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  1. #1
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    Default 2008, 6.8SPC PERFORMANCE TEST- follow up report now posted

    The Setting: This test was performed at Wild River Ranch near Goliad, TX. The 1000 acre ranch provides ample room, a lodge, a range and wild hogs, so that my guests, Art Kalwas (from Silver State Armory, hereafter “SSA”), and Harrison (Constructor) would have something fun upon which to test their loads, besides just gelatin.

    Disclaimer: This test was not intended as a recommendation for reloaders to follow. We are not promoting duplication of these loads. You should always start at lower published, safe powder charges, and work your way up for your own barrel. Yes, we took some measure of risk, but these were calculated risks based upon data gleaned from 2 years of previous testing. Neither Silver State Armory, nor any of the participants in this test recommend its duplication, and thereby accept no liability for personal injury or loss of life, limb, or property resulting from its duplication or emulation. This was not a test of “brands,” but rather technical specs. I did not mention all the brand names used since we do not want to give the impression that we compared them against one another. Also we did not seek permission from manufacturers to test their products, except of course for SSA ammo. Your barrel is an individual, and while it will probably perform similarly to other barrels of like manufacture, you should always adjust reloads based upon how they perform in your barrel.

    Our goal: to help answer some questions, regarding 6.8 SPC. Namely, 1) what is its maximum potential for its velocity? 2) Does twist rate limit powder charges when loading for maximum velocity? 3) Does a suppressor change the gun’s ability to handle pressure, and reduce the safety margin of max loads? We hypothesized that the 1 in 11” twist would not show pressure signs in brass with max reloads with which we all had experience, but that the 1 in 10” might show ejector swipes with these same loads. Ejector swipes are considered a high pressure sign and it is one of the earliest and easiest ways to establish a ceiling past which you should not load for that gun.

    Silver State Armory supplied both commercial and combat loads in 6.8 SPC, and their guidance in this test was invaluable. We shot only their “combat” pressure rounds. We wanted to see how different twist rates might relate to the combat pressure rounds. Many of you have heard, anecdotally, that some uppers / barrels might not be able to safely shoot the 6.8 combat ammo, because they have too fast a twist rate, and are chambered to SAAMI specs. We consider it a given that the 1 in 9.5” barrels are too fast and increase pressure, while offering no advantage over slower twist barrels. I personally would not shoot the loads we tested in a SAAMI chambered, 1:10 twist barrel. I would not recommend that you do so. Also, we hope that manufacturers of barrels will now begin using 1 in 11 or 1 in 12”, as we believe that those twists offer the lowest pressure, and allow the cartridge to reach its maximum velocity potential.

    A little History: Manufacturers of 6.8 SPC guns and ammo have been confused about the barrel specs because Remington had problems with the initial loads released for consideration of military adoption. They used a SAAMI drawing and barrel blueprint which showed tight chamber, along with the 1:10 inch twist, and that, exacerbated by powder that was temperature sensitive, resulted in pressure problems right from the start. Unfortunately, SAAMI seems to have followed the “Greenhill formula” recommending a faster twist than was necessary, without anticipating that this could limit the velocity potential of the cartridge

    Then came the SSA X-treme SSA was asked to make the bullet by an outside source. The bullet was copper plated lead and not jacketed, and this allowed for the swelling of the bullet upon firing, which caused obduration in the throat, and thus a small ring of copper could be shaved off into the throat. Upon firing subsequent rounds, the bullet would encounter this reduced “luminal” diameter and a pressure spike would occur, causing blown primers, and stuck casings.

    Finally, some pretty ingenious folks designed a chamber that would allow for extra room, so that the broad shouldered bullet would not spike pressure. The diameter of the “SPC II” ** chamber as we will call it, is slightly larger than the SAAMI chamber. It is also a little longer. However, since so many guns had the tighter SAAMI chamber and 1 in 10” twist, SSA finally withdrew the X-treme from the market. It seemed, at that point, that the future of the cartridge was in question, but it wasn’t just because of one bullet. It had more to do with the confusing SAAMI specs, and a twist rate that was based upon a long action bolt gun cartridge, using bullets which were 40% heavier.

