?

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 ........

OK, just for fun, I also went and checked Hodgdon's site as well.
They list the 63 Sierra SP using 25gr pf H335 as making 50,000 CUP and 3051fps in a 24" barrel loaded at 2.200" long.
How is this for a Quickload validation/comparison?
Quickload estimates 47,916 PSI and 3021fps from a 24" barrel.
As Quickload is just an estimation and Hogdon's data is from actual testing, it's safe to assume that Quickload is VERY close/accurate.
It's well known that the same load in different barrels can vary 50 FPS.
I could probably tweak the rifling bore/groove dimensions to get quickload within a few FPS and it would correlate to Hodgdon's published test results with other powders/bullets as well.

If you like playing with internal ballistics, bone up the $150 and buy a copy.
It's the coolest program you will ever own.

ArticWolf,

The greater body taper of the 7.62 X 39 over the 5.56 Nato cases leads to better/cleaner extractions. Furthermore, since this is a Colt upper/barrel, the chamber is more on the par with a true military chamber, and not the tighter produced (side wall dimensions) then what is being offered by some of the non-colt suppliers (again, another bonus).

In regards to the 7.62 X 39, coated steel case ammo is the standards and is no concern with the colt rifle, and truth be told, should not be a problem for the other non-colt systems as well.

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).

To sum it up for you, if you are going to run an AR type receiver in 7.62 X39 and already have the colt upper system/barreled action, selling it off for any price is going to bite you in the ass at the end trying to find a cheap reliable replacement kit type rifle to replace it. On the other hand, if you are looking to go exotic for a replacement rifle for it, then someone like AR15barrel would be a great source to have the barrel made, or even have a complete upper/rifle built to your specs.

Articwolf-

Good to hear that you have some good reliable mags with your 7.62x39 upper.
My only suggestion is to get yourself a spare bolt.
7.62x39 bolt thrust runs above the design parameters of the M16 design.

Here is my bolt thrust page that allows you to play with pressures and see how they affect bolt thrust:

Bolt Thrust