A Couple of Questions on Rifle Barrel Attachment/Design

AFluffyWalrus · Apr 29, 2017

Hey guys, new to the forum, so apologies if I'm breaking any rules.

I've been working on a project to see if I can't make a sort of quick-change barrel retrofit for the M14/M1A series of rifles (or any designs utilizing a screw-on barrel), and had a couple questions to ask before I finalize any design specifics.

First off, is there a way to determine the amount of forward force that is exerted on a rifle barrel when a round is fired? I know about the principles of bolt thrust and the rearward pressure on a bolt, but I haven't been able to find a way to calculate the amount of force that is trying to "push" the barrel out of the receiver. I should also specify that I'm trying to find out the figures for a barrel that does not enclose the bolt inside some sort of barrel extension (IE AR-pattern rifles), but instead for the forward pressure for rifles whose receiver takes the rearward pressure from the bolt. (IE M14/M1A, AK, most bolt actions)

Secondly, where does the chamber wall thickness rule apply? From this source, the minimum wall thickness for 308 should be .1232". However, does this only apply in front of the neck of a cartridge, or are you required to have this thickness throughout the chamber, even where the main body of the cartridge case sits?

Lastly, for more of a general metal strength question, is it feasible for a set of screw threads to not be "wrapped" either on the inside or outside of some cylinder, but to instead look more like some sort of coil? Relating back to my first question, I'm trying to figure out if some sort of receiver extension could be held onto a rifle by just having it almost snake through the receiver's threads and not having any sort of support from a central piece.

Sorry for the somewhat long-winded post. If you have any questions or need some clarification, please let me know.

Thanks in advance for the help.

GGaskill · Apr 30, 2017

Secondly, where does the chamber wall thickness rule apply? From this source, the minimum wall thickness for 308 should be .1232". However, does this only apply in front of the neck of a cartridge, or are you required to have this thickness throughout the chamber, even where the main body of the cartridge case sits?

Using that number would be absolutely absurd. It is based on an unrealistic assessment of chamber pressure and has no safety factor. If you have seen any serial pressure test results, you will find pressures at least 50% greater than the minimum due to unknown causes. The recommended chamber wall thickness is 2/3 of the maximum cartridge diameter, which on a .308 Win would be .470" X 2 / 3 or .313" for a diameter over the chamber of 1.097". .123" would be questionable for the muzzle from an overall strength perspective.

MIBill · May 2, 2017

AFluffyWalrus said:
Hey guys, new to the forum, so apologies if I'm breaking any rules.

I've been working on a project to see if I can't make a sort of quick-change barrel retrofit for the M14/M1A series of rifles (or any designs utilizing a screw-on barrel), and had a couple questions to ask before I finalize any design specifics.

First off, is there a way to determine the amount of forward force that is exerted on a rifle barrel when a round is fired?

In practical terms, the answer to this question is NO. It is a function of the friction coefficient of the barrel and the projectile. Since these numbers very by type of barrel and type of bullet there is no practical way to know these numbers for any particular cartridge. Was the barrel hammer forged??? Button rifled??? Cut rifled??? Lapped or not? What is the jacket material of the bullet? Is it jacketed or is it a solid bullet??? Lead??? Brass???

The ONLY force acting to move the barrel forward (under normal conditions) out of the receiver is the friction between the projectile and the barrel. It is much less than the force acting on the bolt. However, the data necessary to calculate that number is not readily available. This number, if it were calculated from know values would be independent of the type of connection used (i.e. barrel threaded into the receiver such as a Rem. 700 v.s. barrel threaded into an extension such as a M16 / M4)

AFluffyWalrus · May 3, 2017

GGaskill said:
Secondly, where does the chamber wall thickness rule apply? From this source, the minimum wall thickness for 308 should be .1232". However, does this only apply in front of the neck of a cartridge, or are you required to have this thickness throughout the chamber, even where the main body of the cartridge case sits?

