User Panel
Posted: 10/14/2004 10:34:44 PM EDT
I would like to see a 6.8 case necked down to a 6.5 grendel spec bullet and the case lengthened slightly since the bullet is smaller. The 6.5 bullet would then have to be redesigned to fragment like the 6.8. You would get the superior ballistic coefficents of the 6.5 design in a package as efficient as the 6.8 case. A 6.5 bullet designed to tumble and fragment would be equally as devastating. The bullet would have to be lighter than the 6.8 to get similar muzzle velocities, but the design of the 6.5 bullet generally always offers higher ballistic coeficients. Just a curiosity to me. Throw it around guys Its just that to me the concept of the 6.8 is great...Improved performance out of the AR platform, but its seems the bullet diameter and shape (not design) should have been done a little different. |
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The guys behind the SPC tried a 6.5, but decided against it. I don't know why.
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I've posted the reason why at least two dozen times... Many folks who were on the SPC development team *assumed* that the 6.5mm bullet was going to be the winner. Everyone certainly liked the accuracy and trajectory that the 6.5's are known for, and there was actually a bias towards 6.5mm when it started. But the 6.5 bullets simply did not meet the performance requirements. Specifically, the 6.5 bullets penetrate way too far before they begin to yaw and fragment. Depending on the bullet, the "neck" of the wound will be anywhere between 4-8". That's WAY outside of the desired profile. The testing showed that the 6.8mm was very good, and that the 7mm was a bit better in terminal ballistics, but that the 7mm's trajectory was somewhat poor, it was less accurate, and had a shorter fragmentation radius due to the lower initial velocity compared to the 6.8mm. Thus, 6.8mm turned out to be the "sweet spot". Had the AR been designed around a longer cartridge (say, halfway between 5.56 and .308), the 7mm would have occupied the sweet spot (good enough infact to REALLY replace both 5.56 and .308), but cases small enough to fit into a 5.56 weapon simply don't have enough case capacity to make the 7mm work well enough. The 6.5 cartridges, such as the Grendel, are still fantastic match rounds, and will work well for hunting applications that require more penetration than the "combat vs humans" profile, but they simply don't work well as a combat bullet. And while I won't say that making a 6.5mm bullet that gives the required profile isn't possible, I *can* say with a fair amount of confidence that it isn't practical. The already thin bullet jackets would have to be made even thinner, yet maintain a higher degree of consistancy, and still be tough enough not to be shredded by the rifling. I think most people understand that manufacturing costs rise *significantly* as the required level of precision is increased, and that *assumes* that thinner jackets would still be tough enough, which may not be the case. Certainly, you could design or use a bullet that is *designed* to fragment, such as a V-Max, but such a bullet wouldn't be JAG-legal, and would then not have a military application. The 6.8mm SPC has been shown to be 1 MOA accurate at 600m from a 12" barrel at the US Army's fully-instrumented Parks shooting range, so it has plenty of accuracy to do the job. Certainly no one can honestly suggest that an AR should be relied upon to make "stops" on point targets beyond 600m, as NO bullet available for the AR platform has enough energy left at 600m to incapacitate reliably with a torso shot. That's really .308 or larger territory in the first place. The great thing about 6.8 SPC is that it does all of the jobs that the AR platform is realistically required to perform, from CQB with very short weapons to "Designated Marksman" roles out to 600m, and it does them all very well. No other cartridge that currently exists does THOSE things better. -Troy |
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Shame that the 280 British never was adopted, isn't it? 1.75" long case, 7m bullet, 2.5" overall length .... |
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Marty: I agree. We'd be 50 years ahead of the game. Ballistics are similar:
280" British (7x43mm, 7x44mm, 7mm EM2) 139gr, 2530fps, 1,986 fpe. 6.8 SPC 115gr, 2800 fps, 2002 fpe. 7mm-08 140gr, 2860 fps, 2542 fpe. The EM2 rifle had a 24" barrel, so the 6.8 SPC numbers are from Remington's 24" test barrel (factory magic dust applied). Imagine an FN-FAL or AR-10 in a light-load 7mm-08 or 260 Remington? Machineguns and marksmen could use a full-power round. I think the whole process is quite interesting: the development of the cartridges and guns vary all over the place, especially with new technologies, then settle down as they seek a golden mean. .30-06 too big, .30 Carbine too small, 7.62x51 too big, 5.56 too small, 6.8 just right? 7.62x54R too big, 7.62x25 SMG too small, 7.62x39 too big, 5.45x39 too small.... .38 too small, .45 ACP "too big", 9mm too small, 10mm too much, 40 S&W just right? This is just my opinion, but is the pattern visible? (Edit) I found some numbers on the .276 Pedersen which I can't verify until I reach Cartridges of the World: 125gr, 2700fps. We could have been here eighty years ago. |
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It's getting old, isn't it? Maybe you should set a macro function on your keyboard to automatically post your response |
