Gunsmithing Minimum AR barrel thickness

[flyfisherman246]

I see these pencil barrels and ultra lightweight ar15 and ar10 barrels available. They are around .750" from about 2.5" in front of the barrel extension to the gas block. I understand some of these are probably chrome moly and can handle the pressure with the thinner wall thickness. I have spoke with both bartlein and kreiger about minimum wall thickness and they both said somewhere around .300" at 6" from the chamber end. So on a .308 that would be .908 and a .223 would be .823" at 6" in front of the chamber. Those are pretty heavy contours especially on the .308. My question is how are places like lija spinning up ar10 barrels that are .750" at 2.5" from the chamber end all the way to the gas block with stainless barrels. Thats a wall thickness of .225 which is significantly thinner than the minimum recommended by bartlein and kreiger. Then on their sporter contour bolt gun barrels their 1,2, and 3 contours are .700, .750, and .800 at the 6" mark. I'm not saying one company is right and the others are wrong, but that's a big difference of opinions. I would like to be able to cut a little meat off ar15/10 barrels, but there has to be a reason bartlein and krieger are giving the minimum thicknesses they are stating. Any input?

 

[CrabsandFootball]

Its 100 year old tech they have it figured out.

Its basic physics and metallurgy.

Everyone has different risk profiles and bartlien and kreiger are peanuts compared to gun barrels produced every year across the globe. Various proof houses have much stricter requirements yet we have pencil barrels handling high pressure magnum calibers.

They say what they say becuase thats what they sell.

Your looking for a problem where none exist.

 

[flyfisherman246]

I wouldn't say I'm looking for a problem, I'm just trying to get a better understanding of the situation. I find it interesting how different the thickness requirements are between barrel manufacturers. There has to be something bartlein and kreiger know that would cause them to come up with the numbers they have right?

 

[Mark Buettgen]

Hey guy's,
Some of the reasons we set the standards we do is, we're ruled over by Attorney's. we have to make sure what we ship out will be safe in almost every conceivable scenario, heat, cold, how it's run, how it's cleaned--Box ammo or hotter'n hell handloads.
Anytime someone tries to bring something down to minimum tolerances, it invites disaster, and the last thing we want is for the Widow to bring suit.
we've got 50 people and their families that would all be affected and we always are keeping that in mind when making these decisions.
Stay Safe,
Mark Buettgen
Bartlein Barrels Inc

 

[BradyW]

we're ruled over by Attorney's.

This. I do a lot with the ARs in the shop. We want to make sure you don't get blow'd up!

Now, Paul @ Craddock Precision will make you a thicker barrel and then flute it - that's his deal, and he can lighten them up quite a bit - if you want something like that done, talk to him!

Brady @ Bartlein

 

[godofthunder]

I would assume the manufacturers calculated, or had someone do it for them, the hoop stress for various calibers to know what the barrel needed to withstand at different locations given data (or using collected data) on the pressures seen within the barrels at said locations. Then they decide how stupid they think people are and apply a safety factor accordingly.

 

[flyfisherman246]

I figured it had to be some sort of lawyer involvement type thing. That being said, for personal use I'm trying to figure out how to cut a little weight of these things. A .900" barrel up to a rifle length gas system turns into a pretty heavy rifle. So I ran some numbers base off of barrel thicknesses of both types.

Based off the best research I can find online, it looks like the yield strength of barrel grade 416SS is somewhere around 105 KSI. A .308 Winchesters max saami chamber pressure is 62 KSI and proof loads are 75.275 KSI.

BARTLEIN/KRIEGER:
Based off the recommended wall thickness of .300" with a .308 bore, the OD would be .908". The hoop stress of a max saami load of 62 KSI is 78.123 KSI and a safety factor of 1.34. This same barrel with a proof load has a hood stress of 94.85 KSI and a SF of 1.107.