Using that number would be absolutely absurd. It is based on an unrealistic assessment of chamber pressure and has no safety factor. If you have seen any serial pressure test results, you will find pressures at least 50% greater than the minimum due to unknown causes. The recommended chamber wall thickness is 2/3 of the maximum cartridge diameter, which on a .308 Win would be .470" X 2 / 3 or .313" for a diameter over the chamber of 1.097". .123" would be questionable for the muzzle from an overall strength perspective.

Thanks for the response. I thought that .1232" seemed a bit thin, so your basic formula to determine the approximate required diameter helps greatly. However, I realized that I didn't word my original question that well, in that I'm more concerned with is where in the chamber that thickness is required (I:E at the bullet itself versus where the brass is supporting the chamber) instead of how much thickness is required. Luckily, based on the below response, I shouldn't have to trim down the barrel enough for it to matter.

MIBill said:
In practical terms, the answer to this question is NO. It is a function of the friction coefficient of the barrel and the projectile. Since these numbers very by type of barrel and type of bullet there is no practical way to know these numbers for any particular cartridge. Was the barrel hammer forged??? Button rifled??? Cut rifled??? Lapped or not? What is the jacket material of the bullet? Is it jacketed or is it a solid bullet??? Lead??? Brass???

The ONLY force acting to move the barrel forward (under normal conditions) out of the receiver is the friction between the projectile and the barrel. It is much less than the force acting on the bolt. However, the data necessary to calculate that number is not readily available. This number, if it were calculated from know values would be independent of the type of connection used (i.e. barrel threaded into the receiver such as a Rem. 700 v.s. barrel threaded into an extension such as a M16 / M4)

Thanks a lot for this. I had a suspicion that the only force pushing the barrel out of the receiver would be the drag from the bullet passing through the rifling, and confirming this makes me a lot more confident in the strength of my current leading design.

Again, thanks to both of you for answering my questions.

GeneT · May 3, 2017

MIBill said:
In practical terms, the answer to this question is NO. It is a function of the friction coefficient of the barrel and the projectile. Since these numbers very by type of barrel and type of bullet there is no practical way to know these numbers for any particular cartridge. Was the barrel hammer forged??? Button rifled??? Cut rifled??? Lapped or not? What is the jacket material of the bullet? Is it jacketed or is it a solid bullet??? Lead??? Brass???

The ONLY force acting to move the barrel forward (under normal conditions) out of the receiver is the friction between the projectile and the barrel. It is much less than the force acting on the bolt. However, the data necessary to calculate that number is not readily available. This number, if it were calculated from know values would be independent of the type of connection used (i.e. barrel threaded into the receiver such as a Rem. 700 v.s. barrel threaded into an extension such as a M16 / M4)

At any instant in time the pressure on the bolt is the same as the pressure on the bullet and on the chamber walls and everywhere inside that enclosed space. The thrust imparted to the barrel is that pressure acting on the base diameter of the bullet, as a first approximation. Friction relates only in the sense that it regulates pressure by determining how quickly the volume containing the pressure changes.

GsT

Monarchist · May 3, 2017

AFluffyWalrus said:
Hey guys, new to the forum, so apologies if I'm breaking any rules.

I've been working on a project to see if I can't make a sort of quick-change barrel retrofit for the M14/M1A series of rifles (or any designs utilizing a screw-on barrel), and had a couple questions to ask before I finalize any design specifics.

First off, is there a way to determine the amount of forward force that is exerted on a rifle barrel when a round is fired? I know about the principles of bolt thrust and the rearward pressure on a bolt, but I haven't been able to find a way to calculate the amount of force that is trying to "push" the barrel out of the receiver. I should also specify that I'm trying to find out the figures for a barrel that does not enclose the bolt inside some sort of barrel extension (IE AR-pattern rifles), but instead for the forward pressure for rifles whose receiver takes the rearward pressure from the bolt. (IE M14/M1A, AK, most bolt actions)

Secondly, where does the chamber wall thickness rule apply? From this source, the minimum wall thickness for 308 should be .1232". However, does this only apply in front of the neck of a cartridge, or are you required to have this thickness throughout the chamber, even where the main body of the cartridge case sits?