    By 2006, manufacturers were claiming “a .270 Winchester uses 1 in 10” twist, so that’s what I’ll make mine.” That, in conjunction with the shorter SAAMI chamber, combined to give the highest possible pressure environment for even moderate loads. For this reason, shooters got factory ammo from Hornady and Remington (now just a bystander), that had to be “watered down” to assuage liability concerns…and actually, you can’t really blame them, as most of us would have done the same thing, not knowing who would shoot what, out of what gun, and with 3 or 4 different barrel designs on the market..

    **Dimensions of the 6.8 SPC

    *****************new case----resized case ---SAAMI-------SPCII-------DMR
    base dia. ********** .4155*****.418******* .422****** .422 *******.4205
    shoulder dia. *******.402***** .402 *********.4028*****.4028 *****.403
    neck @ shoulder dia***301 **** 301 **********.3085**** .3085 ***** .305
    neck @ throat dia **** .301 *****.301*********.307 *****.308 ****** .304
    leade dia. ********************************.2781******.2781 **** .277
    length of leade **************************** .064**** *.105 ****** .095


    Recently, several enterprising manufacturers took the 6.8 SPC barrel back to the drawing board, allowing the cartridge to deliver more of its potential. The combination of both extended chamber and 1 in 11” twist, clearly gives the lowest pressure environment for the hottest loads. Ko-Tonics also took the rifling down to 4 grooves (since this was what the designers in 5th Special Forces originally specified), and a 3-groove 1 in 12” test barrel is now being planned by Harrison and me. Noveske uses polygonal rifling, and there are others who claim that the ratio of the lands to the groove height might be equally important. Others use one improvement but not all.


    The guns tested: We shot four guns, which represent a small sample of those currently on the market. We did not seek to test multiple gun brands, because our hypothesis was that a barrel with 1 in 10” twist with SPC II chamber would exhibit higher pressure than would a 1 in 11” SPC II barrel. We were certain enough about the SPC II and DMR chambers helping relieve pressure that there was no SAAMI chambered gun used for the test, especially given the very high powder charges we tested.

    In the photo, from top are a 1) An 18”, DMR chambered, 1 in 11” twist barrel (see chart above). 2) A 16 inch, SPC II chambered, 1 in 10” twist upper. 3) A 6mm Banshee, built by Harrison (a 6.5 Grendel necked down to 6mm) and thus, not part of the 6.8 test, just there for our excitement. 4) A 16”, SPC II chambered upper, with 1 in 11” twist and 4 groove rifling, also using a JET titanium .30 cal suppressor, both on and off the gun for the suppressor portion of the test.

    Methods: 1) We fired at least 10 rounds of each load in the 1 in 11” guns looking at the brass carefully, for ejector swipes or flattened primers. In the photographs you will notice brass is shown in great detail, and this accentuates the ejector marks, but there are clearly a few rounds that exhibited deep gouges and if you look carefully, some mild primer cupping. We then fired 3 rounds from the 1 in 10” barrel, looking at the brass for ejector swipes. We also looked for velocity increases with barrel heating, and we shot 3 shot strings about 30 seconds apart to calculate average velocity. We took the hottest loads from our handload stock, to test in each barrel with the following bullets, then chronographed them, showing average velocity. The exception to this was in the suppressor test. (*note: We did not intend to post SD or accuracy figures in this test):

    - Speer TNT, 90 gr. reload,SSA brass and 31.5 gr. of H322 (compressed) Remington 6.5 BR primer.
    - Speer TNT 90 gr. reload with 29 gr. of RE7, SSA brass, and CCI BR primer.
    - Barnes TSX, 110 gr., combat SSA factory
    - Sierra Pro Hunter, 110 grain soft-point reload, 32 gr. of H322 (compressed), Remington 7.5 BR primer
    - Sierra Pro Hunter, SSA 110 gr. combat factory
    - Sierra Match King enhanced frag., 115 gr., SSA combat factory


    We strongly recommend these rounds never be fired in SAAMI chambered guns with 1 in 9.5” or 1 in 10” twist.” SSA also uses this array of bullets in its factory offerings in either the commercial or combat pressure. Commercial offerings may be fired in any 6.8 SPC. Combat pressure is designed for guns with SPC II chambers.
    **Once again, I should reiterate that these loads have been fired safely in 1:11” twist guns, even one WOA barrel owned by Harrison, that had a SAAMI chamber, without popped primers or even ejector swipes.
    .

