Is a mid length gas system really better in a 16" AR?

[RX-79G]

Keep digging - still doesn't change the fact throttling gas flow is regularly used in multiple firearms designs and bringing up the FN pre-cycle design of the M96 has absolutely nothing to do with the reliability conversation between a mid-length or carbine length AR carbine.

I don't understand why all of your responses are rude. I assume you don't act like this when talking to customers.

There are many ways to build a 16" carbine - and all of them work. Your way is not the only one, but you can't seem to discuss the validity of the different methods without resorting to insults.

 

[Varminterror]

How would you suggest building an AR upper with an FN style regulator (which the operations manual DOES incorrectly call a "regulator")? I've seen it done, haven't seen the parts in over a decade.

My "way" of tuning AR's is not "one way," but rather a scientific method of balancing gas flow, carrier and buffer weight, charge weight, cartridge, barrel length, port diameter, spring rate, port position, and block port size to meet the customers needs. Gas flow isn't the only link in the chain, but most often, it's the most violated design parameter in barrel maker and rifle manufacturers design scheme.

If a customer came into my shop and argued about the gas system on an M96 as applicable to that of an AR, yes, I'd invite them to leave.

 

[Varminterror]

None of the systems used in self loading rifles regulate pressure. They regulate flow.

Reducing flow with orifice restrictions in non-steady state flow systems does reduce pressure. But I understand where that confusion comes from.

ETA: Since we're mincing words, self-loading rifles don't "regulate" flow either - they restrict it.

 

[RX-79G]

How would you suggest building an AR upper with an FN style regulator? I've seen it done, haven't seen the parts in over a decade.

 

[Varminterror]

Nice! I haven't seen a bleed block in years! I'd love to have one on one of my AR-10's which I bounce suppressed too often, but didn't know anyone was making one.

AGB's work great, and again, don't lose velocity like a bleed block, BUT, adjusting under a 15" or 17.5" handguard isn't handy. Bleeders tend to be easier to tune with a can.

 

[MistWolf]

Regulating flow with orifice restrictions does regulate pressure. But I understand where that confusion comes from.

They regulate flow. If it was a closed system, the orifice would not prevent the pressure from equalizing in the two sides.

A pressure regulator means it can be set so that one side will have less pressure than the other. It also means that the pressure can vary on the high side but the pressure on the low side will be constant (until the pressure is bled off). This is not true with a flow regulator.

If the system regulated pressure, we would not experience a change in carrier speed when using ammo that generates more or less pressure.

Anyway, the bottom line is that while the system is designed to change pressure, it does not regulate pressure

 

[Ibmikey]

Wow five pages of posts on a question that appears every couple of weeks. My deeply researched conclusion after building over a hundred and having about thirty five AR's in the racks and spending hours shooting both Carbine and mid length is : Buy what you want they both work great and can provide hours of trouble free fun

 

[Varminterror]

[T]he bottom line is that while the system is designed to change pressure, it does not regulate pressure

Right - and fair point on my incorrect use of the term. I got hung up on "regulator" after reviewing my M240 and M96 manuals as they both incorrectly call their adjustment systems "regulators." I SHOULD have used the word "RESTRICT" pressure, rather than "regulate." I'll edit my posts to correct my incorrect use of term to eliminate future confusion from this.

They regulate flow. If it was a closed system, the orifice would not prevent the pressure from equalizing in the two sides.

Same incorrect use of "regulate" here - it's easy to do so, however. They restrict flow as well as pressure. This isn't a closed, steady state system.

I design pressure systems for a living. I'll offer this one final confirmation of the science here because I want to be clear the pressure against the carrier IS reduced relative to the bore pressure.

As you stated - if this were a steady state closed system, the pressure would equalize over time on both sides.

However, the transient state AR-15 gas system is not a steady state, closed system. The orifice represents a "back pressure" device, which is how it limits flow. Across the orifice,

Here's an example of orifice restrictions restricting flow AND pressure compared to a non-restricted flow (these aren't my diagrams) - bear with me, these are steady state open systems, not a transient system like the AR, but the application applies:

Non-restricted flow - pressure across all 3 pipe diameters is the same, hence the water is spraying the same distance:

Same system, but with orifice regulator plates installed in the two larger lines - note, both pressure AND flow rate are reduced in the lines with the orifice restrictions, so the water doesn't spray as far in the flow AND pressure restricted lines. In this analogy, the top line is the bore, where the bottom two lines would be analogous to the gas system:

 

[RX-79G]

You really can't use an non-compressible liquid to model what happens to gases as they pass through nozzles. Boyle's Law doesn't work with water. Gas systems are not hydraulic.