Lastly, for more of a general metal strength question, is it feasible for a set of screw threads to not be "wrapped" either on the inside or outside of some cylinder, but to instead look more like some sort of coil? Relating back to my first question, I'm trying to figure out if some sort of receiver extension could be held onto a rifle by just having it almost snake through the receiver's threads and not having any sort of support from a central piece.

Sorry for the somewhat long-winded post. If you have any questions or need some clarification, please let me know.

Thanks in advance for the help.

I'll not call your concept 'daft', but it does have that flavour.

First - there is 'prior-art' on quick-change barrels, 7.62 NATO.

Can't see any sort of 'adapter' having a place. Wants a purpose-built receiver and barrel system. Most especially if it is to remain gas-operated, not top-break or turnbolt. A Mattel Toys glorified twenty-two it is not.

Second - 7.62 arms were sold for use outside of NATO as well. More than just a few. Scary-large numbers, actually. Both sides of the battle line, Falklands War, both sides of other battles.

And then.. NATO went over to twenty-twos.

Result is that there is not really a shortage of some rather well-executed designs in 7.62 NATO. A 'civilian' would have to be quite wealthy and blessed with heavy bones and meaty shoulders to be able to STAND the cost and pain required to "shoot out" a Steyr hammer-forged barrel, for example. What's to be changed?

BTW.. all I ever asked of mine was that it help keep me alive. It did so. Otherwise, the M14 is not even rated all that well amongst some right worthy competition. FN-Herstal, Steyr, and others.

More research suggested...

GGaskill · May 3, 2017

First off, is there a way to determine the amount of forward force that is exerted on a rifle barrel when a round is fired?

It is essentially the same as the back thrust on the bolt less some fraction of the force applied to the bullet. This has to be the case as the pressure inside the case is the same in all directions and the front facing area is the same as the rear facing area except for the bore area.

AFluffyWalrus · May 3, 2017

Thanks again for all the responses guys.

GeneT said:
At any instant in time the pressure on the bolt is the same as the pressure on the bullet and on the chamber walls and everywhere inside that enclosed space. The thrust imparted to the barrel is that pressure acting on the base diameter of the bullet, as a first approximation. Friction relates only in the sense that it regulates pressure by determining how quickly the volume containing the pressure changes.
GsT

GGaskill said:
First off, is there a way to determine the amount of forward force that is exerted on a rifle barrel when a round is fired?

It is essentially the same as the back thrust on the bolt less some fraction of the force applied to the bullet. This has to be the case as the pressure inside the case is the same in all directions and the front facing area is the same as the rear facing area except for the bore area.

If I understand correctly, it seems like what you're saying is that the forward pressure on the barrel is a combination of the bullet being pushed through the rifling and the force on the shoulder of the case, which in turn tries to force the barrel out of the receiver by pushing on the corresponding shoulder in the barrel, unless I'm looking at it completely wrong.

Monarchist said:
I'll not call your concept 'daft', but it does have that flavour.

First - there is 'prior-art' on quick-change barrels, 7.62 NATO.

Can't see any sort of 'adapter' having a place. Wants a purpose-built receiver and barrel system. Most especially if it is to remain gas-operated, not top-break or turnbolt. A Mattel Toys glorified twenty-two it is not.

Second - 7.62 arms were sold for use outside of NATO as well. More than just a few. Scary-large numbers, actually. Both sides of the battle line, Falklands War, both sides of other battles.

And then.. NATO went over to twenty-twos.