    2) We wanted to see if the SSA combat loads had a velocity comparable to the reloads that Harrison and I have been developing over the last 2 years. We chose SSA because they have, in our opinion, the most highly evolved 6.8 SPC ammunition, and brass, on the market.

    3) We sought to find out if the suppressor would add “back pressure” and change the behavior of the loads. IOW, would the suppressor add enough pressure to push the hot loads over the edge and then cause swipes in a 1:11” barrel with SPC II chamber? We fired 33 rounds from the suppressed KT-68, then cleaned the gun and shot 5 rounds of each load unsuppressed. If pressure signs were absent we would reattach the suppressor and fire another 5 rounds of the reloads and combat loads.


    The day’s conditions: Beautiful! We got a cool front, taking temps down to 34 Friday night and we froze our butts off in a blind, while hunting hogs using my PVS-14. The Saturday test was done under sunny skies, 5 mph north wind, 77 degrees and at about 107 ft above sea level with approximately 50% humidity. We managed to damage a Chronograph in the process. Ahhh, the challenges of gun and load testing!




    Results:

    - Speer TNT, 90 grain reload with SSA brass and 31.5 grains of H322 (compressed) Remington 6.5 BR primer.




    Avg. velocity in SPC II, 1:11”, 4 groove barrel – 2770 FPS, no sig. marks unsuppressed. (left photo)
    Deep ejector marks & primer cupping after 33rd suppressed round (right photo)
    Avg. velocity in SPC II 1:10 barrel - N/A

    - Speer TNT 90 grain reload with 29 grains of RE7, SSA brass, and CCI BR primer.
    Avg. velocity in DMR, 1:1, 18”barrel – 2921 FPS, no marks
    Avg. velocity in SPC II, 1:11, 16” barrel- 2881 FPS, no marks (no photo necessary)

    - Barnes TSX, 110 grain, combat SSA factory


    Avg. velocity in SPC II, 1:11”, 4 groove barrel – 2681 FPS, no sig. marks both suppressed and unsuppressed, before and after cleaning. (left photo)
    Avg. velocity in SPC II 1:10 barrel - 2642 FPS, no significant swipes seen (right photo)

    - Sierra Pro Hunter, 110 grain soft-point reload with 32 grains of H322 (compressed), Remington 7.5 BR primer



    Avg. velocity in SPC II, 1:11”, 4 groove barrel – 2771 FPS , initially deep gouges & primer cupping in dirty suppressed chamber, then no sig. marks both suppressed and unsuppressed, after cleaning. (top two photos)
    Avg. velocity in SPC II 1:10 barrel - 2701 FPS, deep ejector swipes and primer cupping seen (bottom photo)

    - Sierra Pro Hunter, SSA 110 grain combat factory


    Avg. velocity in SPC II, 1:11”,4 groove barrel – 2709 FPS, no sig. marks both suppressed and unsuppressed. (left photo)
    Avg. velocity in SPC II 1:10 barrel - 2642 FPS, moderate swipes seen (right photo)


    - Sierra Match King enhanced fragmentation, 115 grains, SSA combat factory


    Avg. velocity in SPC II, 1:11”,4 groove barrel – 2651 FPS, no sig. marks both suppressed and unsuppressed, after cleaning. (left photo)
    Avg. velocity in SPC II 1:10 barrel - 2625 FPS, significant swipes seen (right photo)


    1) 33 rounds of all the various loads were fired Suppressed, in the SPCII / 1:11” upper with no deep swipes or gouges, including the compressed handloads. Then on firing round # 34, the 32 grain H322 / Sierra pro hunter load began having deep ejector swipes. We then detached the suppressor, cleaned out the extension, chamber and leade/throat of carbon fouling (and there was a LOT). With the suppressor off, we fired 5 more rounds. This time, no swipes. We reattached the suppressor to the clean rifle and fired 5 more rounds of the 32 grain H322/ Pro Hunter load, and this time no swipes. Conclusion: Use CAUTION when firing rounds of this velocity and pressure with a suppressor. Either don’t use a suppressor with your max loads, or don’t fire more than a few rounds before cleaning the gun, chamber and throat area. Every load fired in the suppressed gun, after cleaning, behaved similarly, so we feel this is pretty consistent evidence that the suppressor, at least in a DI gun, deposits more carbon back into the gun, causes a reduction in the luminal diameter of the throat, and thus raises pressure as subsequent rounds are forced through a smaller area. Loads this hot can be increased in pressure by mere hundred thousandths of an inch of fouling. It remains to be seen if a piston system will ameliorate this problem.

