 hi-tech-rancher
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Joined: Jan 2007  TX, USA
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Posted: 3/17/2008 10:37:21 PM
[Last Edit: 9/5/2008 5:25:26 PM by hi-tech-rancher]
**UPDATE: As of 9/5/2008, The new report is up. We tried to address some of the shortcomings of this first test, adding pressure trace data, and more new handloads with very striking velocities. 2 Years Ago, when I joined this board, I said that I thought 2800 FPS from a 16" barrel was possible. Some people replied "let the guy dream." I think we all just woke up today..... The GS custom, 80 grain all copper bullet, screamed out of my Cardinal Armory 16" barrel at 3250 FPS!! It also reached 3412 FPS from the "upgraded spec barrel": a 12 twist, 3 groove I told you all about (which Constructor and I custom designed a year ago). The other, more mundane bullets, like the SPH, hit the 2780 mark from the Denny's 16" barrel. That is 20 FPS from the HOLY GRAIL, folks. Now, let's see if the MAJORS will give us the new, improved 6.8 SPC. It is truly a thing to behold. Enjoy!! 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: The very first tests of this load came from DocGKR, Chris Murray and Steve Holland. It is believed that they actually specified a chamber longer than the SAAMI specs which Remington released to the public. They used 1 turn in 10" twist blanks, because this was all that was available, but they felt that this probably would be too fast. As DocGKR has added to this post, we see now that they were able to generate 2700-2750 FPS out of the original AMU guns, so these velocities are certainly possible. You should also know that these were Mike Rock 5R blanks, which are superior barrels and finished in such a way that the bore is much smoother when delivered, than many other factory barrels (I own one). Even Randall Rausch did velocity testing with these same barrels, with a SAAMI chamber, and got velocities in the 2700 FPS range, so it shows you that there are a lot of factors that will determine pressure. However, I have never seen his results duplicated, because IIRC, that was with Remington 115 grain OTM's loaded with their proprietary powder, which only exhibited high pressure when exposed to temperatures that might be seen in a desert (*take from that what you will*), or perhaps South Texas,  Manufacturers of 6.8 SPC guns and ammo have since been confused about the barrel specs because it is rumored that Remington had problems with the initial loads released for consideration of military adoption. They used a SAAMI drawing and barrel blueprint with what is now considered to be a 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. 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 bullet.  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, 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. Also, note that the "significant" ejector marks actually scored the brass enough to distort the lettering on the case. "Insignificant marks" made just a little smudge or surface mark, but did not distort the lettering. 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: For reference, here is a photo of the most badly damaged case next to an unfired cartridge. Despite deep ejector marks, there appeared to be no significant swelling of the case rim. I have not yet measured the case wall yet, but will post that later as an edit. (note: the two cases were not perfectly vertical when I took the photo, so there maybe a little distortion caused by the perspective.)  - Speer TNT, 90 grain reload with SSA brass and 31.5 grains of H322 (compressed) Remington 6.5 BR primer.   Avg. velocity in 16", SPC II, 1:11”, 4 groove barrel – 2770 FPS, no sig. marks unsuppressed. (left photo) Deep ejector marks & faint 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, 1:11, 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 16", 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 16", 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. (first two photos from left)Avg. velocity in SPC II 1:10 barrel - 2701 FPS, deep ejector swipes and primer cupping seen (far right 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, except for the Barnes TSX combat load. 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. Weaknesses of this test: ETA: ** I should have added that a better way to isolate twist rate as an independent variable in this test would be to have the following barrels, all made by the same MFG, sequentially, on the same equipment, with the same tooling on the same day, to test in a side by side comparison (practically speaking, this of course, would have been very difficult): -a SPC II, 1:10", 4 groove and 6 groove -a SPC II, 1:11", 4 groove and 6 groove -a SPC II, 1:12", 4 groove and 6 groove Furthermore, I would like to challenge any barrel maker to send us these uppers (or do this exact test themselves) to test so that we can, once and for all, determine if the twist rate or number of grooves is / are an independent variable responsible for increasing pressure. ** 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.  Here are links to those companies we mentioned, with their permission: www.ko-tonics.com www.ssarmory.com/ www.jetsuppressors.com/ |
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Owenslee
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Posted: 3/17/2008 11:16:58 PM
Well Done...!!
More evidence that the 1:11 twist barrel is the fastest twist to be considered by any 6.8 SPC barrel manufacturer. Hopefully all manufacturers will get the message and buyers will no longer buy from or support those who only offer 1:10 and faster twists. Kerry |
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Cold
AR Variants Mod and Juris Doctor (in training)!