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My copy of Sierra's Infinity program shows that a 75 grain HPBT launched at 2700 fps will reach the 600 yard mark with roughly (IIRC) 450 lbs-ft of energy. Very simmilar to the ME of a 45 ACP. I thought the 45 ACP was a good fight stopper. What am I missing? I will double check the 600 yd energy tonight when I get home. |
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Energy itself doesn't do crap to incapacitate. It's what the bullet actually does. At 600 yds a .223 bullet won't fragment/expand, and most likely won't even yaw. You'll have a .22 inch wound all the way through. A 45ACP makes a .45 inch wound. A .308 won't expand either (JAG legal rounds), but at least it makes a .30 inch hole, and will go deeper. In theory, a 223 has the same energy at 1000yds as a .22 LR has at the muzzle (around 150 ft/lbs). It can certainly kill you (ignoring wind drift/bullet drop), but it wouldn't be reliable enough for a combat situation. |
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Everyone who wants to understand how bullet wounding (and "stopping" of targets) actually occurs should read this article: www.firearmstactical.com/hwfe.htm and others on the FTI web-site.
blikbok, The AR-10 platform has too much reciprocating mass to match the speed possible with the AR15 platform. You can load 7-08 or 260R as light as you want, but that mass still has to move back and forth. -z |
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Agree
Probably right
I disagree. The bullet is very likely to tumble due to its mass distribution and lack of stability/velocity downrange. |
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In the Grendel, the higher BC is mostly because it can use longer bullets. You'll lose that advantage if you use the 6.8 SPC case. IMO, the other way around would be more interesting. Use a 6.5 Grendel case with the 6.8 magic bullet. That way, you'd be able to use high BC bullets long range, and still be able to use the magic bullet for CQB. |
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I thought that's what the 6.8 SPC gelatin tests were done with, a VMAX without the polymer tip? If that's not the case, then what bullet did they use? |
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115gr OTM, which *is* JAG legal. Gel tests have also been done with the 110Vmax as well, of course, along with the 115gr FMJ. |
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I agree. But my point was, the .276 Pedersen M1 Garand existed in 1930, and the 7x41mm FN-FAL was around in the 1950's. The "light load" comment was to compare the power levels of the early intermediate cartridges to current rounds. I would assume that in the succeeding years, they could have trimmed the mass of the weapon down. Loading improvements would cut the size of the round down. The M-14-style successor to the .276 Garand could have been closer to the Mini-14 in size. Heck, the Russians had a select-fire rifle firing a 139gr bullet at 2500 fps (6.5x50SR Arisaka) in 1916. It even had a forward pistol grip :) Link to external pic, 27K |
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DINGDINGDING!!! WE HAVE A WINNER!!! |
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Correct
Once again, you'd have case capacity problems, as heavier bullets need slower powder, and you wouldn't have enough for an optimal load, so your velocity would be low, negating any BC advantage. Plus, you'd have further lowered mag capacity, and the over-all length of the round would be too short with the standard 6.8 bullets. This is yet another example of how all the components of a gun and its ammo work together as a SYSTEM. Many times, it isn't possible or practical to change a component of that system without making significant changes to other components in order to keep things working. -Troy |
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It actually will yaw, but only after a LOT of penetration, so that isn't going to do a whole lot of good. It certainly will be very poor in comparison to a 45 ACP, which uses bullets that are designed to work at low velocity/energy levels. Pistol bullets and rifle bullets work very differently, and what is true for one is not going to be true of the other. This is one of MANY reasons why using energy figures to compare different bullets and ESPECIALLY different calibers a bad idea. The amount of energy a bullet has at a given distance only tells you how it is likely to work, given that bullet's design and construction, and is useful mostly for comparing the bullet's performance against itself over distance. It is a poor tool for comparing performance against dissimilar bullets. -Troy |
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once again, troy, i would like to thank you for crushing my hopes and dreams.
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It was originally done with the 110gr OTM, which was available off-the-shelf, as the development team had no money or support (initially) to develop a new bullet. It worked very well, but along the way, a couple of the big ammo companies lent their support to the project, and developed some 115gr bullets to further optimize the cartridge, which is how we got the 115gr OTM and FMJ bullets. The excellent (but not JAG-legal) 110gr VMax is an off-the-shelf bullet, of course. -Troy |
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I recall that the size of the 6.8 SPC case was determined to be optimum as well as the bullet diameter-- that a .422" case fit and fed better than the .445" case.