    2) Velocity in this test was higher in the 1:11” twist barrel than in the 1:10” barrel, using every load tested. The 1 in 11” barrel was faster by 25-75 FPS.

    3) None of the loads tested showed deep ejector marks in any of the 1:11” twist barrels (except as mentioned in the fouled suppressed rifle).

    4) All reloads and combat loads shot in the 1:10 twist showed at least some swipes. However, with the SSA combat rounds, they were not deep marks but just shiny spots and occasionally a small nick in the brass. The hot reloads showed deeper marks, and some primer cupping, so we discontinued the strings of those loads in the 1:10 immediately. Clearly, the 1:11 was capable of shooting the reloads whereas the 1:10 was not. Velocity tended to increase as the barrel heated in both twists. However, this was much more pronounced in the 1:10” twist, with the SSA Pro Hunter Combat load increasing in velocity by as much as 130 FPS in a rapid, 3 shot string.

    5) When shooting at targets, every load tested in every barrel, stabilized with no key-holing. This was as expected with bullets from 90 to 115 grains.

    6) Barnes TSX bullets do seem to decrease pressure. The “gas checks” in the bullet are thought to reduce bearing surface and thus friction. Even with a dirty suppressed gun the Barnes TSX brass was not marked.





    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.

    Weaknesses of this test:


    1) we used only “reading” of brass, and various methods of calculation for pressure testing. Further pressure testing should be done by manufacturers with access to the most sensitive equipment.

    2) we would like to have tested a piston upper to see if this reduced carbon fouling, or if it extended the time it took for carbon fouling to cause higher pressure, when the gun was suppressed.

    3) we did not test any 130 grain bullets. Some of you may say that the 1:10 twist is necessary if 120 or 130 grain bullets become popular in the 6.8 SPC. I disagree and I have personally seen 130 grain bullets stabilize satisfactorily in the 1:11” twist, out to 100 yards, and I suspect they will in the 1:12”. More testing needs to be done here.

    4) we did not test for accuracy

    5) We did not test any SAAMI chambered uppers with 1 in 11 or 1 in 12” twist barrel

    6) We used only one brand of suppressor in the suppressor test

    Thanks for reading this test and review. Please feel free to make comments or suggestions. It is my hope that we will perform some testing along these lines at least once a year. I would like to also thank my new friends Art Kalwas and Harrison (constructor) for their extensive input into the test.

    Last, but not least, here’s a picture of one of the hogs we killed. Around 250 lbs. It was killed with a single Barnes TSX combat SSA factory load, among my all-time favorite loads for hogs. As usual, it was DRT. We smoked a little 30 pounder the first afternoon. Wow, was that good. We also lost one hog shot with the 90 grain Speer TNT reload. Though the velocity is around 2900FPS, we conclude that it is too lightly constructed to kill these tough beasts. I recommend the following three in this order, for hogs**.

    1) Barnes TSX

    2 ) Sierra Pro Hunter

    3) SMK

    **When hunting deer, I usually prefer the Sierra Pro Hunter.

    Last edited by paulosantos; 05-28-2009 at 12:15 PM.

  2. #2
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    Default Re: 2008 6.8 SPC AMMO AND BARREL TWIST RATE TEST (plus Hog hunt

    Nice report Chris, it's good to see all of the variations compared at the same time using the exact same ammo lot at the same temperature so no one can say it was due to conditions or other variables.
    Did you find that 18 hogpack?
    Have a few things you may want to try. 30gr H335 and those 130 Sierras loaded to 2.3"-2521fps from the 18" and a 5/8" group today.
    I shot the revised 12" pistol with the 12 Twist and pistol length gas system today. I still need to adjust the loads but using the same 130s 8.5gr #9 hit 1040fps and cycled the action, 5.3gr "clays" hit 840fps and ejected the cases. by the time I get both up to 1150 they will cycle no problem.
    I am going to try 7.2gr of clays next, it's a flake and fills the case better and it's faster so it should be quieter than the ss loads you were shooting when I was there.