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Posted: 3/17/2008 11:20:10 PM
[Last Edit: 3/17/2008 11:48:26 PM by Cold]
God I miss Texas.
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Molon
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Posted: 3/17/2008 11:49:22 PM
Nice work!
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Punkkin
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Posted: 3/17/2008 11:50:47 PM
Cool, thanks.
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Tim_W
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Posted: 3/18/2008 12:26:41 AM
Very well done there HTR. Was worth the weight and good job with the pics of the cases. Very well written.
To add the most recent testing has shown the 130s tested did properly stabilize in the 1:12 barrels. I can still remember the taste of the hog from this past X-mas that we cooked on the pit when I was done there. damn good eating  Great Job Tim |
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moose-hunter 
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Posted: 3/18/2008 1:40:07 AM
Your efforts are to be commended!!! Thank you so much!!! My only regret is that there was no 1-11 SAAMI chambered gun used. My DPMS 1-11 SAAMI 20" upper is on it's way to Harrison as we speak... Feel free to put a few rounds through it if it would help with your testing. Don't worry if it blows up. Harrison can just build me a new one...
Again... GOOD JOB!!! And thank you!! |
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RedFalconBill
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Posted: 3/18/2008 1:57:08 AM
Well then, DRT and TSX seem to go hand in hand.
Mmmm, Bacon, Ribs, AND Roast..... File this info for future use. Oh, thanks for the report!! |
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inkaybee
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Posted: 3/18/2008 9:15:18 AM
Nice. Thanks
Could you add a picture of an unfired case as well? |
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hi-tech-rancher
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Posted: 3/18/2008 9:42:05 AM
[Last Edit: 3/18/2008 10:00:56 AM by hi-tech-rancher]
ink, Yes, I will. I have a picture of two cases side by side, to show that there was no significant swelling of the base of the case near the rim, in any of the rounds fired from any gun. The photos were the hardest part of the write up, and in the interest of fulfilling my promise to have the post up last night, I excluded them. ETA: another photo added this morning....weary...must........sleep  |
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RRA_223
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Posted: 3/18/2008 9:49:17 AM
tagaroonie - I am shopping for a 6.8 barrel myself, this (and your other posts) have been exceptionally helpful and influential.
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cav_scout_tj
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Posted: 3/18/2008 10:02:06 AM
I think a lot of people have been misguided by 5.56 and the obsession with 1:7 twist. Some how this has led to the idea that faster is better. Which is totally wrong. We forget that 5.56 had its best performance at 1:14, but that was too slow for Alaska, so we got 1:12 (which some have said was 40% less effective). Later we got longer bullets and the 1:7 was born. I have a 1:9 bushmaster which works great.
I am a little curious about a couple things: Given what we know, wouldn't it logical to use the slowest twist for 6.8? The absolute minimum, is that 1:12 or 1:13? Why is twist seemingly more important to 6.8? In 5.56 I have shot from 1:12 to 1:7 with no ill effects. Same in .308 from 1:12 to 1:10. Why does this larger twist range not matter where in 5.56 or 7.62, but 1:10 vs 1:11 matters so much? I dont get it. Is it just newness? Are the others standardized to the point where the end user can just buy one and not really know or care about twist? In 5 years will that be the case with 6.8? |
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hi-tech-rancher
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Posted: 3/18/2008 10:19:15 AM
Cav scout, Damn good questions. These stretch the limit of my knowledge of ballistics and barrel manufacture, so all I can tell you is that it must have something to do with the: -length of 6.8 SPC bullets. Generally short, and stubby for this caliber. - BC of current bullets used. Most have relatively poor BC's, i.e. < .400 - very limited testing of the caliber and guns prior to release, led to a LOT of misconceptions about what twists were / are appropriate. the slowest appropriate twist is likey to be 1:13". However, even this would probably be OK, as long as you used the 90-115 grain bullets. As we all know, bullets that keyhole badly in 5.56, 1:12" barrels DO limit the performance of the platform, but for different reasons. Once everyone decided that 55 grain bullets were too light for 2 legged varmints, the heavier bullets were introduced as an "upgrade" to the performance. Once this occurred, and it was found that heavier bullets didn't stabilize in the 1:12" barrels, everyone started moving away from them. I can only hope that manufacturers will run from 1:10 twist like good conservatives running from Hillary and Osama Hussein.  |
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TurboFC3S
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Posted: 3/18/2008 4:15:59 PM
Excellent post ... the 6.8 n00b's like myself VERY MUCH appreciate the time and effort that went into not only the test, but posting them here. Especially as a 6.8 reloader I thank you!