The 6.8 SPC seems to have been developed with a hard eye towards the end requirements, and every variable including case design, bullet selection and diameter have been chosen too meet those ends. |
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For years I'd thought that when the US went from the 7.62X51mm to the 5.56X45 that they had gone too far. Not a unique observation, certainly, but one that seems accepted in ever widening circles today.
I've always thought that a perfect case for an experimental cartridge that falls about exactly between the 7.62 and the 5.56 would be the old .250 Savage cartridge. Granted, it can't be fitted into an existing M16 magazine and is really too short to be considered in any of the 7.62mm rifles either. But the cartridge case at 48mm is right between the 7.62X51 and the 5.56X45 and is versatile enough to lend itself to bore diameters from it's designed .25 cal. up to 7mm. If the .250 case were to be changed slightly to the version of the Ackley improved with the 30 degree shoulder angle with the straigtened case walls, the cartridge would closely resemble a slightly smaller 7.62X51mm (or slightly larger 5.56X45mmm). The case capacity would be proportionate to the smaller bullet diameter in the 6.5mm - 7mm as well. As is, the .250-3000 Savage cartridge is one of the most efficient cartridges that I have ever loaded for and it has taken it's place as my favorite deer rifle due to it's power and low recoil. I use a handload that I found in the 47th edition of the Lyman Reloading Manual that pushes a Sierra 100 gr. BTSP at a legitimite 3100 feet per second out of my 1984 Remington 700 Classic with a 24" barrel (chronographed @ 3109fps/5 shot average). With the case walls straightened and the case necked up to a 6.5mm-7mm, the cartridge could become a formidible military cartridge using bullets in the 115-120 grain range. One of these days I'll get the bug to try a wildcat like this cartridge as I've described. I know that some experimentation has already been done with this case as I've described it in 6.5mm, but I've only found limited information written about it. And I don't believe that any of the experimentation utilized modern powders capable of a full military load. Ah well...so many ideas, so little time. |
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Really, they did go too far. But politics, budget issues, and lack of any real scientific understanding of terminal ballistics, the limited testing that was done showed that the round (M193) did pretty well (from a 20" M16). That combo would still work as well today, but that's NOT the rifle the military wants to carry today. They want ever-shorter barrels, and short barrels kill the 5.56's performance. It's just taken FAR to long for the military to admit and acknowledge that fact, and they still have not fully done so, in order to prevent a loss of confidence by the troops who are still issued those weapons. -Troy |
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Yepper there:
One of the comments made early on to me in the 6,8mm program by one of it's designers was that: "If we adopt this cartridge, the British should come over here and kick our A$$e$!" to which I agreed with him about. BTW last time I checked, Parks range was only 300 yards or meters (Can't quite remember), has it changed? I have not been down there in 2 years. It has a great instumentation system computerized that gives you a printout of your shots in order of impact, velocity, position from center and the extreme spread and vert/horizontal spread. Quite possibly the best external ballistics set-up I have seen yet (Now, mind you I have not been to Picatinney or RIA yet). The 6.5mm has never been a good performer on humans from what data I have seen and collected. Most countries that fought with them opted for larger caliber cartridges (Japanese, Greeks, Italians). The Japanese even modified their later 6,5 bullets to tumble and fragment, but I would guess they had the same problems i.e. deep yaw cycle etc. that are had nowadays. I was not a big fan of the 6,5 from the beginning from the historical aspects that were available. But you might as well test it to see if things had changed. And the first ballistics tests showed that the 6,5 while having a higher velocity and great accuracy just came up short in the terminal department. I think the cartridges both do what the are designed to do well to me it is hard to compare them because they were both designed for different purposes. The 6,5mm bullet has its good and it's bad. So does the 6,8mm. We shall see what the future has in store for both. |
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Sorry, man, but that was REALITY doing all the crushing. I'm just the messenger. -Troy |
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If you take the true average of the 223 and 308 cartridges: Base diameter: avg of .378 and .473 = 0.4255 (remarkably close to the SPC) Case length: avg. of 1.7600 and 2.0150 = 1.8875 Bullet diameter: avg. of .223 and .308 = 0.2655 Loaded length: avg. of 2.255 and 2.810 = 2.5325 Now, if you do some looking around, you find the 6.5-225 Winchester (or 6.5 JDJ) 100 gr Sierra HP 2714 fps from 14" 120 gr Speer 2467 fps from 14" 129 gr Hornady 2342 fps from 14" Base diameter 0.419 *rimmed case Case length 1.93 Bullet diameter .264 Loaded length - variable While it won't fit the AR-15, it sure seems similar to the 280 British, SPC and Grendel .... So, suppose I had built an AR that would accept the 280 British |