 

[boom boom]

I believe that the greatest importance for the debate on the length of gas systems in an AR is for those building AR's from parts or if you are buying a gun assembled by the small no-name builders. If you do not select the right compatible components that go with your chosen gas system, then you will have functioning problems.

 

[Varminterror]

Never said anything about Boyle's law - but pressure drop across an orifice still applies regardless of compressibility, especially in transient systems (i.e. a bullet fired from a gun and the gas exposed to the gas port). Gas systems aren't "hydraulic," but pneumatic systems aren't really so much more complicated to understand when it's a simple pressure/flow reducing orifice in an open system.

Flow area restrictions are used to create pressure drop in all kinds of gas systems - let down valves are an example, most common for most folks in welding gas cylinder regulators. These are REAL regulators, as they balance spring force against gas pressure, but the principle is the same - they let down pressure by restricting the flow area (an annular/conical area around the poppet spindle). The gas pressure on the cylinder side might be 300psig, while the pressure on the outlet - in an open system, could be anywhere from 0psig when fully closed to 300psi if fully open with an obstructed line. ANYWHERE in between.

Grab some duct tape, a piece of paper, a coffee straw, and a regular drinking straw. Make a spitwad and press it into the drinking straw, blow it out. The pressure you generate creates enough force to blast out the spitwad - welcome to 3rd grade "physics". Now stick the coffee straw partially into the end of the drinking straw, and seal the two together with duct tape. Repeat the spit wad shot. Can you shoot it as far with the restriction in the line? PRESSURE doesn't care about flow, it's a force per square inch - your body is able to generate the same exhale PRESSURE regardless of what's in your mouth, so you have the same pressure on one side, but aren't able to generate the same pressure on the spitwad simply because there's a restricting orifice in the line - the coffee straw.

THAT is how an AGB reduces pressure.

 

[RX-79G]

Never said anything about Boyle's law - but pressure drop across an orifice still applies regardless of compressibility, especially in transient systems (i.e. a bullet fired from a gun and the gas exposed to the gas port). Gas systems aren't "hydraulic," but pneumatic systems aren't really so much more complicated to understand when it's a simple pressure/flow reducing orifice in an open system.

You didn't mention Boyle's Law, I did. The problem with comparing to a tank of compressed gas is that the gas is at 300psi, not 26,000psi, and it is cold with no particulate dissolved in it.

While you don't have to worry about what happens when you push room temp pure oxygen through a needle valve, you have other concerns when the pressure is 100 times that, hot and full of condensing particulates of carbon, silicate and metal. Push that through a nozzle and the gas will rapidly rise in temp at the restriction, then plummet on the other side.

The AR15 was designed to tap gas with a .096" hole and let it move through the .120" tube so the temp, pressure and flow would be as smooth as possible, preventing the particulate from condensing out of gas and fouling the system. If you have a carbine with a .0625 gas port, you've already restricted your gas path by nearly 60%. Add more restriction on the other side of that, and then open the gas path back up to .120" in the tube and you have created a hot flame that is then rapidly cooled, condensing the fouling onto the tube walls, as well as whatever material was burned out of the restriction.

That will work for quite awhile, but is pretty much the opposite of smooth, non-turbulent anti-fouling flow the 20" rifle was built to have. Using a vent system allows you to go back to the smooth, unrestricted flow of the original design while lowering pressure and temp across the entire gas system to rifle levels. Fouling then moves through the system and out, rather than condensing, and erosion doesn't make as much change in system operation over time.

 

[Varminterror]

And everything you're touting about the 20" model remains absolutely irrelevant for the 14.5" M4 - which was designed with a smaller gas port and a shorter gas system, as was the civilian 16" M4 clone when it was released - and they've been running fine for decades.

