How to design gas system?

The design aspect is simple. You either have an op rod system, a Stoner type system or true direct impingement.

It's the specifics of port size, piston size, length of cylinder, etc. that need to be calculated, and it's just a matter of putting variables together. For a one-off build, I'd say it'd be far less frustrating for most to base their figures on something already extant and modify as necessary.

I'm sure that's not as specific as what you're seeking, but I am not aware of any tech books specific to small arms gas systems engineering. Just falls under the umbrella of mechanical engineering and applied physics.

http://www.amazon.com/dp/1857531078...TF8&colid=3J0YYCLYO36PI&coliid=I259EQ19XOGKXR

Brassey's Essential Guide to Military Small Arms: Design Principles and Operating Methods

CMC said:

http://www.amazon.com/dp/1857531078/ref=wl_it_dp_o_pC_nS_img?_encoding=UTF8&colid=3J0YYCLYO36PI&coliid=I259EQ19XOGKXR

Brassey's Essential Guide to Military Small Arms: Design Principles and Operating Methods

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And the cheapest one is ONLY $783.79!

Probably a good idea to get 2 or 3 at that price ....

Go to DTIC:
http://www.dtic.mil/dtic/

The Document you want is AMCP 706-260 "Automatic Weapons". There is no direct link, you have to use the DTIC search engine, a highly confusing and inefficient search engine, to find the pdf you can download.

This document is highly technical and mathematical. It has models of the M60 gas expansion system.

I highly recommend going to Milsurps.com and downloading the Machine Gun book series by Chinn. Vol 4 describes the theory of operation for all semi automatic weapons and includes the constraints you have to live within. While this book is not mathematical, there are lots of graphs and tables. Read the section on Blowback operation before you think of working on a gas system design.

http://www.milsurps.com/content.php?r=347-The-Machine-Gun-(by-George-M.-Chinn)

If you plan to do real design, buy the Brassey's. It will point you in the correct direction.

China's the Machine Gun has lots of information about designing firearms, including examples of extractors, ejectors, bolts, feeding mechanisms, and other firearm parts.

Once you decide you want a self loading firearm, you need a source of energy. Only recoil and gas operation have ever been successfully used in self loading firearms.

BSW

briansmithwins said:

China's the Machine Gun has lots of information about designing firearms, including examples of extractors, ejectors, bolts, feeding mechanisms, and other firearm parts.

Once you decide you want a self loading firearm, you need a source of energy. Only recoil and gas operation have ever been successfully used in self loading firearms.

BSW

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Solely electrically operated rotary machine guns and cannons are not gas or recoil operated.

*Chinn's

Nom, those also aren't man-portable or legal semi-autos

TCB

Nom, those also aren't man-portable or legal semi-autos

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The argument could also be made that they're not really self loading, since they require a drive motor and an external power source.

barnbwt said:

*Chinn's

Nom, those also aren't man-portable or legal semi-autos

TCB

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I was responding to the statement "Only recoil and gas operation have ever been successfully used in self loading firearms", emphasis added. Besides whata ya mean "aren't man-portable"? Haven't you ever seen "Predator"? Jesse Ventura cuts down half a jungle with a man-portable rotary machinegun! Yes, I know, "Only in the movies".

http://www.youtube.com/watch?v=O_lnq0HYBQE

Not legal semi-autos? "Now why spoil the beauty of the thing with legality"* The man isn't intending to start manufacturing he is only seeking knowledge.

*Apologies to John Milius for stealing Brian Keith's line as Teddy Roosevelt in "The Wind and the Lion".

MachIVshooter said:

The argument could also be made that they're not really self loading, since they require a drive motor and an external power source.

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My aren't we picky. I think a check of Jane's would show the drive motor is considered part of the gun. External power source? Isn't a belt of ammunition the external power source for the internal combustion engine that is a gas operated gun?

What I want to know is why you and barnbwt didn't comment on the efficiency of the design of the Alimentary System for gas operation?

eldon519,

Something that has not been mentioned is there also are operating systems that use both gas and recoil combined. I think gas and electricity has been combined in some small (20mm to 30mm) autocannons. Good luck with your search, it will be arduous in finding the reference materials and even more arduous wading through them.

Edit: Regarding gas and electricity, I don't just mean electric primers in the ammunition.

Solely electrically operated rotary machine guns and cannons are not gas or recoil operated.