Result is that there is not really a shortage of some rather well-executed designs in 7.62 NATO. A 'civilian' would have to be quite wealthy and blessed with heavy bones and meaty shoulders to be able to STAND the cost and pain required to "shoot out" a Steyr hammer-forged barrel, for example. What's to be changed?

BTW.. all I ever asked of mine was that it help keep me alive. It did so. Otherwise, the M14 is not even rated all that well amongst some right worthy competition. FN-Herstal, Steyr, and others.

More research suggested...

I can certainly see why my idea seems seems illogical, or at best pointless, and I can't really disagree, but I'm not trying to make the next generation of battle rifle here. I personally prefer the M14/M1 design to most others that I know of, but there are plenty of merits to the AR-based designs that are so prevalent in the modern rifle design world. This is really more of a fun project for me, and if it fails, at least I gained some experience from it. To address your other points, I don't really want to start a caliber war or a rifle war here, as there's plenty around to find comparing every little facet of the respective designs. The point of making it a adapter for an existing design is mostly due to the fact that I don't particularly want to deal with the creation of a completely new design, and would instead prefer to deal with a solid base that already exists. Maybe it would be easier to do so on an AR-based design, but as I said before, I prefer the M1 type rifles. As to why to make a quick change barrel for a battle rifle anyway, I prefer the added modularity and eased maintenance, not to mention that it makes it a bit more interesting to design.

Sorry for maybe being a bit to wordy in my response. Thanks for the feedback and advice.

MIBill · May 3, 2017

GeneT said:
At any instant in time the pressure on the bolt is the same as the pressure on the bullet and on the chamber walls and everywhere inside that enclosed space.

Absolutely correct.

GeneT said:
The thrust imparted to the barrel is that pressure acting on the base diameter of the bullet, as a first approximation.

Think about this for a second. Assuming the projectile is set to engage the lands when the round is chambered, this would be true ONLY for the first few microseconds until the projectile overcomes the initial friction. Once the projectile starts to move, whether before engaging the lands or after, there is nothing PUSHING the barrel forward.

Assume for a moment that there were no friction between the projectile and the barrel... What then would be acting to move the barrel forward? It is a tube closed at one end (by the case and bolt) and other than the moving projectile, open at the other.

What IS trying to move the barrel forward is the friction between the projectile and the inside wall of the barrel. Or put another way, The projectile is trying to drag the barrel along with it.

GeneT said:
Friction relates only in the sense that it regulates pressure by determining how quickly the volume containing the pressure changes.

GsT

See above. The above statement would be true except for the word "only".

MIBill · May 3, 2017

GGaskill said:
First off, is there a way to determine the amount of forward force that is exerted on a rifle barrel when a round is fired?

It is essentially the same as the back thrust on the bolt less some fraction of the force applied to the bullet.

"The force applied to the [base of the] bullet" is the SAME as the force applied to the bolt, is the same applied radially to the inside of the barrel.

GGaskill said:
This has to be the case as the pressure inside the cartridge is the same in all directions and the front facing area is the same as the rear facing area except for the bore area.

THIS has to be the case as the pressure is the same in all directions. The geometry of the case and the bore have no effect in terms of pressure. The AREA that the force is acting on is irreverent in terms of pressure (psi). It is only relevant in terms of absolute force.

In a bottleneck cartridge such as the .308 there would be some forward push as a result of the gas acting on the shoulder area of the case. However, the case itself is taking some of that force such that the resulting force on the barrel is proportionately small.

But again there are a boat load of variables involved in such calculations and some of the variables are simply not available. I went through this exercise with a couple engineers at GDLS when I was designing my .50. They have empirical data that they use since the calculations are not practical.

GGaskill · May 5, 2017

From the OP:
... is there a way to determine the amount of forward force that is exerted on a rifle barrel when a round is fired?

He is asking about force.

The AREA that the force is acting on is irreverent in terms of pressure (psi). It is only relevant in terms of absolute force.