So when I exhale through a coffee straw into a drinking straw vs. a straight drinking straw, I'll see greater moisture condensation from my breathe (since no human exhales "pure oxygen," but rather a mixture of atmospheric gases - a little heavy on CO2 - effectively at their saturation point) falling out into the tube, and my spitwad will be a little extra spitty. The moisture in my breathe will condense in the drinking straw as well, but not as greatly so as the throttled system. No question, at a few thousand feet per second, those droplets - which represent moisture condensate and unburned powder particulates - will have a much greater erosion effect than they do at a few feet per second in my breath.

BUT carbines with greater gas pressure have greater condensation potential, but nobody has ever actually proven they have any increased rate of failure in civilian semiautomatic firearms giving them a LESS THAN acceptable firearm service life. We're not talking about an M16A2 lasting for 100,000rnds and an M4A2 lasting for 1,500. Both gas systems will last longer than their throat, both longer than their recoil spring, both longer than their fire control group... Replace a barrel with a bad throat and you pick up a new gas port along the way... Throw in $7 on top of your $100-500 investment to get a new gas tube while you're at it if it makes you feel good - less than sales tax...

So now we're back to where we were days ago - we've now debated pressure restriction orifices for you to come back to the same fallacy - the original AR-15 design is NOT the only functional design, so the differences in gas pressure, port erosion, tube condensation, blah blah are all moot... The M4 works, the civilian 16" carbine works, and the original 20" AR-15 design was NOT on the ragged edge of function such that it would not reliably function with other cartridges, system lengths, buffer weights, carrier rates, spring rates, port diameters, or gas port positions (gas system lengths).

That will work for quite awhile

Yeah - it will work for "a while," and in this case, M4's have proven the length of "a while" is longer than the required service life of the firearm, or at least longer than other major components - so complaining about it is a waste of breath.

So now you're in an academic argument with no real world application - if the rifle with the shorter functional ife lasts longer than it's REQUIRED service life, it doesn't really matter that the other rifle's gas port will last longer. Both last long enough.

 

[MistWolf]

Varmint, I see your point about the gas port being a flow restrictor rather than a flow regulator.

RX, while water isn't compressible and gas is, they are both fluids. One thing thatdoes change the rules inside a self loading rifles is that the gas flow in the bore is supersonic before it passes through the gas port. Supersonic gas flows passing though a restriction like that drop to a little less than the speed of sound

 

[Malamute]

He should have said INTERPOLATE instead of extrapolate. But his principle stands.

It doesn't take 50yrs to prove a firearm design is as good as an M16. plenty of rifles in almost every possible combination imaginable have been tested and proven - whether it's different length barrel and gas lengths, or even other cartrisges. Most shooters are unfamiliar with the idea of "wearing out" a firearm in their lifetime, let alone within a year, so it seems to them like "proving" a platform takes generations - frankly, that's not the case. Arms designers have proven metrics based on longevity studies of past firearms, and testing protocols to prove out new firearms to meet or exceed minimal acceptable standards.

Most shooters won't even burn out ONE barrel in their life, where as some others wear out a few barrels a year. Couple with that fire control groups, extractor springs, gas rings, barrel bushings, recoil springs, ejectors, operating rods, etc etc... Arms manufacturers can proof test a design within months.

PRODUCTION lines tend to be where a design fails, but the product design in itself is easily tested.

I missed the post that thought it took 50 years to prove a variation.

Let me see if I can distil my thoughts to the lowest possible denominator.

Until someone, anyone, does a statistically significant level of test, for example say the .mil takes 500 rifles, fires them 50,000 or more rounds each, with otherwise identical components and ammo, then concludes that X type gas system bolts last 20K rounds on average over the test sample, and Y type gas system bolts last 25K (or whatever), then the difference is theoretical, not "proven", even though the elements that comprise the difference can be shown to have differing components(such as port pressure). Using inferred info is not data, an actual test result with a large enough number of guns involved with exactly comparable components would be data. So far, we don't have that. It keeps being brought up that there is some sort of data, but nobody has presented it. How many rounds between bolt failures are there with what test sample size that is a measureable improvement over carbine gas? "Data" would have real measurable numbers arrived at by test methods, theory and inference is guessing based on the different component factors. Commercial success isn't data either, its marketing success. The unrelated mass of various commercial guns in use isn't relatable as data, it hasn't given us provable, hard numbers of what the parts life difference is with the different systems, because no such attempt to gather or develop such data has never been done. Theres no reason to when mids sell without the effort to build the data base.

