Minimum barrel thickness for 9mm

[milktree]

I'm building a 9mm carbine, including profiling the barrel.

How thin can I make it and maintain safety?

I don't mean "at the chamber", I mean "at least 2.5 inches from the breech"

I want it to be light, but not at the expense of being sloppy/floppy, so I want to put flutes in it.

How deep can I make the flutes before it's a risk?

A Glock 26 barrel has a wall thickness of about .11", so I know I can easily go that deep.

But how much of that is because it needs to be stiff in the Glock action?

For instance, if I used a .70" OD with flutes deep enough for a .08" thickness minimum, is that a problem? the .700" OD would make it nice and stiff, the flutes would make it lighter.

Like this:

 

[ReluctantDecoy]

I'm no engineer, but if you have a program like Solidworks, you should be able to at least determine the factor of safety using SAAMI specs of the desired ammunition and design/composition of the barrel. Still though, things get more complicated between schematics and fabrication when heat treatment strength factors come into play. What looks good on paper might not translate to the real world if your steel quality or heat treatment aren't up to spec. I'd assume you'd want to err on the side of caution with regards to over engineering as opposed to minimum tolerances.

That being said, I did find this document. See page 5+. Again, I'm not an engineer and barely understand what I'm looking at, so don't take this as the bible or anything. Just a good reference piece for further research:

http://www.imaximo.com/ATTACHMENTS/Barrels.pdf

 

[mibro]

Are you a injuneer?

Here's a hoop stress calculator. You will need the material properties of the steel you're using and the peak pressure generated by the round. Maybe we can crowdcalculate it. I'm too lazy to do it right now.

Pressure Vessel , Thin Wall Hoop Stress Calculator Strength of Materials | Engineers Edge | www.engineersedge.com

 

[tripletaco]

For pressure vessels the thing your usually concerned with is hoop stress. Its usually the direction of the greatest stress. Thats why pipes, tanks ect tend to split their sides (axially) like hotdogs on the grill.

The calculations for "thin wall" vessels are simpler but I think technically a gun barrel does not qualify as thin. You need a wall thickness that is substantially smaller than the inside diameter (20x or more maybe?).

Its been a while since Ive had to do calulations like this. I dont have time to totally dig into this now, so its very possible Ive made an error. Equations and calulator for thick wall vessels here:
Stress in Thick-Walled Cylinders - or Tubes

Assuming......the inside diam, (ID) of our vessel will be .355", wall thickness is (t) is .08", outside diam. (OD) .515" and max pressure as given by SAMMI is 35,000psi. I am getting about 98,000psi max stress. Now consider the strength of your material. Just recalling some rough numbers from memory....properly heat treated and annealed 4150 steel probably has a yield strength in the 100,000-140,000psi range and an ultimate tensile of 150,000psi or more...I think.

So what does that mean.....We're over simplifying this problem but probably taking a more conservative approach. Assuming these numbers are correct, youve got close to a 1.5x safety factor. That doesnt sound so bad...maybe 2x would be better. Its kind of up to you, unless you can dig up some industry data on best practices. I think this will work...you probably wont die....probably....

 

[milktree]

I'm no engineer, but if you have a program like Solidworks, you should be able to at least determine the factor of safety using SAAMI specs of the desired ammunition and design/composition of the barrel. Still though, things get more complicated between schematics and fabrication when heat treatment strength factors come into play. What looks good on paper might not translate to the real world if your steel quality or heat treatment aren't up to spec. I'd assume you'd want to err on the side of caution with regards to over engineering as opposed to minimum tolerances.

That being said, I did find this document. See page 5+. Again, I'm not an engineer and barely understand what I'm looking at, so don't take this as the bible or anything. Just a good reference piece for further research:

http://www.imaximo.com/ATTACHMENTS/Barrels.pdf

Well! That's pretty cool!

It turns out I made a lucky guess!

9mm is 2414 PCrmax according to Wikipedia
6.5x52R is 2430 according to that page.
2.1mm (the thickest on that chart) is .083", just a hair over the .08" I'd guessed!

