NEW 6.8/SR-556

[Tokarev]

Has anybody done any laboratory testing of the SAAMI chamber against the SPCII chamber to test differences in chamber pressure? Seems most things I've seen regarding pressures are reloaders who are looking at their primers for signs of cratering, flattening, etc. Has anybody submitted anything to HP White Laboratories?

[Boomer]

I'm thinking very few people have access to laboratory testing facilities, I know I don't. From personal experience, I started with a Spec I chamber and SSA combat loads would flatten primers and leave ejector marks with this barrel. I switched to a SpecII chamber and the pressure signs went away. The case head expansion on the brass was less, too. Sure, if your shooting nuetered ammo through a spec I there won't be any issues but your pressures will still be higher than if you put the same ammo through a spec II chamber. If you half the free bore (leade) in a chamber it will raise pressure. A Spec I has a leade of .50 and a Spec II has a leade of .100. Its the same concept with .223 chambers vs. 5.56 chambers. I wouldn't put Lake City ammo through a .223 chamber. And yes, I do reload, but most of my reloaded ammo is for training, most of it moderatly loaded. Even then, a SpecII chamber will reduce pressure and add life to your brass and rifle. It's nice being able to get eight to ten reloads from brass. Add .13$ Nosler OTM bullets to this and you suddenly have training ammo that is only around .25$ each.

As for the difference in chamber pressures between the two chambers I don't know, I've heard 4000 PSI difference through the grape vine but I'm not positive. I like to load 115 grain OTM's with 28.2 grains H322, Hodgdon's max load for this bullet for the old SpecI chambers. In my old barrel it was on the ragged edge of high pressure, the primers were flattening, faint ejector marks on the brass as it heated up. With my SpecII's this same load has nice rounded edges on the primer, zero ejector marks, even after getting the rifle nice and hot, and the case head expansion is barely existent. Remington brass has a case head measurement of .418" and after firing this load through the correctly chambered rifles it either remains at .418" or barely expands to .419". I've gone over Hodgdon's max with this powder/bullet combo and the results were the same. I mostly just stick with Hodgdon's data though.

[Tokarev]

Thank you for an honest and detailed reply. Seems much of this debate degenerates into, "Because I said so!"

Have you played with what happens to ammo that's loaded to SPCII pressures after being exposed to high heat? A scenario I envision is a carbine and ammo in a gun bag in the truck of a squad car or patrol vehicle out in the Arizona sun. All these types of things need to be taken into account if we're going to see any resolution to which is truly the better chamber.

I don't have any inside information on this but I'm curious to know if Ruger looked at SPCII and rejected it for some safety reasons or if they went straight to the SAAMI chamber right off the bat. I know the guys at Ruger are aware of the new chamber so I'm sure they did at least some homework on the subject before making their decision. I do know they shoot the crap out of their prototype guns. Maybe they found some longevity issues with guns shooting hotter ammo?

[Todd.K]

Has anybody done any laboratory testing of the SAAMI chamber against the SPCII chamber to test differences in chamber pressure? Seems most things I've seen regarding pressures are reloaders who are looking at their primers for signs of cratering, flattening, etc. Has anybody submitted anything to HP White Laboratories?

SSA tests pressure with a universal receiver and pressure test barrels.

[Boomer]

Thank you for an honest and detailed reply. Seems much of this debate degenerates into, "Because I said so!"

Have you played with what happens to ammo that's loaded to SPCII pressures after being exposed to high heat? A scenario I envision is a carbine and ammo in a gun bag in the truck of a squad car or patrol vehicle out in the Arizona sun. All these types of things need to be taken into account if we're going to see any resolution to which is truly the better chamber.

I don't have any inside information on this but I'm curious to know if Ruger looked at SPCII and rejected it for some safety reasons or if they went straight to the SAAMI chamber right off the bat. I know the guys at Ruger are aware of the new chamber so I'm sure they did at least some homework on the subject before making their decision. I do know they shoot the crap out of their prototype guns. Maybe they found some longevity issues with guns shooting hotter ammo?

Much of my "I said so" testing was done with this hotter ammunition sitting in my truck for hours in summer heat, 100 +, before shooting it. And shooting it after heating the gun up with some tactical training, at least 100 rounds fired in a minute or two. Defend Ruger all you want, but they chose a chamber that was never supposed to be, it was a mistake, submitted by the AMU (or Remington depending on who you talk to) to SAAMI by accident. The 6.8 was orginally designed with a SpecII type chamber, .100 leade, and were successfully tested with 115 grain Hornady OTM's at 2650 fps with 16" barrels with no problems. Along comes this saami chamber with half the leade and pressure spikes caused serious problems with this ammunition. This is why Remington 6.8 ammo is so slow. Their 115 grain rounds barely make 2400 fps from a 16" barrel. I used to shoot SSA's older combat loads at 2580 fps with no problems. Remington ammo is laughable, both in performance and price, compared to SSA and Hornady.

[Tokarev]

Boomer,

Thanks again for the info.

[carbinero]

Seeing all that 1:10 fail makes me feel like I'm in a time machine.

Looking forward to those in-spec 10 round mags.

[deadduck357]

Finally got the SR-556/6.8SPC to the range. It was 100 degrees farenheit and the elevation approx 471 ft. Hot as hell and Sept 1st - opening day of dove season so I didn't have much time. I tried to get a good variety of 6.8spc ammunition. Brands tested were Double Tap 100gr Nosler AccuBond, Double Tap 115gr FMJ-BT, BVAC 115gr HPBT Match, Remington 115gr OTM, Hornady 110gr BTHP Match, and Silver State Armory 115gr OTM.





The first thing I wanted to do at the range was velocity testing. Shooting Chrony F-1 Master was set up at 12 ft.




Top velocity honors go to Hornady 110gr BTHP Match


Barrel was cleaned before any shooting took place and again after every brand was fired to prevent any advantage or disadvantage due to fouling.


Off to the 50yd range to sight in the irons.


After a few rounds of sighting-in & about 7 clicks to the right, three rounds per brand were fired.


Goal of the session was more testing for function, so results should not be taken as indicative of the rifle's capacity for accuracy. Given more time I probably could have improved the results. Three rounds each per brand as I did not want to burn up all my ammunition supply on the 50yd test. Saving the majority of the supply for the 100yd test.



Results may not be fair to Double Tap ammunition as it was first fired and groups got progressively tighter. This could be due to barrel break-in. However, all ammunition chambered and cycled reliably on regulator setting No.2.

All spent casings show sufficient primer strikes.



This was a short range session. I was already running late to meet a buddy for a scheduled dove hunt and had to run. A 100yd range test will be added soon with glass. Did notice that the recoil was greater than the 5.56mm but nothing to cry about. Recoil compares roughly to shooting a 7.62x39mm.

[rsilvers]

Has anybody done any laboratory testing of the SAAMI chamber against the SPCII chamber to test differences in chamber pressure?

Yes. This has been done.

[rsilvers]

Seeing all that 1:10 fail makes me feel like I'm in a time machine.

There is no downside to 1:10 or 1:9 twist. The upside is that it makes more bullets stable under a wider range of conditions, and the terminal effects with certain bullets is greater. If you like 1:7 twist on 5.56mm, then 1:9 is the 6.8mm equivalent.

There is a downside to a SAAMI chamber, but not as much as people think.