Please note, that at no time have I criticized or said I didn't believe that mid gas was a good idea or improvement, I do believe its a good idea, and I like them. What I have said is so far we don't have solid data on exactly what the difference is in parts life. It makes sense that theres an improvement, but repeatedly going over all the component differences doesnt prove exactly what the difference in parts life is. Its a given that if someone says anything challenging in any way to peoples beliefs, even if those beliefs don't have the rock solid foundations they believe they have, they take offense, even when the different viewpoint has made zero offensive remark. I don't need educated in mid theory or history, Ive probably read all that's been mentioned more than once in researching mid gas, I like the mid concept, it just hasn't had the level of actual testing under the same type circumstances carbine gas has because of the .mil use of that system for as long as its been around. Reading any more than that into my posts is making incorrect assumptions. Once somebody of note makes an actual, real life test of statistically significant numbers to compare the parts life and port life differences, then we'll be there. Someday someone probably will. Nobody has come up with such a test so far.

Edit: Much of what I was attempting to address was covered in V-T's post above. The real life difference in functional service lifetime of the various AR types is beyond the required service life of the guns, and is basically not very important. All work. All continue to work with pretty minimal parts replacement to continue service even farther.

 

[Malamute]

BUT carbines with greater gas pressure have greater condensation potential, but nobody has ever actually proven they have any increased rate of failure in civilian semiautomatic firearms giving them a LESS THAN acceptable firearm service life. We're not talking about an M16A2 lasting for 100,000rnds and an M4A2 lasting for 1,500. Both gas systems will last longer than their throat, both longer than their recoil spring, both longer than their fire control group... Replace a barrel with a bad throat and you pick up a new gas port along the way... Throw in $7 on top of your $100-500 investment to get a new gas tube while you're at it if it makes you feel good - less than sales tax...

So now we're back to where we were days ago - we've now debated pressure restriction orifices for you to come back to the same fallacy - the original AR-15 design is NOT the only functional design, so the differences in gas pressure, port erosion, tube condensation, blah blah are all moot... The M4 works, the civilian 16" carbine works, and the original 20" AR-15 design was NOT on the ragged edge of function such that it would not reliably function with other cartridges, system lengths, buffer weights, carrier rates, spring rates, port diameters, or gas port positions (gas system lengths).



Yeah - it will work for "a while," and in this case, M4's have proven the length of "a while" is longer than the required service life of the firearm, or at least longer than other major components - so complaining about it is a waste of breath.

So now you're in an academic argument with no real world application - if the rifle with the shorter functional ife lasts longer than it's REQUIRED service life, it doesn't really matter that the other rifle's gas port will last longer. Both last long enough.

Ok, you sir, won the Internet for today. Absolutely on point post. Completely agree on all points here. You said several things Ive wanted to address, and did so perfectly.

 

[RX-79G]

And everything you're touting about the 20" model remains absolutely irrelevant for the 14.5" M4 - which was designed with a smaller gas port and a shorter gas system, as was the civilian 16" M4 clone when it was released - and they've been running fine for decades.

The M4 was "designed" to be able to accept an XM177 forearm, bayonet and M203. Every detail of its barrel length, gas system and cyclic rate are the result of those goals, not the point of the mechanism. The barrel is 14.5" long because that carbine forearm + bayonet = 14.5. The gas tube is the length it is because of the forearm. The port is the diameter it is because of the barrel and gas tube length, and the cyclic rate is a direct product of barrel length and port pressure.

The M4 works fine, but the Marines put it off for quite awhile because it was observed to be not quite as reliable as the M16A4. Better mags and revised feed ramps make up for enough of the deficiency that the M4 has become fully accepted.

However, this thread is not about the 14.5" barrel M4 but its retarded 16" cousin, which uses the same port diameters despite an extra 1.5" of pressure raising barrel length. You make it sound like you have a business selling parts that "fix" gas pressure problems, so I don't understand your insistence that there is no problem at all.

But if there is indeed something to improve about the AR15 carbine, we can certainly discuss ways to do it in terms of their efficiency and durability. I don't understand why you keep vacillating between that discussion and saying "everything is fine, stop talking about this".

 

[Varminterror]

I don't understand why you keep vacillating between that discussion and saying "everything is fine, stop talking about this".

