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Posted: 9/19/2012 10:04:25 AM EDT
[#1]
6.8 SPC Max Pressure is: 54 KPSI SAMMI and 58 KPSI CIP (Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives or Permanent International Commission for the Proof of Small-arms, sometimes referred to as the International Proof Commission).
I have not seen SAAMI nor CIP pressure standards for the Grendel, but I suspect that because of it's larger rim diameter it's pressure will be a touch lower than the 6.8 SPC |
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Posted: 9/19/2012 10:31:01 AM EDT
[#2]
I have seen from MAP's of 42,000 PSI to 52,000 PSI for the Grendel.
Originally AA set it @ 42,000 PSI with Lapua SRP Brass in 2003... I have seen the Powder Companies rate it as high as : 52,000 PSI...which may be pushing it to +P+ levels in a AR-15... There seems to be little in the way of the Info. dept. of SAAMI as to the max pressure standaerd for the Grendel ctg. |
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Posted: 9/19/2012 11:25:01 AM EDT
[#3]
The AA reloading data for the Grendel (on their website and the 65Grendel.com forum) is right around 48-50K psi.
On the old Grendel forum, BA said that the Grendel max was ~52,000 psi. In the Archives on this site, Art (SSA) said that he kept his 6.8 SPC 'Combat' loads right around 52,000 psi, which would mean a max around 55,000 psi, though I have seen published load data in the 58,000 psi range. |
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Posted: 9/20/2012 3:35:45 AM EDT
[#4]
6.5 Grendel pressures per Bill Alexander in a post at 6.5Grendel.com:
Originally Posted By Bill Alexander: Grendel has the MAOP set at 50,000 psi. This was established as the ideal working load for the rifle to allow for a durability cycle that matches the existing military needs for a fleet size sample. It may be a little conservative. The maximum pressure for the round at STP is 51,800 psi. This will account for production variations, storage variations for a batch and to a certain extent a temperature/instrument wander.
The case is developed with a 58,000 psi web. Depending upon manufacturer the offset for the case varies between 5,600 psi and 9000 psi which is an indicator of the ability of the case to operate in a automatic mechanism, which opens with some level of residual pressure. The rifle itself is rated to the same as the case at 50,000 psi for operation but has to also be able to fire proof loads without sustaining damage. In this instance 67,340 psi. The difference between the MOAP and the proof loading constitutes the safety factor and allows for the durability. |
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Posted: 9/20/2012 8:28:55 AM EDT
[#5]
Quoted: 6.5 Grendel pressures per Bill Alexander in a post at 6.5Grendel.com: Originally Posted By Bill Alexander: Grendel has the MAOP set at 50,000 psi. This was established as the ideal working load for the rifle to allow for a durability cycle that matches the existing military needs for a fleet size sample. It may be a little conservative. The maximum pressure for the round at STP is 51,800 psi. This will account for production variations, storage variations for a batch and to a certain extent a temperature/instrument wander. The case is developed with a 58,000 psi web. Depending upon manufacturer the offset for the case varies between 5,600 psi and 9000 psi which is an indicator of the ability of the case to operate in a automatic mechanism, which opens with some level of residual pressure. The rifle itself is rated to the same as the case at 50,000 psi for operation but has to also be able to fire proof loads without sustaining damage. In this instance 67,340 psi. The difference between the MOAP and the proof loading constitutes the safety factor and allows for the durability. Thanks for posting that. |
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Posted: 9/20/2012 9:00:34 AM EDT
[#6]
So ... sifting through the above, is it accurate to say that the MAOP is 50,000 psi for the 6.5 and 54,000 psi for 6.8?
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Posted: 9/20/2012 9:22:36 AM EDT
[#7]
Quoted: So ... sifting through the above, is it accurate to say that the MAOP is 50,000 psi for the 6.5 and 54,000 psi for 6.8? Since there is load data out for the 6.8 that is around 58,000 psi, I would push that a bit higher. I do not load my 6.8's that hot, but YMMV. |
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Posted: 9/20/2012 10:43:29 AM EDT
[#8]
Quoted:is it accurate to say that the MAOP is 50,000 psi for the 6.5 and 54,000 psi for 6.8?