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Ko-tonics
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Posted: 3/18/2008 8:04:08 PM
[Last Edit: 3/18/2008 8:10:14 PM by Ko-tonics]
h-t-r,
Well done, sir. I am green with envy, and wish I had been there. But I am flooded with a constant flow of orders, and we are putting a new Web site up, and I have at least two new products I am working on. I really wish I could have met you and constructor face-to-face, having already met kalwasart. As an aside, where would 6.8 be without the WWW? I get almost every combat-rifle-related periodical in the gun business, and it is AMAZING how the writers for the gun magazines absolutely have their head in the sand when it comes to 6.8. I saw an article just this week telling people that, contrary to some reports, you really don't need a 6.8 mag, just use your 5.56 mag. That sentence alone tells you that the writer HAS NEVER TRIED IT. Instead he took some second-hand hoo-hah from some yokel and writes it as fact. (for the uninitiated, it just plain don't work.) But I did want to throw something in here for your consideration. Bullets are stabilized when they are spinning above a certain number of revolutions per minute. When you are looking at rifling twist rate and bullet stability, the factor which finally determines the rate of spin is not just the barrel's twist rate, but the twist rate combined with the velocity. Let me repeat that in a different way: Take a certain barrel with a twist rate of 1:X. You work up a load that gives you a velocity of V. And for that bullet, the rate of spin (twist rate combined with velocity) stabilizes the bullet. Now incrementally REDUCE THE LOAD, and reduce the velocity. You will eventually come to the place where the barrel of twist rate 1:X will no longer stabilize that same bullet. This may seem obvious to some. I write it here because a LOT of things affect velocity, which in-turn will affect stability. For instance, by changing powders for a given marginal/unstable bullet, you may be able to bump velocity up to the point where it becomes stable. I bring this up because 6.8 is a remarkable cartridge, in that it achieves so much velocity so quickly, and remains lethal even in short barrels. My sales reflect this: 16" barrels by far and away outsell 18" and 20" barrels. And it looks like changing the powder can remarkably tailor the velocity for a specific bullet and barrel-length. H322 burns a LOT different from H4198, and you can play a lot of valuable games with different bullet weights, powders and fairly short barrel lengths. paulosantos deserves a LOT of credit for the great work he has done documenting a lot of loads, and Tim_W for consolidating so much loading information into one place. h-t-r, constructor and kalwasart, have added a lot of data points here. As well as all the people who contribute on an on-going basis to the 6.8 reloading thread. I think the 6.8mm lesson is, DON'T just assume something is going to work or not work a certain way, unless you have factored a lot of elements into the mix. There is a LOT going on in a chamber and barrel when the firing pin hits the primer. It will never come down to one or two factors which will determine success. Every small feature which decreases pressure and increases velocity can have a small, incremental effect that, together with other factors, can change "okay" performance into great performance. For instance, Tim_W, I am pretty sure that one of my barrels cut to 12.5" will still be able to stabilize 130 grain bullets. I also think in the future a VERY SHORT BARREL with a 1:13" or 1:14" twist could be developed that fires <90 grain bullets at good velocities, very good terminal performance, and low recoil. Such an optimized design would make a terrific PDW. (NO, THAT'S NOT A NEW PRODUCT TEASE, I'VE GOT TOO MANY IRONS IN THE FIRE ALREADY. )Just some thoughts. - Tim |
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hi-tech-rancher
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Posted: 3/18/2008 9:12:05 PM
Tim,
Thanks for posting here with your input. I hope to hear from you often! This write up was certainly designed to stimulate discussion among experienced users as well as to provide info to those considering a 6.8 gun. If we can spread enough good information around, then I believe that the other major manufacturers (who obviously have already picked up on the idea) will start making 6.8 barrels that optimize the cartridge's performance. I would like to believe that this work is an evolution and that we are completing research and development that was not finished when the cartridge was released. You, of course, are WAY ahead of us |
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DocGKR
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Posted: 3/19/2008 1:15:53 AM
Thank you, great job!