LILJA:
Lilja has an ar10 .308 contour with a OD of .750 at 6" from the chamber end. With saami max pressure loads it has a hoop stress of 87.154 KSI with a SF of 1.2. With the proof load it has a hoop stress of 105.815 KSI and a SF of .992. So theoretically, this barrel could bulge at the 6" mark with a proof load based off these numbers.

Can somebody check or make since of these numbers to me? What would be a reasonable safety factor to be working with when dealing with rifle barrels under pressure, and would that number be based off saami max pressure of proof loads?

I know I'm going way down the rabbit hole here, I'm more curious than anything now. Based off these numbers (assuming they are accurate) what would be a safe minimum barrel thickness?

 

[CrabsandFootball]

Why not just buy a proof prefit and be done with it?

 

[flyfisherman246]

Why not just buy a proof prefit and be done with it?

Because they don't fucking shoot!!! Ask me how I know!! hahaha

 

[CrabsandFootball]

Because they don't fucking shoot!!! Ask me how I know!! hahaha

 

[Yondering]

I wouldn't say I'm looking for a problem, I'm just trying to get a better understanding of the situation. I find it interesting how different the thickness requirements are between barrel manufacturers. There has to be something bartlein and kreiger know that would cause them to come up with the numbers they have right?

Not so much what they know, but more about their market (precision accuracy barrels) and maybe a little about what they don’t know and haven’t tried.

Thin pencil profile barrels do work and are fairly common, and dangerous failures of them are not. It’s not just in ARs either; if you’ve ever seen a Steyr AUG barrel for example they are a very thin straight profile starting just in front of the chamber. You can profile barrels that thin if you want, I’ve done a few myself, just don’t be surprised if POI wanders a bit as it heats up.

I did exactly what you’re asking about on one of my 308 barrels for the same reason. You can take quite a bit of weight off a heavy profile. A profile I’ve used a few times is to taper from .825” near the chamber to .700” behind the gas block, then from .700” to .625” between gas block and muzzle. Don’t forget to leave a flange for a barrel thread shoulder. You can go a fair bit thinner than that too.

 

[godofthunder]

I figured it had to be some sort of lawyer involvement type thing. That being said, for personal use I'm trying to figure out how to cut a little weight of these things. A .900" barrel up to a rifle length gas system turns into a pretty heavy rifle. So I ran some numbers base off of barrel thicknesses of both types.

Based off the best research I can find online, it looks like the yield strength of barrel grade 416SS is somewhere around 105 KSI. A .308 Winchesters max saami chamber pressure is 62 KSI and proof loads are 75.275 KSI.

BARTLEIN/KRIEGER:
Based off the recommended wall thickness of .300" with a .308 bore, the OD would be .908". The hoop stress of a max saami load of 62 KSI is 78.123 KSI and a safety factor of 1.34. This same barrel with a proof load has a hood stress of 94.85 KSI and a SF of 1.107.

LILJA:
Lilja has an ar10 .308 contour with a OD of .750 at 6" from the chamber end. With saami max pressure loads it has a hoop stress of 87.154 KSI with a SF of 1.2. With the proof load it has a hoop stress of 105.815 KSI and a SF of .992. So theoretically, this barrel could bulge at the 6" mark with a proof load based off these numbers.

Can somebody check or make since of these numbers to me? What would be a reasonable safety factor to be working with when dealing with rifle barrels under pressure, and would that number be based off saami max pressure of proof loads?

I know I'm going way down the rabbit hole here, I'm more curious than anything now. Based off these numbers (assuming they are accurate) what would be a safe minimum barrel thickness?

Those are max chamber pressures though. Pressure will only drop as you increase the volume behind the projectile, so I would guess that those safety factors are higher than what you calculated.

 

[flyfisherman246]

Those are max chamber pressures though. Pressure will only drop as you increase the volume behind the projectile, so I would guess that those safety factors are higher than what you calculated.

I’ve heard that somehow 6” from the chamber end is the location of max pressure.