In a bottleneck cartridge such as the .308 there would be some forward push as a result of the gas acting on the shoulder area of the case. However, the case itself is taking some of that force such that the resulting force on the barrel is proportionately small.

Nominal chamber diameter at the shoulder is .455"; case wall is maybe .025" (a guess; I will section a case when I get home and correct this if necessary.) I doubt that a .025" ring of brass cartridge case is going to contribute much to resisting the forward force.

It is a tube closed at one end (by the case and bolt) and other than the moving projectile, open at the other.

This is mostly true only with a straight wall case (and they are not really cylindrical, they have a slight taper which means the rear end is larger than the front end.) With a bottleneck case, you have the shoulder area under pressure as well as the body taper. The case wall is in intimate contact with the chamber walls (including the shoulder area and the forward component of the body taper) and the pressure is causing force in the forward direction among others.

I agree that the solution is not readily amenable to calculation, but if the force were trivial, we would not see the robust designs used to retain the barrel in the receiver.

CalG · May 5, 2017

GGaskill said:
We would not see the robust designs connecting barrel to receiver if the forward force was trivial.

Inertia forces are enough to justify the robust joint between barrel and receiver. Barrels are Heavy!

Regarding any "forward push" on the barrel during the firing cycle, lets take it to the extreme.

First the scenario:

The hammer drops and drives the cartridge deep into the chamber.
The primer ignites the powder charge that instantly builds pressure, expanding the case "intimately" within the chamber walls, effecting a gas tight seal (the elasticity of the brass reduces the cart dimensions after the pressure subsides, allowing extraction )

The burning powder releases hot gasses and pressure builds behind the boolit that sends it down the barrel.

Now the extreme part.

Stop the boolit mid barrel with a rod down the muzzle and butted up against an immovable object.
If the barrel and action are stout enough not to blow, How much "force" is trying to propel the barrel forward?

Answer? NONE!

The breach end of the barrel is sealed by the cartridge (supported by the bolt for sure).
The walls of the barrel are supported by material thickness enough to contain the pressure.
And the muzzle end is stopped by the boolit and supported by the "immovable object".

There are no forces pushing the barrel forward due to pressure. It's just a closed high pressure vessel.

The boolit friction description has merit.

GGaskill · May 7, 2017

It's just a closed high pressure vessel.

What makes it a closed pressure vessel? The barrel threaded into the receiver. Someone with a trashable action needs to remove the threads from an old barrel and perform the demonstration on youtube.

Stop the boolit mid barrel with a rod down the muzzle and butted up against an immovable object. If the barrel and action are stout enough not to blow, How much "force" is trying to propel the barrel forward?

Answer? NONE!

Ignoring the fact that most barrels and actions are NOT stout enough not to fail in this scenario, what you have done is ignore the pressure on the forward facing component of the cartridge (mostly the shoulder) which will still apply force to the barrel via the front of the chamber. What you have taken away is the frictional component (bullet against barrel) from the overall equation, nothing else.

magneticanomaly · May 13, 2017

IMO GGaskill is correct. Force trying to separate barrel form receiver is the sum of

1.) chamber pressure acting on the annulus between chamber maximum diameter and bullet diameter

2.) frictional resistance of bullet (maximum before it engraves)

3.) resistance of the mass of the barrel to being accelerated rearward by recoil.

By the seat of my pants, I think #3 is the largest term.

A very complicated dynamic, elastic analysis.

Experimental data would not be that hard to collect. Build a test gun with barrel tenon a slip fit in adapter, restrained by some lashup which transfers the load on bearing balls against washers. Brinnell hardness calculation gives you the force.

But large empirical safety factors would have to be developed since every variation of chamber friction and extractor function, bullet/barrel friction, loading, mass of action and mass of barrel would change the number.

If the OP's quick-change barrel design uses ball detents, he may (inadvertently) be building an approximation of the test rig.

Test-fire it under a blasting mat.

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A Couple of Questions on Rifle Barrel Attachment/Design

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