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The 6.5mm JDJ is indeed an interesting cartridge. There would appear to be some drawbacks that might prevent it's consideration as a military cartridge though. The case is rimmed for starters. I know that the base of the case could be re-designed as a semi-rimless, but that's another step in the process. Secondly, the case has a very short neck which limits the bullet selection. I realize that the .280 British had it's bullet seated in such fashion as to cause a sizable section of the bullet to pertrude from the case. Not completely bad, but certainly not ideal when considering an auto loading military weapon. Lastly, the advantage that the 6.5mm JDJ cartridge has over a hybrid of the .250 Savage case in a 6.5mm/30 degree shoulder angle is in the powder capacity of the case. The (inside) length of the case from the base to the beginning of the shoulder's angle in the JDJ is 1.630 (41.40mm) as opposed to 1.500 (38.10mm) in the improved Savage case. But due to the short neck on the JDJ case, heavier bullets would have to be seated well into the case, thus limiting the powder capacity of the cartridge. Combine this with the fact that the I.D. width of the improved Savage case is .470 (11.94mm) at the base, and .456 (11.58mm) at the shoulder. The JDJ measures .422 (10.72mm) and .409 (10.39mm) respectively. Also a consideration would be the JDJ's sharp shoulder angle of 40 degrees, as opposed to the Savage hybrid's 30 degrees. While that difference certainly favors the JDJ to the handloader in the way of longer case life/less case stretching, it is usually not considered a positive feature regarding feeding of the cartridge in an automatic rifle. I believe that these factors favor the hybrid Savage case over all. I'd love to hear the opinions of Troy and others on this. I'm not so much concerned with having my opinions questioned or disputed as I am eager to hear the well reasoned alternate points of view from others with significant knowledge on the subject. BTW MartytW...I didn't mean this as any kind of a flame of your post. I appreciate your opinion and enjoy the sharing of differing points of view. |
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I totally agree that the 6.5 JDJ is not the way to go, but it offers a starting point, in that it shares base (but not rim) diameter with the SPC, and gives viable performance. Several aspects, as you pointed out, make it less than ideal for a military cartridge. I offered it merely because it has published performance numbers. The 280 Brit would still seem better, however, currently the AR magwell will not properly accomodate a double stack mag for that case diameter, nor will the bolt withstand firing that size rim at elevated pressures. These are just points to consider when designing the next great cartridge .... Isn't this fun? Marty |
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Upon closer examination of my original premise, it would appear that an improved 6.5/250 case would be nothing more than a slightly attenuated 7.62 X 51 case. There's only a .06" difference in interior case length from base to shoulder and the width at the shoulder is only .002" greater on the improved case (ID). The case capacity would be significantly greater than the average between the 5.56 X 45 and the 7.62 X 51. The 6.5mm JDJ indeed would be closer to a midpoint in case volume and overall dimension between the 7.62 X 51 and the 5.56 X 45 then, although not a good parent cartridge for the agreed upon reasons. I suppose there are two arguements here. One revolves around a replacement cartridge for a weapon that would accept an M16 length magazine. The other would be about a new weapon designed from the ground up as as totally different replacement without regard for interoperability with the M16. It appears that the current US Military thinking is wedded to the 5.56mm M16 length magazine - and probably a magazine that is actually interchangable with the current M16 mag. This despite the expressed desire by the US Military to adopt a completely new weapon to replace the M16. If we're talking about a replacement cartridge for the 5.56mm that retains the OAL of the current 5.56mm cartridge, then it becomes a moot point. They've already found their replacement - the 6.8mm. And judging by the testing that's been done to date, it appears that the 6.8mm is indeed a worthy candidate. At least in this instance the experimentation process was conducted properly with the conclusion being reached after the data was compiled. That alone is such a refreshing change from the SOP that would have been used had the US Military been responsible for the trials. I'm still rather intrigued by the demonstrated superiority of the 6.8mm over the 6.5mm as a bore diameter. Conventional wisdom always pointed towards the 6.5mm as an "optimal" bore diameter of sorts. It may be that the 6.5mm still retains it's touted superiorty when used in hunting cartridges where expanding bullets are the norm...but the evidence seems to clearly show that the 6.8mm has the lethality edge when non-expanding, military acceptable bullets are used. I really shouldn't be so amazed by the 6.8mm's performance though. I've never been even slightly disappointed in the performance of my .270 Win as a deer cartridge. To the contrary, it's been effective to the point where I find it to be overkill for my average deer hunting ranges. Ok, so it'll be a 6.5mm/.250 Improved deer hunting cartridge then.... I guess now I can abandon all military considerations and just go with a 40 degree shoulder angle to boot...hmmmm, sound like a beltless Weatherby mag. in the works. Yes, this is fun... Keith |
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DANG.. thanks for all the responces guys. I had heard the reason the 6.8 won over 6.5 when the 6.8 craze was first starting, but forgot it since then. Thanks for all the feedback.