You've challenged the science behind the 16" civilian clone as inferior, using moot issues and partial-science. I want to be sure others are not mislead by your unsubstantiated fears of anything except an M16A2 or M4A2 system design, as almost ALL of them work.

You make it sound like you have a business selling parts that "fix" gas pressure problems, so I don't understand your insistence that there is no problem at all.

The tuning I do to AR's allow them to slide around on a matrix of performance potential based on the users desires. 99% of the time, I'm not fixing issues, I'm optimizing the firearm for the application. It's really quite rare to find AR's which need their gas or recoil system "fixed" from a non-operational state which does not have some part out of specification tolerance. The "fixing" I tend to do on AR gas and/or recoil systems is typically a matter of balancing a customer's rifle and load (which I also offer load work-up service), OR doing repairs which should have been done as PM - guys often don't realize recoil springs do need replaced. And of course, that "fixing" tends to be a low volume workload beside other "fixing" of fire control groups, gas rings, peened firing pins, fatigued springs, etc - which is all low volume to those "if you clean this more often, it'll run right." Optimization and modification work is the name of the game - as AR's are really pretty reliable, whether you're running a suppressed 17Rem or a 458 socom, or anything in between.

 

[RX-79G]

You've challenged the science behind the 16" civilian clone as inferior, using moot issues and partial-science. I want to be sure others are not mislead by your unsubstantiated fears of anything except an M16A2 or M4A2 system design, as almost ALL of them work.

No one is going to be mislead because I have also stated several times in this thread that all of them work. So you don't have to keep harping on that non-issue with my posts.

I enjoy talking about how things work and it sounds like you enjoy the same, up until anyone posts something that makes it look like you might be in error.

 

[lobo9er]

wow this has unraveled a bit...

You've challenged the science behind the 16" civilian clone as inferior, using moot issues and partial-science. I want to be sure others are not mislead by your unsubstantiated fears of anything except an M16A2 or M4A2 system design, as almost ALL of them work.
.

i don't think RX is saying "some don't work" only that are room for improvements and other ways that also work.

 

[azrocks]

Personally, I'd rather have a carbine length system with an AGB than a mid-length system. I can always throttle down, but you can never throttle up without reaming the port, and even then, it's a limited game.

This statement makes one heck of a lot of sense, and combined w/ everything else I've learned in this thread, has gone a long way towards changing my perspective of the 'perfect' AR (for my use). At least when considering rifles where easily accessing an adjustable block is possible.

Thanks for the continuing education.

I've enjoyed and appreciated everyone's input in this thread, even if it ran counter to my beliefs.

 

[Arizona_Mike]

In my opinion carbine has such a long track record for 14.5" (or is it 14.7?) and Rifle has such a long track record for 20" that interpolating between the two would indicate that Intermediate is just about right. I've gone intermediate on all my 16" rifles including conventional and piston.

Mike

 

[Malamute]

This statement makes one heck of a lot of sense, and combined w/ everything else I've learned in this thread, has gone a long way towards changing my perspective of the 'perfect' AR (for my use). At least when considering rifles where easily accessing an adjustable block is possible.

Thanks for the continuing education.

I've enjoyed and appreciated everyone's input in this thread, even if it ran counter to my beliefs.

...or just using a known quality carbine properly gassed barrel (such as a Colt) and living with it as is. Just a thought.

 

[azrocks]

...or just using a known quality carbine properly gassed barrel (such as a Colt) and living with it as is. Just a thought.

No... in that case I'd prefer the middy. If I can get reliable cycling with less wear & tear on the rifle, then that's the route I'm going to go. It doesn't matter to me if I can quantify the impact of the increase in heat/pressure. If I can get the same functional reliability and be kinder to the weapon, that makes the most sense to me. There's always the issue of premature failure to contend with... it isn't just a matter of component wear. Without an AGB, I see absolutely no advantage to a carbine, and the potential for a disadvantage. Since they'll both cost the same, I still think a mid-length makes more sense (in 16") in this case.

Does this line of thinking make sense, or am I missing something (sincere question)?

 

[NWcityguy2]

Del-ton publishes the size of their gas port on all of their barrel models. That will give you an idea where the commercial market is when it comes to drilling gas tubes at Carbine/Mid/Rifle length systems.