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Discussion Starter · #1 ·
Anyone here ever try Silver State Armory's 6.8 combat loads in your XCR? If so, how did it perform? Any pressure signs? I have a few boxes of it I would like to try when my conversion kit arrives and wanted to get your opinions first.
 

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Is it the 115GR OTM?? If so that is all I shoot and no signs of anything out of the ordinary. Great shooting ammo, easily shot an inch at 100 prior to cutting the barrel.
 

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Discussion Starter · #3 ·
Is it the 115GR OTM?? If so that is all I shoot and no signs of anything out of the ordinary. Great shooting ammo, easily shot an inch at 100 prior to cutting the barrel.
My understanding is that you have to specifically request the "combat loads" when you order from SSA. It is not advertised on their website. They just load whatever round you requsest a little hotter than normal (Pro hunter, 115 OTM, etc.). Otherwise, SSA is ammo is standard pressure.
 

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Well, I am not sure. When I ordered he said they had just finished making a big order and pulled 2K aside for me. I posted some numbers from the Chrono a while back, those may be able to to tell you something.
 

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I spoke with Nancy at SSA yesterday and she said that a 1 in 11 twist rate barrel is required to use the combat loads. As far as I know the XCR has a 1 in 10 barrel.
 

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Discussion Starter · #7 ·
I spoke with Nancy at SSA yesterday and she said that a 1 in 11 twist rate barrel is required to use the combat loads. As far as I know the XCR has a 1 in 10 barrel.
Thanks for the info M38. I'll hold off on trying the combat loads for now and leave that experiment for someone with more experience and equipment, etc. As far as I know from reading about 6.8 barrels, three things are ideal for hotter loads, 1:11 twist, TypeII chamber, and 4 groove rifling. The XCR has two out of three, which is better than most manufacturers IMO.
 

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I wonder if Alex and RobArm can make barrels with a 1:11" twist. That is if they are making their own barrels now? :-\
 

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I was disappointed, I thought the spec II chamber & 4 grove would do it. You have to call to get the combat loads and they won't sell them to you unless your rifle meets their requirements.
 

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So who has barrels that meet the requirements?
 

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for some reason I thought theirs weren't 4 groove but I may be wrong.
 

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I spoke with Nancy at SSA yesterday and she said that a 1 in 11 twist rate barrel is required to use the combat loads. As far as I know the XCR has a 1 in 10 barrel.
Thanks for the info M38. I'll hold off on trying the combat loads for now and leave that experiment for someone with more experience and equipment, etc. As far as I know from reading about 6.8 barrels, three things are ideal for hotter loads, 1:11 twist, TypeII chamber, and 4 groove rifling. The XCR has two out of three, which is better than most manufacturers IMO.
Someone please correct me if I am wrong but here is what I remember from reading an article. The 1:11 twist allowed the use of all current factory loads whereas the 1:10 does not (most AR's) but with Robinson or any rifle with the improved chamber the high pressure issue is alleviated even though they may have a faster twist barrel. The modified chamber also explains why Aziator has not noticed anything out of the ordinary. I did the initial research because I plan on buying the combat ammo after my rifle is broken in. The gal at SSA may not have understood that the XCR has the correct chamber for the combat ammo. Google it like I did and you should be able to find the article. I think I entered 6.8 SPC combat ammo.
CMD
 

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Discussion Starter · #14 ·
Some recent testing I read about over on the .68Forums.com indicated that the twist rate and number of grooves also affected pressure more than previously thought. Might be worth checking into. :2cents:
 

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I was just at the 6.8 Forums and my memory was correct. The spec II chamber (improved like XCR) with either 1:95 or 1:10 can use any commercial load including the combat. A SAAMI chamber (original Remington spec) will create too much pressure. The site also said if you have the 1:11 SAAMI chamber fire a few rounds and check for pressure. So the 1:11 is not necessarily the answer to end all issues and Robinson like I suspected knows what he is doing. :duh:

CMD :2rifle:

P.S. You will need to join the forum to get the info.
 

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Discussion Starter · #17 ·
This is an excert from the thread:

Our conclusions:
1) 1:11 inch twist appears to be one of the most important factors in ameliorating high pressure of hot 6.8 SPC loads. 1 turn in 10 inches is unnecessarily fast with the current 6.8mm bullets, including 130 grains, and it offers no advantage over slower twists. 1:10 twist may exhibit higher pressure than barrels of slower twist, and thus may limit the performance of the cartridge significantly.

2) SPC II and DMR chambers are somewhat important in ameliorating pressure, but may be less a factor than the twist rate.

3) The 6.8 SPC will achieve the highest velocity, safely in SPC II or DMR chamber if paired with 1:11 or 1:12” twist. This combination, were it to become the standard, could allow ammo manufacturers to increase the performance of the 6.8 SPC substantially. Most current factory ammo is loaded to accommodate the SAAMI chamber and 1:10 twist, rendering it with reduced performance.

4) The 6.8 SPC may be capable of achieving high velocity in SAAMI chambers, but probably most safely when paired with a 1:11” or slower twist rate.

5) There seems to be little reason for manufacturers to continue to make 1:10” twist barrels in 6.8 SPC. SBR’s may be the exception, but we fail to see how a 1:11” twist could be a disadvantage in barrels longer than 11 inches, with bullets ranging from 90 to 130 grains. We do not yet have any opinion about pistol length barrels because not enough testing has been done.

6) A suppressor could potentially tip your max loads over the edge of the pressure safety margin, even if you have a 1 in 11” twist, DMR or SPCII chambered gun. When fired in a suppressed weapon, cartridges reloaded beyond current SAAMI or ammo /component manufacturer’s specifications, could be considered inherently dangerous.
 

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Discussion Starter · #19 ·
Madog, This is the part I was refering to:

"2) SPC II and DMR chambers are somewhat important in ameliorating pressure, but may be less a factor than the twist rate."

Even so, I still think that XCR 6.8 barrels are absolutely fantastic and that Robinson did their research. The testing mentioned above came along after the XCR barrels were already on the market (and are still being conducted). I would, however, like to see them come out with a 1:11 twist as an option.
 

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I talked to Alex a month or so ago about building a custom barrel with the correct configuration and testing the XCR for him, he sounded interested in testing and making the changes if necessary. Since then another company made me an offer so testing the XCR would be a conflict of interest. I hope he does the research and continues with the 2 projects we discussed.
 
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