A couple of things to note: --Remington advertised 2800 fps for the 115 gr loads out of a 24" test barrel, never out of a 16" barrel weapon. --The original AMU loaded 115 gr OTM ammo we began testing in 2002 was running up to 2700-2750 fps out of our 16" test weapons without pressure problems; however, these were Mike Rock 5R 1/10 barrels using the original "Murray" chamber, not a SAAMI chamber. --As stated, the progenitors of 6.8 mm, Steve Holland and Cris Murray, have always felt the 1/10 was too fast and only used it because no other barrel blanks were readily available in early 2002 when the project was first started--1/11 is much better and 1/12 is probably optimal. |
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hi-tech-rancher
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Posted: 3/19/2008 1:53:26 AM
Doc, Thank you for posting here.  I greatly appreciate your input, and I would welcome any other information that you think should be added. This was/is certainly a collaborative effort.From someone of your expertise, I am humbled to know that you liked the write up. |
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9divdoc
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Posted: 3/19/2008 9:46:46 PM
Thanks very much for your work on this...greatly appreciated.
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Warthog86
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Posted: 3/20/2008 10:47:29 PM
Wow! I've wanted to see what folks were refering to when describing ejector swipe marks. I certainly learned quite a bit from your excellent post.
I can honestly say that I've not experienced the deep swipes while shooting some compressed loads of H322/enhanced frag 115gr. SMK's out of a SAAMI chambered 1-11" WOA barrel. The most I've seen were flat primers and the slight enlargement of the primer pockets (lg. prmr SSA brass). Of course, after continued use of the same brass with normal loads the primer pockets loosened up alot and had to be discarded. I have also shot some 115 gr. SMK combat loads through this barrel with good results (slight flattening of primers, excellent accuracy). Thank you, hi-tech-rancher & guests for sharing this wealth of information with us. Jeff |
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hi-tech-rancher
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Posted: 3/21/2008 10:29:58 AM
You're welcome, Jeff. And thank you for your input here. As I mentioned, I have seen high pressure loads shot safely out of the SAAMI chambered WOA barrels. They are very high quality, IMO. I hope that we can add one of those barrels in our next test, and if IIRC, Denny already offered the use of his previously tested upper (new ARguy's writeup) in our next one (whcih I greatly appreciate). The SSA combat loads are safe to shoot in almost any barrel, as long as it isn't a 1 in 9.5", or 1 in 10" SAAMI chambered barrel, and Art will tell you this if you call to order the loads. Seeing moderate ejector swipes isn't necessarily dangerous in itself, but it tells you that 1 in 10" twist barrels are right on the ragged edge of pressure and I hope that this twist rate will eventually be phased out by all MFR's, altogether. Maybe next time, we can test the following: 1 in 10" SPCII barrel 1 n 11" SAAMI chambered WOA barrel Ko-Tonics 12.5" (SBR), 1 in 11" , 4 groove SPCII chambered with PWS piston, both suppressed and unsuppressed. Since nobody currently makes a mid length piston system (PWS's is on the production line right now, IIRC) we'll have to wait until those hit the market. |
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081420popo
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Posted: 3/22/2008 8:56:55 AM
Thank you for your work. I also am especially grateful for the swipe mark photos. I have been working up loads and appreciate your data.
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LWRC_Jesse
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Posted: 3/26/2008 12:34:56 AM
If you can compare two identical barrels (1-10 twist with SPC II chambers) except chrome line one and not the other. You will be amazed at what you find in regards to pressures. Anytime we have had pressure issues, it had involved chrome-lined barrels. I've yet to see an issue with stainless 6.8 barrels.
Hi-tech IM sent |
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JTJ
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Posted: 3/27/2008 3:24:45 PM
Would a progressive twist barrel be better than a 1:11 twist barrel for reducing pressures?
JT |
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Gamma762
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Posted: 3/27/2008 3:34:24 PM
Progressive or gain twist doesn't work well in small arms. The bearing surface length to diameter ratio is very high in comparison to large caliber arms which use driving bands to engage the rifling. The more limited bearing surface length of the driving bands is what makes gain twist feasible. |
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hi-tech-rancher
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Posted: 3/27/2008 9:25:05 PM
I would agree. I think we can keep this very simple in the 6.8 SPC. When you reduce the energy required to spin the bullet, you then can translate that energy into velocity. Friction caused by additional rifling grooves (6 grooves are unnecessary) also adds to inertia, and thus increases pressure. Simply reducing the grooves to 4 or even 3, reduces bearing, friction and inertia, and thus pressure, while still allowing for adequate stabilization. We needn't complicate matters any further....I still firmly believe that the 1 in 11" and 1 in 12" twists are optimal for the 6.8 SPC. When combined with the SPC II or DMR chamber, we have the optimum environment for lower pressure and we can then push the powder charges and maximize velocity. It is just that simple. |
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