 

[godofthunder]

I’ve heard that somehow 6” from the chamber end is the location of max pressure.

Interesting, I've never heard that. I would assume the pressure would be highest at the throat when the bullet contacts the rifling, then after that it would be fairly easy to [roughly] estimate pressure using the ideal gas law - P1V1 = P2V2 --> P2 = (P1V1/V2). It would be an oversimplification as there are a lot of variables to consider, but if you took those numbers you would get a pressure of around 25kpsi 6" from the chamber. That would give you a hoop stress of about 30kpsi and a safety factor of 3.5

 

[godofthunder]

I found this when doing some searching. I haven't had the chance to read it, but probably would give you the answers you are looking for: Stress Analysis of Gun Barrel Subjected to Dynamic Pressure :: Science Publishing Group

 

[Yondering]

I’ve heard that somehow 6” from the chamber end is the location of max pressure.

The fact that pencil profile barrels are available and don’t blow up should be a significant factor in your estimations. Don’t just go by the fact that certain manufacturers cater to a heavy barrel market.

 

[flyfisherman246]

Interesting, I've never heard that. I would assume the pressure would be highest at the throat when the bullet contacts the rifling, then after that it would be fairly easy to [roughly] estimate pressure using the ideal gas law - P1V1 = P2V2 --> P2 = (P1V1/V2). It would be an oversimplification as there are a lot of variables to consider, but if you took those numbers you would get a pressure of around 25kpsi 6" from the chamber. That would give you a hoop stress of about 30kpsi and a safety factor of 3.5

That would make since to me as well, not sure how pressure would be higher down the barrel. Appreciate your input and will read that article you posted. Thanks!!

 

[Wannashootit]

Some 30,000,000+ Mosin Nagants have been/are in service chambered in 54R
28-1/2" barrel that's roughly .770 just ahead of the chamber to .580 at the muzzle.

Supposedly, a bit lower pressure round...but trust me- they can be and are loaded to .06 velocities.

 

[flyfisherman246]

Appreciate everyone's (almost everyone's) helpful input. I feel better about experimenting with taking some meat off barrels in the future. Thanks

 

[sirhrmechanic]

Barrel thickness is also about absorbing harmonics and not getting bent just dropping it or getting in and out of a vehicle.

Look at a FN 5.7 carbine barrel? Or look at a cylinder wall of a large caliber revolver like a .44 mag. At its thinnest point, that cylinder wall is only about .060-.080” thick. And it is milled for the lock right at the thinnest point.

Barrels can be made very very thin if simply containing pressure is the goal. But they also have to maintain shape. Dissipate heat (and absorb heat and store what it cannot dissipate immediately). Survive handling and field conditions. Add to the balance and handling characteristics of the rifle. And deal with the interior ballistics such as harmonics and pressure waves. Those things require a substantial piece of steel, not a thin tube just required to keep in pressure.

Lawyers certainly play some role. But engineers most certainly know what they are doing, esp. when it comes to mil.spec parts that have incredible requirements for them, not the least of which is that they have to be private-proof.

It would be an interesting exercise to take an at barrel and, on a lathe, turn off .020 increments while firing the barrel in between. My thought experiment says that first, you are going to get a dramatic loss of accuracy (after a few passes.). Then you will get a barrel that warps and you get huge groups with a shifting poi (and probably find the barrel is warped after a few shots and won’t chuck up true). Last, you will get a burst crack, longitudinally along the barrel, though perhaps a break at the shoulder where your taper to the chamber begins. But if you machine a long and gentle taper, betting on the longitudinal crack, propagating at a rifling groove.

In my thought experiment, I am also imagining that firing one shot and letting barrel cool would extend the number of rounds before these events happen considerably. A mag dump in an at is a pretty stressful event for even a new barrel

Bet somewhere, someone has done this. Be an interesting practical experiment. I’d also bet that the barrel, long before it bursts, would be a lot thinner than most people think? Just look at shotguns!

Interesting thread!

Sirhr