    I checked my notes when I got home from the hunt, using the TNTs 34gr of 335 gave me 3028fps and 30.5gr of RE7 hit 3066.

    The Barnes should work well with H335 maybe around 31.5-32gr.
    later
    BTW- everyone that is worried about stabilization, the 130 sierra is the same length as the 110 Barnes and will both shoot from a 12" twist.
    The 120 Barnes solid is as long as most 150's and will shoot 1/2" groups from the 11 twist I have.
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  3. #3
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    Default Re: 2008 6.8 SPC AMMO AND BARREL TWIST RATE TEST (plus Hog hunt

    H,

    Thanks for the info. I just need some time to load up the 130's and I will try the 335.

    What's a 18 hogpack?? :lol:

  4. #4
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    Default Re: 2008 6.8 SPC AMMO AND BARREL TWIST RATE TEST (plus Hog hunt

    If you guys encountered a pack of 18 pigs and didnt lay waste to them, I am going to be unhappy!!!

    If there is one thing which gets my heart going its coming upon a pack of hogs in a wallow... :twisted: Nothing like the follow up of an AR for such situations.

    Good read, thanks HTR for all the work!!
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  5. #5

    Default Re: 2008 6.8 SPC AMMO AND BARREL TWIST RATE TEST (plus Hog hunt

    Paulo,

    No pictures are showing up. I will see if I can get it working.

    Tim

    HTR,

    As I said over on ARFCOM very good and thru report. Well put together.

    Tim

  6. #6
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    Default Re: 2008 6.8 SPC AMMO AND BARREL TWIST RATE TEST (plus Hog hunt

    VERY WELL DONE!!!

    Thanks to everyone involved for your efforts!!!

    BTW... When is the 68Forums pig hunt going to be held? :wink: :wink: :mrgreen:
    Dan

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  7. #7

    Default Re: 2008 6.8 SPC AMMO AND BARREL TWIST RATE TEST (plus Hog hunt

    Paulo,

    Ok I have added all the pictures. Now I know why you didn't fix the pictures. There weren't any to fix. I pulled them from the arf thread.

    HRT,

    I duplicated the formatting and colors you used on arf. Hope you didn't mind me adding the pics and formatting. You spent some time posting that over there with the pics and formatting I figured you didn't feel like doing it twice. :wink:

    Tim

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    Default Re: 2008 6.8 SPC AMMO AND BARREL TWIST RATE TEST (plus Hog hunt

    Good work Tim.

    Chris, great work. I bet the piston AR's would work much better with the max/combat loads. The piston AR's have either a self-regulating piston or have the adjustable piston which allows you to adjust for a suppressor. That would dump less carbon into the upper.
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  9. #9
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    Default Re: 2008 6.8 SPC AMMO AND BARREL TWIST RATE TEST (plus Hog hunt

    Quote Originally Posted by Cold
    If you guys encountered a pack of 18 pigs and didnt lay waste to them, I am going to be unhappy!!!

    If there is one thing which gets my heart going its coming upon a pack of hogs in a wallow... :twisted: Nothing like the follow up of an AR for such situations.

    Good read, thanks HTR for all the work!!
    Actually there were 18 in one herd and 6 in another heading straight for Chris and they were 300 yds or more from me. I thought I'd let Chris pop a few and then they would come back the same way and by then I would be right in their path....combat pig hunting :lol:


    Chris,
    I think you have a few guys that want to book a pig hunt, why don't you give them some details.
    SIX5 -6.5mm on a 6.8 case
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    http://www.msrhunt.com/

    The 6.8 is the #1 choice for hunting deer and hogs with an AR15

  10. #10

    Default Re: 2008 6.8 SPC AMMO AND BARREL TWIST RATE TEST (plus Hog hunt

    Quote Originally Posted by paulosantos
    Good work Tim.

    Chris, great work. I bet the piston AR's would work much better with the max/combat loads. The piston AR's have either a self-regulating piston or have the adjustable piston which allows you to adjust for a suppressor. That would dump less carbon into the upper.
    They still get dirty as hell. It's just a lot easier to clean up. It's like a fine soot when you shoot LWRC's suppressed. When you shoot them supressed smoke oozes out every damn hole and crack in the gun. Like I said though it just takes a wipe down to clean it off
    OCD Sucks


 

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