50k is correct for 65G. You heard it from the horse's mouth. For the 68SPC, depends on which ammo you're talking about. 54k is probably correct for SAAMI spec ammo, but, for example, SSA loads their "tactical" rounds to 58k. Anybody have the 6.8 SAAMI spec to verify the MAOP? John |
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Posted: 9/20/2012 11:28:01 AM EDT
[#9]
Quoted: Anybody have the 6.8 SAAMI spec to verify the MAOP? John Since there has been no changes in the SPC by SAAMI since it was adopted in 2004, I would think that Max is 54,-55,000 psi. |
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Posted: 9/20/2012 1:11:38 PM EDT
[#10]
No one has had the money and influence to update the SAMMI specs for 6.8SPC to 6.8SPC II, IIRC 6.8SPC II is in the 57-58k psi range for the MAX pressure, with very hot handloads going into the 59k range. Some bolt gunners have pushed past 61k but that is not advisable unless your a very experianced handloader with a very well built gun.
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Posted: 9/24/2012 11:28:34 AM EDT
[#11]
In talking with the Lapua guys at a booth in Europe this year, they took a Sako bolt gun chambered in Grendel and ran it 15% over max of what .300 Win Mag chamber pressure allows. Brass was ruined, but they pushed the 123gr Scenar to over 3200fps from a 24" barrel.
The Grendel in a bolt gun can handle 58,000+ pressures, and makes an excellent lightweight hunting rifle. The CZ527 action is ready to go for this purpose when using the 7.62x39 model, as the bolt face and extractor are set-up for the Grendel head dimensions-it being a descendant of the 7.62x39. |
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Posted: 9/24/2012 3:03:55 PM EDT
[#12]
Quoted:
No one has had the money and influence to update the SAMMI specs for 6.8SPC to 6.8SPC II, IIRC 6.8SPC II is in the 57-58k psi range for the MAX pressure, with very hot handloads going into the 59k range. Some bolt gunners have pushed past 61k but that is not advisable unless your a very experienced handloader with a very well built gun. Nicely put! SAAMI for the 6.8 is 55k. That was what the .mil guys wanted. It has come up many times. 6.8 Barrels are proof tested at 70k. But, Like said above 57-58k is the working max for the SPC II. I do believe SSA uses the same pressure limits as the M855A1 govt. specs or something to that effect. |
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Posted: 9/24/2012 5:52:41 PM EDT
[#13]
M855A1 will run chamber pressures from +62,000 psi measured by direct reading transducer at the case mouth. Apart from potential problems with obturation due to the projectile construction, this puts the cartridge in the same class as the Mk262 mod 0. The original mod 0 severely stressed the M4 platform and drove the work that resulted in mod 1 configuration that dropped the pressure to 58,700 psi, again measured at the case mouth.
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Posted: 9/24/2012 6:14:09 PM EDT
[#14]
Quoted:
In talking with the Lapua guys at a booth in Europe this year, they took a Sako bolt gun chambered in Grendel and ran it 15% over max of what .300 Win Mag chamber pressure allows. Brass was ruined, but they pushed the 123gr Scenar to over 3200fps from a 24" barrel. The Grendel in a bolt gun can handle 58,000+ pressures, and makes an excellent lightweight hunting rifle. The CZ527 action is ready to go for this purpose when using the 7.62x39 model, as the bolt face and extractor are set-up for the Grendel head dimensions-it being a descendant of the 7.62x39. Lapau have tested the case and the web is suitable for a working pressure of 58,000 psi. The margin between this and the 50,000 psi MAOP is part of the safety factor built into the system. Typically we proof rifles at 67,340 psi using a standard case so there is more margin in the case than expected. Again this is a nice safety blanket. One of the factors that governs a caliber in the AR is the hoop strain generated in the breach. When calculating, one must consider that the minor thread diameter of the breach is only 0.738". There are certain things that may help in this but regardless the current 5.56 pressure limits(58,700 psi) are very close to a maximum for the gun. |
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Posted: 9/25/2012 4:18:38 AM EDT
[#15]
Quoted:
Quoted:
In talking with the Lapua guys at a booth in Europe this year, they took a Sako bolt gun chambered in Grendel and ran it 15% over max of what .300 Win Mag chamber pressure allows. Brass was ruined, but they pushed the 123gr Scenar to over 3200fps from a 24" barrel. The Grendel in a bolt gun can handle 58,000+ pressures, and makes an excellent lightweight hunting rifle. The CZ527 action is ready to go for this purpose when using the 7.62x39 model, as the bolt face and extractor are set-up for the Grendel head dimensions-it being a descendant of the 7.62x39. Lapau have tested the case and the web is suitable for a working pressure of 58,000 psi. The margin between this and the 50,000 psi MAOP is part of the safety factor built into the system. Typically we proof rifles at 67,340 psi using a standard case so there is more margin in the case than expected. Again this is a nice safety blanket. One of the factors that governs a caliber in the AR is the hoop strain generated in the breach. When calculating, one must consider that the minor thread diameter of the breach is only 0.738". There are certain things that may help in this but regardless the current 5.56 pressure limits(58,700 psi) are very close to a maximum for the gun. Are you saying that for military use? There are several cartridges being used in the AR platform that have much more performance than the Grendel that aren't blowing up rifles. The 458 Socom, WSSMs(which operate at a high PSI), 30 RARs and cats based on the 284 and BR cases, Or are you saying Arne correctly chose the PPC case as the largest usable safe cartridge in the AR platform for that reason? |