So it seems like I'm in a not-insane ballpark.

 

[milktree]

Are you a injuneer?

Nope, but I get accused of being one pretty regularly.

Here's a hoop stress calculator. You will need the material properties of the steel you're using and the peak pressure generated by the round. Maybe we can crowdcalculate it. I'm too lazy to do it right now.

Pressure Vessel , Thin Wall Hoop Stress Calculator Strength of Materials | Engineers Edge | www.engineersedge.com

That web page says 97k psi hoop stress.

I'm using a barrel blank from Green Mountain Barrels: Muzzleloader Barrels, Ruger 10/22, BPCR, Muzzleloading Supplies | Green Mountain Rifle Barrel CO. specifically this:

9MM-Blank 9mm Raw Blank 17" x 1" 1:10

"4140 steel, stress relieved"

Now I have to find what kind of stress 4140 can take...

 

[milktree]

For pressure vessels the thing your usually concerned with is hoop stress. Its usually the direction of the greatest stress. Thats why pipes, tanks ect tend to split their sides (axially) like hotdogs on the grill.

The calculations for "thin wall" vessels are simpler but I think technically a gun barrel does not qualify as thin. You need a wall thickness that is substantially smaller than the inside diameter (20x or more maybe?).

Its been a while since Ive had to do calulations like this. I dont have time to totally dig into this now, so its very possible Ive made an error. Equations and calulator for thick wall vessels here:
Stress in Thick-Walled Cylinders - or Tubes

Assuming......the inside diam, (ID) of our vessel will be .355", wall thickness is (t) is .08", outside diam. (OD) .515" and max pressure as given by SAMMI is 35,000psi. I am getting about 98,000psi max stress. Now consider the strength of your material. Just recalling some rough numbers from memory....properly heat treated and annealed 4150 steel probably has a yield strength in the 100,000-140,000psi range and an ultimate tensile of 150,000psi or more...I think.

So what does that mean.....We're over simplifying this problem but probably taking a more conservative approach. Assuming these numbers are correct, youve got close to a 1.5x safety factor. That doesnt sound so bad...maybe 2x would be better. Its kind of up to you, unless you can dig up some industry data on best practices. I think this will work...you probably wont die....probably....

OK, this is pretty encouraging.

Given that maximum pressure is in the chamber and it drops pretty dramatically as the bullet travels down the barrel, and due to the way AR barrels have to be built simply to fit, the flutes wouldn't even start until about 3" from the bolt face.

This gives me way too much to learn about, but it also doesn't seem bat-shit crazy!

I'll take pictures.

 

[Waher]

Barrels should be designed to survive a double charge.

 

[amm5061]

Barrels should be design to survive a double charge.

Especially in a rifle barrel I would think that would be important.

 

[straightshooterjake]

Barrels should be designed to survive a double charge.

I have read that many firearms are designed to withstand double their expected operating pressure. This is a design standard that provides a wide margin of safety under normal operating conditions.

I believe that withstanding a load that was accidentally double charged with propellant is a completely different thing. Depending on the load and propellant, I think that a double charge could potentially give much more than double pressure. So I am not sure if a double charge could always be contained.

I know that some people here know much more about this subject than I do, so I would be happy to hear more about these points from the experts.

 

[milktree]

I have read that many firearms are designed to withstand double their expected operating pressure. This is a design standard that provides a wide margin of safety under normal operating conditions.

I believe that withstanding a load that was accidentally double charged with propellant is a completely different thing. Depending on the load and propellant, I think that a double charge could potentially give much more than double pressure. So I am not sure if a double charge could always be contained.

I know that some people here know much more about this subject than I do, so I would be happy to hear more about these points from the experts.

You're absolutely right that "double charge" is vastly more dangerous than "double pressure".

Smokeless powder doesn't burn linearly with pressure, so a small increase in powder (or bullet weight or obstruction) leads to a huge increase in pressure.

Designing for double pressure isn't insane. Designing for double powder is.