04-07-2014, 04:26 AM
I noticed the same oddity, Stan. I'm also noticing that they chose to show the 144gr FMJBT at normal COL, not a 123gr.
I'm losing track of how many wildcats there are off the 6.8 case now. This one requires cutting, forming, and firing.
The SIX 5 is just a size job, but case length limits you to short ogive pills in the AR15 magazine well. Don't let that stop anyone from claiming it "smokes" or "beats" the Grendel in performance. A 120gr SMK with .406 BC was recently used to highlight that the SIX 5 works with the long 120gr bullets. The 120gr SMK is not one of the long ones, and lacks a full numerical value in BC compared to the 123gr pills (.510).
I've also recently been looking at hoop stress in the AR15, as well as bolt thrust. Another claim that I accepted at face value has been discovered to not be supported by the results I found when comparing bolt thrust and hoop stress calculations.
http://lassengunsmithing.com/html/CylinderStress.htm
If you push the pressures over 55,000psi and let 58,850psi be your maximum probably mean with any of these cases that have a diameter of .421" or larger (at the base above extractor groove), the hoop stress gets close to or exceeds 5.56 NATO 70,000psi proof loads. If you push the 6.8 SPC II to the pressures I'm seeing listed in Accurate Powder's tables, like 58,850psi chamber pressure, you've exceeded the hoop stress generated by the SAAMI MAP for the Grendel easily, and you have more bolt thrust than the 6.5 Grendel.
These pressures are what are being advertised as the saving grace for some of these SPC wildcats in the AR15, allowing them to blow the Grendel away, yet case capacity is still the same or less, and 3-5kis is not going to give you the performance advantage you're looking for, while running the ragged edge of what is safe in the AR15.
One cartridge I think took a better approach to this is Hornady's 6mm Hagar, which uses case capacity, not pressure, to get more performance. They increased the SPC's case length to do that, which limits mag-fed options for longer target bullets, but spits the 90gr stuff out very fast.
I've gained a new found respect for why the Grendel was kept "on a leash" at 52,000psi, and the bolt is the least of my worries. Bill has said that for years, but you really get an eye-opener when you run the bolt thrust and hoop stress calcs. If we stay with the AR15 thread tennon diameter on the barrel, then we have to accept a series of constraints. The Grendel overcomes these to a large extent with case length being short enough to fit long projectiles in the AR15 mag well, and push them at moderate velocity inside of safe pressures in the system.
If one wants to remove the constraints of the AR15 barrel extension, thread tennon, and upper receiver ID for the tunnel, one needs to design another rifle, that is if you want to push the pressures like a 5.56 NATO does. I'm becoming more and more interested in lower pressures myself. More throat life, longer critical parts life, larger factors of safety...
The thing I still laugh at is that I'm getting way more performance than I ever thought I would at distance.
I'm losing track of how many wildcats there are off the 6.8 case now. This one requires cutting, forming, and firing.
The SIX 5 is just a size job, but case length limits you to short ogive pills in the AR15 magazine well. Don't let that stop anyone from claiming it "smokes" or "beats" the Grendel in performance. A 120gr SMK with .406 BC was recently used to highlight that the SIX 5 works with the long 120gr bullets. The 120gr SMK is not one of the long ones, and lacks a full numerical value in BC compared to the 123gr pills (.510).
I've also recently been looking at hoop stress in the AR15, as well as bolt thrust. Another claim that I accepted at face value has been discovered to not be supported by the results I found when comparing bolt thrust and hoop stress calculations.
http://lassengunsmithing.com/html/CylinderStress.htm
If you push the pressures over 55,000psi and let 58,850psi be your maximum probably mean with any of these cases that have a diameter of .421" or larger (at the base above extractor groove), the hoop stress gets close to or exceeds 5.56 NATO 70,000psi proof loads. If you push the 6.8 SPC II to the pressures I'm seeing listed in Accurate Powder's tables, like 58,850psi chamber pressure, you've exceeded the hoop stress generated by the SAAMI MAP for the Grendel easily, and you have more bolt thrust than the 6.5 Grendel.
These pressures are what are being advertised as the saving grace for some of these SPC wildcats in the AR15, allowing them to blow the Grendel away, yet case capacity is still the same or less, and 3-5kis is not going to give you the performance advantage you're looking for, while running the ragged edge of what is safe in the AR15.
One cartridge I think took a better approach to this is Hornady's 6mm Hagar, which uses case capacity, not pressure, to get more performance. They increased the SPC's case length to do that, which limits mag-fed options for longer target bullets, but spits the 90gr stuff out very fast.
I've gained a new found respect for why the Grendel was kept "on a leash" at 52,000psi, and the bolt is the least of my worries. Bill has said that for years, but you really get an eye-opener when you run the bolt thrust and hoop stress calcs. If we stay with the AR15 thread tennon diameter on the barrel, then we have to accept a series of constraints. The Grendel overcomes these to a large extent with case length being short enough to fit long projectiles in the AR15 mag well, and push them at moderate velocity inside of safe pressures in the system.
If one wants to remove the constraints of the AR15 barrel extension, thread tennon, and upper receiver ID for the tunnel, one needs to design another rifle, that is if you want to push the pressures like a 5.56 NATO does. I'm becoming more and more interested in lower pressures myself. More throat life, longer critical parts life, larger factors of safety...
The thing I still laugh at is that I'm getting way more performance than I ever thought I would at distance.