Mil Spec FoS

[cory]

I am not an engineer so this is just an interesting read for me. I do fond it intresting Lilja has the consumer crunching #'s. I would think that liability would lie with them and they wouldn't produce anything less than what their #'s concluded was safe. I apologize about the aircraft talk!

LILJA does not and has not asked me to crunch these numbers.

I'm doing this for my sanity, and so I can present a safe contour to Lilja with our request. I'm sure Dan will do his own math or already has to determine a minimum barrel thickness.

And I don't do this because I don't trust Dan. I do it because I can, I do it because I NEED to. Variable presented me with a problem, "how thin can we go with a Grendel barrel?" A problem has been presented, I won't have piece of mind until I solve it.

It's the curse of the engineer.

[Variable]

Heh heh. I like that thinking. I'm sure the bolt would blow first, but it's good to know anyway.

I'm typically a "hold my beer and watch this" kinda guy, but I will look for info that others can figure out too. Helps me get to enjoy future beers that way.

Then what do you get for a barrel that tapers down to .625 past the chamber, and stays that way all the way out (it'd have two bumped shoulders-- one for the gas block shoulder, and one for the muzzle thread shoulder)? Much thinner would be impractical I'd think.

[LRRPF52]

The main area of the barrel we are concerned with is the thread relief cut on the tennon. That is the weak link in the pressure containment vessel, in the area where pressures are highest. Since that is already engineered and limited based on AR15 barrel extension geometry, which is limited by AR15 upper receiver tunnel geometry, you're good to go there. That is why the Grendel has the 50,000psi max operating pressure.

Here is the area in question:



See the narrowest thread relief cut just before the shoulder of the barrel...that is your limiting OD to determine FoS for the chamber pressure. You will have different burn curves for different loads, so there needs to be a buffer as you move forward, but you would really have to drastically cut the barrel profile close to the chamber to go under the FoS for that area forward of the chamber.

When I did the hoop stress calcs for .222 Remington and 416R stainless, it came out very clean, since I was looking for the numbers that Armalite engineers would have been working off of in 1957, before 5.56x45 and .223 Remington even existed. The rifle was built around those dimensions.

The nominal minimum for the thread relief cut is .7385" IIRC. If I were cutting the barrels, I would hand-fit the extensions to them and go with the least amount of thread relief, or no thread relief at all. For those wondering about some of the terminology, here is a diagram of basic thread terms:



You still will have the root diameters to work with. For mass production, variances have to be accounted for, even with high quality control standards, so deviations don't go anywhere near FoS thresholds. This is why the 6.8 SPC is still un-vetted. Doesn't look like anyone did the engineering analysis on it. If you run the pressures that are listed for it, it exceeds FoS bigtime in 416R, and also exceeds bolt thrust of the Grendel.

[Variable]

I'd say something else, but I think you pretty much just stuck a fork in that one Paul. That theoretical potato seems to be done.

Maybe you guys can compare barrel stiffness at different lengths and diameters?

Like: If a twenty inch .750"dia. barrel is stiff enough for a particular purpose, then how stiff by comparison is a .625" barrel that is only 12.5" long?

Dan Lilja has a bunch of math about that stuff here: http://www.riflebarrels.com/articles/bar...rifles.htm
It's way above my paygrade, I wouldn't stand a chance of figuring out something like that.

[Keep The Change]

Good points LRRPF52.

You mentioned the pressures will be highest at the chamber and the highest stress will be at the minor diameter next to the barrel shoulder in picture. So are you saying there is a pressure spike that could be above said 50ksi. Or is 50ksi the max pressure the barrel/chamber will see and it all happens right here.

So if the max pressure happens back at the chamber, to determine what diameter the barrel can go down too at other locations, you would want to know the pressures at those locations, which should be less than 50ksi.

I think bending at this critical location might be something to consider. This is the anchor point of the barrel which is cantilevered from this location. Barrel whip will put tension stress in the barrel at this location. That is if you want to know what sort of FoS there is for interacting the hoop stress and axial stress due to bending.

For determining how small you can contour the barrel it shouldn't be an issue.

[SG4247]

BE really ideal to get actual pressure transducer traces from a Grendel test barrel, and load them into an exact model of the desired profile, and run some FEA.

The results would be really enlightening...I would expect!