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Great discussion gentlemen. I just wish that the 6.8 program would take off a little faster, so that it would give the bullet makers time to get a suitable hunting bullet for folks like me to get into the mid-bore game. Preferably before next antelope season. I just can't get excited about the 6.5 when I think that the 6.8 might evolve into a hunting round. Surplus ammo for plinking or anti-personel, big game bullets for handloading. It has the potential, but right now it would still be betting on the come. Decisions, decisions. I guess I'll just keep using the old 458 Socom. For now......
Craig |
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The glacier-like pace of Remington getting ammo out to the civilian world HAS certainly been the biggest obsticle for the 6.8mm, but it looks like a couple of additional players will be entering the market shortly, and THAT is really going to be when 6.8mm will start to get interesting. There are bound to be some unexpected problems, and probably some interesting developments as well, as the cartridge sees mass production and has to work in guns of various manufacture and assembly, but in the end, we should really have a great round.
-Troy |
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Yes, the 6.8 won over the 6.5 for terminal effect when originally tested with the same SPC case. Unfortunately this comparison has been used to compare with the 6.5 Grendel which uses a different case and different bullet design. What happens when you put the 6.5 SPC bullet in a 6.5 Grendel Case????? |
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Am I correct in assuming that you meant the 6.8 SPC bullet? If so, that would be an interesting experiment. Since it's already known that the 6.5mm bullets that were tested in the 6.8 mm SPC case hadn't fared well in the lethality department. At least not as tested with OTM bullets for military purposes. Since the 6.8mm SPC case has already been tested with less than optimum results using 6.5mm bullets, it would be fair to compare a 6.8mm bullet in the 6.5mm Grendel case. Again, testing for lethality with OTM type bullets. I believe that the 6.8mm OTM bullet would still be more lethal, even in the Grendel case... despite the 6.5mm's better SD, trajectory, and retained energy. There is only a slight velocity advantage favoring the 6.5mm Grendel over the 6.8mm SPC. I'm not sure how that would translate to the 6.8mm bullet in the Grendel case. Assuming that the 6.8mm bullet was pushed a little faster in the Grendel case (and that's a speculative assumption), I would think that it would just be a little more of the same in regards to the 6.8mm bullets lethality. Predictions like that don't always hold true though. Predictions and assumptions are often the victims of unforseen variables. |
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Actually, I was referring to the shorter, lighter 6.5 bullet that was tested against the 6.8 using the SPC style case. The Grendel case should give the light 6.5 bullet a little more velocity as you stated.
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Oh, I see. You meant what you actually wrote...the 6.5 SPC bullet as used in the SPC case. And here I thought I'd already had enough coffee this morning.... |
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The bullets used in the 6.5 SPC testing could easily be loaded in the Grendel case. You would probably get a bit more velocity from them in the Grendel case, but not a significant amount, as case volume between the two is quite close. Regardless, the terminal ballistics wouldn't change; the 6.5mm bullets ALL seem to penetrate much too far before beginning to yaw. Again, there are known designs that would make that irrelevant due to their wounding mechanism (softpoints), but these aren't JAG-legal, so that's a moot point when it comes to military adoption.
-Troy |
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A very wise man recently recommended I write to Federal and Black Hills to suggest they start loading the 6.8SPC. I would like to suggest we ALL write letters or e-mails. |
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That's what I would have expected also. There's not enough of a difference in the two cartridge cases to compensate for the inherent over-penetration problems with the 6.5mm bullets - in all weights tested. It is interesting though that the 6.8mm has performed as well as it has. I suppose it shouldn't be such a shock that it has performed well, but to show clear superiority to all other bore diameters in it's class is a bit of a suprise. It's like the old .276 Pederson has been revived in another form, and perhaps proven the wisdom of an earlier time? |
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I have spoken to Jeff Hoffman (Black Hills) about this. It's not possible until brass is readily available. Otherwise, he tod me that he is interested in making 6.8. |
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Then let's tell Jeff to start pestering the brass makers. |
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