08-22-2014, 02:11 AM
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:
![[Image: foto_23.JPG]](http://4.bp.blogspot.com/_f1TTfqxVUw4/TCwnD2FiLjI/AAAAAAAABgY/qHmB0VY6tog/s320/foto_23.JPG)
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:
![[Image: definitions_large.jpg]](http://www.keystonethreaded.com/images/definitions_large.jpg)
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.
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.