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I'd argue that those aren't self loading as they require a external power source. You could consider a lever action or bolt action at be 'self loading' if you count the users muscle as part of the gun.

You can use gas to accelerate a recoil operated gun, both the Vickers MG and MG42 use a muzzle booster to accelerate the barrel rearwards.

BSW

briansmithwins said:

I'd argue that those aren't self loading as they require a external power source. You could consider a lever action or bolt action at be 'self loading' if you count the users muscle as part of the gun.

You can use gas to accelerate a recoil operated gun, both the Vickers MG and MG42 use a muzzle booster to accelerate the barrel rearwards.

BSW

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I can see where you and MachIVShooter are coming from with regard to an electric motor and electric power supply (battery or platform supplied) being external power sources. My point is that a self-loading firearm is any firearm that self-loads after firing because a human pulled a trigger without any other physical movement. Bolt and Lever action rifles don’t meet that requirement. Perhaps we should think in terms of a magazine/belt of ammunition as the chemical energy storage container of a M16/M249 being similar to the electrical energy storage container (battery) of the electrically powered XM-214.

You hit the nail on the head with the Vickers and MG42. That is what I was thinking of when I comment on a combination of recoil and gas operation.

I can see where you and MachIVShooter are coming from with regard to an electric motor and electric power supply (battery or platform supplied) being external power sources. My point is that a self-loading firearm is any firearm that self-loads after firing because a human pulled a trigger without any other physical movement.

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A machinegun can complete a cycle of operation with 1 round of ammunition. The energy to do so is entirely contained within a single cartridge of ammunition.

The energy used to complete a cycle of operation can be utilized by either recoil or by tapping gas from the action. It's usually fairly obvious* which is used: If there's a gas piston it's gas operated and if the barrel moves it's recoil.

BSW

*Some recoil guns use gas to accelerate the parts, like the MG42. Some aren't that obvious, like the Benelli inertia operated shotguns.

"My point is that a self-loading firearm is any firearm that self-loads after firing because a human pulled a trigger without any other physical movement."
For a double-action, maybe. Otherwise, all the finger is doing is releasing a (mechanically) needless obstruction in the system, which serves only to make the gun "domesticated" vs. a feral animal running free in the wild. Semi-auto leashes are quite tragic, really

TCB

As far as the "how" of the design angle;
At this point in history, there is such a wealth of successful firearms of nearly every design type at nearly every power level. That means that an educated guess can be made for factors like bolt weight, unlocking timing, and gas piston volume, that will at least get you close where you need to be with very little math involved. For razor's edge design problems like the MP7, there was probably some basic math done to obtain a starting point for the dynamics of the system, but internal ballistics is still such a dark science due to the difficulty in measuring important values that I bet a ton of prototypes and adjustments were needed before that gun was reliable (it supposedly is, at least for 6000 rounds between armory trips )

Some of your system constraints are:
-Returning energy of the bolt assy being high enough to strip a round into the chamber (lower limit on spring force, bolt mass, and bolt over travel
-Cyclic speed being low enough for the mag to keep up (lower limit on bolt mass and bolt over travel)
-Recoil speed being high enough to reliably reach the end of motion (upper limit on bolt mass and spring force)
-Unlocking timing (lower limit on reciprocating mass for recoil actions and blowbacks, upper limit on locked-travel for recoil actions, and lower limit on distance of gas port to chamber in the case of gas-ops)
-Force & distance required for cycle (fixed in the case of recoil & blowbacks, but defines a lower limit for piston head area, with piston head travel set by whatever is needed to obtain the recoil velocity)
-And then finally you have your structural requirements, which will limit how much force you can put on a gas piston of such and such dimensions before it buckles, bolt lug stresses, receiver battering, and cam surface galling

There's so many variables involved at the end of the day, that "educated guesses" based on what we've found to work is the only efficient way to narrow the unknowns down to something than can be hashed out through testing of a prototype. If the only unknown is the gas port size, it's easy to work your way up until the thing runs . For something like a funky-length AR build where you have at least half a dozen variables like bolt/buffer mass, spring strength, gas tube length, a range of ammunition pressure curves, and also gas port size --it can be an exercise in frustration.

Part of the reason recoil and blowback designs are easier to design (thus, were invented first) is because cartridge selection cements many of the variables needed to define the system. But you do typically end up with a narrower band of what works in terms of ammunition, and the designs won't scale as well.