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Posted: 9/25/2012 5:27:47 PM EDT
[#16]
Quoted:
M855A1 will run chamber pressures from +62,000 psi measured by direct reading transducer at the case mouth. Apart from potential problems with obturation due to the projectile construction, this puts the cartridge in the same class as the Mk262 mod 0. The original mod 0 severely stressed the M4 platform and drove the work that resulted in mod 1 configuration that dropped the pressure to 58,700 psi, again measured at the case mouth. +1, I routinely run loads measured at 58,000 PSI in my 6.8. These have been tested on equipment at both Barnes and SSA. I do not get ejector swipes, popped primers or leakage, and have never broken a bolt, or an extractor with these pressures. I use an adjustable gas block to provide for proper timing, even with a suppressor. If the 6.8 is properly built, with upgraded components, like enhanced bolts with lobster claw extractor or made from 9310, they should run fine with normal wear. My use has been mostly slow or very controlled rapid fire (i.e, dumping a single 15 or 25 round mag) and these rifles are performing reliably after > 5000 rounds and up to 5 years of service without parts replacement. I've become very comfortable using a real working max of 58,000 PSI, but I don't recommend that inexperienced reloaders do this. Also, I tend to load long, and use slower twist barrels with 3 or 5R grooves to mitigate pressure as much as is possible. I agree that we do have a good safety margin in the 6.8 SPCII with our current pressures. |
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Posted: 9/25/2012 5:35:23 PM EDT
[#17]
12/3 groove arp barrel?
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Posted: 9/25/2012 6:38:26 PM EDT
[#18]
Quoted:
12/3 groove arp barrel? Older model ARP barrel. |
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Posted: 9/25/2012 7:17:48 PM EDT
[#19]
Quoted:
12/3 groove arp barrel? Yes 12 twist 3 groove ARP. We also had Pac-Nor custom make us a 13 twist, too, for demo purposes. Noveske also makes a 12 twist barrel with poly rifling, and I believe Black Hole Weaponry makes a 12 / 3. Most of the 3, 4 groove or 5 groove designs will assure proper bore area, and these higher quality barrels are likely to be concentric with a pin gauge passing easily, hence no stenotic portions to spike the pressure. Lesser quality barrels were seen in our 2008 test, to occasionally have stenoses ( narrowed spots) along the lumen of the tube, which always results in higher pressure as the bullet passes this. |
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Posted: 9/25/2012 7:26:04 PM EDT
[#20]
Quoted:
One of the factors that governs a caliber in the AR is the hoop strain generated in the breach. When calculating, one must consider that the minor thread diameter of the breach is only 0.738". There are certain things that may help in this but regardless the current 5.56 pressure limits(58,700 psi) are very close to a maximum for the gun. What if you have a larger thread tenon? Lets say around 1". |
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Posted: 9/26/2012 12:32:19 PM EDT
[#21]
Increasing the breach diameter in the area that constrains the cartridge case has a direct effect in reduction of the hoop strain. That is to say the chamber diameter will expand less under peak pressure and will be a better match to the elastic limit of the case material. The analysis is a simple thick cylinder theory which can frequently be simplified by neglecting to consider the axial loads. The current AR15 style set up strikes an excellent balance in not carrying parasitic weight given the operating pressures. We had to drop the Grendel pressures to maintain a similar balance, hence the given specs. There is some ability to offset the plastic limit of the case, especially steel cases which are less elastic than brass but only slightly.
Basically a larger breach is more stable. |
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Posted: 9/26/2012 4:01:41 PM EDT
[#22]
Quoted:
Quoted:
12/3 groove arp barrel? Yes 12 twist 3 groove ARP. We also had Pac-Nor custom make us a 13 twist, too, for demo purposes. Noveske also makes a 12 twist barrel with poly rifling, and I believe Black Hole Weaponry makes a 12 / 3. Most of the 3, 4 groove or 5 groove designs will assure proper bore area, and these higher quality barrels are likely to be concentric with a pin gauge passing easily, hence no stenotic portions to spike the pressure. Lesser quality barrels were seen in our 2008 test, to occasionally have stenoses ( narrowed spots) along the lumen of the tube, which always results in higher pressure as the bullet passes this. BHW uses a 3-poly 1:11. |
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