TCB

barnbwt said:

"My point is that a self-loading firearm is any firearm that self-loads after being fired by a human pulling a trigger without any other Human physical movement."

For a double-action, maybe. Otherwise, all the finger is doing is releasing a (mechanically) needless obstruction in the system, which serves only to make the gun "domesticated" vs. a feral animal running free in the wild. Semi-auto leashes are quite tragic, really

TCB

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Made some changes above, does that make my meaning clearer? You could, heaven knows why you would, create a semi-automatic, electrically operated, rotary multi-barrel, gun. All that is need for that is a trigger that when pulled does not provide a constant supply of electricity, only enough for one shot and positioning of the next cartridge for firing. Why are you making this about semi-automatics? I don't recall Post #1 placing any restrictions limiting the request for information solely to gas operated semi-autos. Post #1 mentions small arms. I think you are aware that the U.S. Army has considered autocannons, .50 BMGs, and mortars small arms. The U.S. Army probably still trains 45B Small Arms Repairmen at Aberdeen Proving Grounds to repair those small arms.

The BATF considers a motor drive weapon to be a machine gun, same LEGALITY as a regular full auto.

I find it more productive to use descriptive nomenclature instead of debating the definitions.
But then I have been known to consciously misuse the Internet Debating Society term "clip," too.
And to be aggravated by apparently ignorant misuse of the Internet Ballistician term "bore diameter."

briansmithwins said:

A machinegun can complete a cycle of operation with 1 round of ammunition. The energy to do so is entirely contained within a single cartridge of ammunition.

The energy used to complete a cycle of operation can be utilized by either recoil or by tapping gas from the action. It's usually fairly obvious* which is used: If there's a gas piston it's gas operated and if the barrel moves it's recoil.

BSW

*Some recoil guns use gas to accelerate the parts, like the MG42. Some aren't that obvious, like the Benelli inertia operated shotguns.

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A machine gun is often defined as a fully automatic weapon no larger in caliber than .50 and at most under 20mm.

Taking your description and making a few changes may help get my point across.

An electrically powered rotary multi-barrel machine gun can complete a cycle of operation with 1 round of ammunition. The energy to do so is entirely contained within a single electric battery.

Is there really any difference in what ultimately occurs (me pull trigger gun go bang) between a machinegun using the stored chemical energy inside a cartridge and the stored chemical energy inside a battery?

Jim Watson said:

The BATF considers a motor drive weapon to be a machine gun, same LEGALITY as a regular full auto.

I find it more productive to use descriptive nomenclature instead of debating the definitions.
But then I have been known to consciously misuse the Internet Debating Society term "clip," too.
And to be aggravated by apparently ignorant misuse of the Internet Ballistician term "bore diameter."

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That is very interesting. So what you are saying is a rifle that only fires a single shot with one pull of the trigger used a electric motor to extract, eject, feed, and chamber the next cartridge BATF would consider that a full auto? Just asking because I am unaware of this having never considered it before this thread. That being said, the thread was about information on designing gas operated small arms. So we have drifted, but regardless of BATF classification, I agree an electrically operated XM-214 is every bit as much of a machinegun and small arm as a gas operated M240.

Or you can do what the Russians are so great at, take an existing design, modified slightly and call it theirs.

eldon519,

barnbwt really knows this gun engineering principles stuff, far better than I can even confirm off the top of my head that he knows this stuff. His posts can really leave me scratching my head a reaching for a book to get a better understanding.

barnbwt said:

Part of the reason recoil and blowback designs are easier to design (thus, were invented first) is because cartridge selection cements many of the variables needed to define the system. But you do typically end up with a narrower band of what works in terms of ammunition, and the designs won't scale as well.

TCB

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Only the above needs a little comment for additional accuracy. John Moses Browning invented his gas operated machine guns before his recoil operated machineguns. Maxim gets credit for coming up with gas operation prior to concentration on recoil operation and prior to Browning using gas operation.

Ron James said:

Or you can do what the Russians are so great at, take an existing design, modified slightly and call it theirs.

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You mean like the U.S. did with Mauser to make the Springfield 1903? That is only the first example I can think of off the top of my head. I hope you are not implying the Russians have not come up with some great original designs. They have designed some of the best MGs ever made.