Twist rates, the Juenke and accuracy.
Posted: Mon Aug 24, 2009 1:00 am
Late last year i aquired a Vern Juenke bullet spinner out of the US. For all these years i have measured and batched bullets by bearing length, ogive profile etc and had some reasonable success at long range with it. I have never fired bullets in competition with the use of the Juenke spinner. So before it goes up for sale i wished to see for myself if there is merit in this devise. Well so far from a short range perspective for me there is, to a point. Recently i did a bit of work with a 22BR and 6BR and a well known commercial bullet brand and did convince myself that bullets that spun less than 5 units did shoot one hole groups and those that were over 10 units on the scale did group rather ordinary. In this case it was possible to batch commercially made bullets that would give true BR accuracy out to 200 yards. The down side was that there was a 75% cull rate where out of a box of 100, 25 were worthwhile and the rest were only good for the crows.
Now here comes the anomaly to this bullet spinning thing. For a number of years i have shot a 30BR with a well known brand of custom bullets. In my 18 twist barrel these bullets will wipeout the dot in a Hunter target with the right driver in good conditions. Groups below 0.250" are easily achieved at 100 yards with this combination. Then a little while ago i decided to spin these bullets on the Juenke, and guess what? They were not all that much better than the above mentioned commercial bullets. Now i checked the batches, and sure enough, they were the same ones that drill holes in the 18 twist 30 BR barrel. So they spin poorly, but still shoot well enough to net a 747.xx at the Innisfail 300 shot a few years back where they shot a 500.32 at 100 and 200 agg. So i know this combination shoots very very well, but the Juenke says they shouldn't. So yesterday i decided to shoot these bullets out of my Maddco 14 twist 3 groove 30 BR barrel. At 100 yards it shot a 249.6, which with the 6x scope class is a reasonably competitive score. Those bullets shot were no better than 10 units and averaged about 15 units. Some were as high as 25 but i did try and use these just for foulers. The one shot dropped was a very skinny 12 o'clock 9. The rest were skinny 10s and there was nothing convincing about this setup. I was surprised that 249 was the score as there were a lot of peripheral shots that just cut the ring. Shot again, i doubt i would have been so lucky.
At 200 yards, the 10 ring was very elusive and 9 points were dropped. No dots were hit, not even close and no single bullet hole was completely inside the ten ring. All 10s were line cutters which is not a good sign for Hunter. These same bullets shot out of the 18 twist just flatline across the target giving a wind weather report of the day. At 200 yards the one target was used and groups of five shots were fired for score on each roundel. All five of these groups were very poor with the same pattern at 100 being evident where the groups were very open and all had some shots that were well outside the 10 ring with one just cutting the 8 ring at 11 o'clock and heading for that seven ring. Lastly i shot a group of these same bullets that did spin 5 units or better and the rifle was completely different. That one group was half of the best of the Hunter targets score groups and more importantly was only a couple of calibers in height.
My conclusion with this is that the Juenke can determine a bullet both good and bad, but i am pretty convinced so far only in a short range application where bullets just spun on this machine will not necessarily be uniform for bearing length or ogive profile. Even using the pointer location as an indication of ogive position is deceiving as a full 15 units of scale is way less than half a thou of an inch of jacket thickness on a 30 cal bullet. Bullets that are less than 5 units in jacket concentricity, but may be 5 units apart on the scale, still shot through the same group when shot at short ranges. With a six mil or 22 that may be different.
As for long range, that is for another test i will be doing shortly for my last hoorah. From what i have seen so far bullets just spin tested will still give ordinary groups, mostly vertical, if shot at 1K with no other batching process. This is where i think the Juenke has become maligned as there have been many disillusioned with the machine as they just do not produce the desired result at 1K. But at shorter ranges, even to 500m they do look very impressive and deceptive at the same time. That much i already know for sure.
There is most definitely an accuracy advantage with twist rates as a bullet that measures poorly still measures poorly no matter what barrel it is shot in. But the difference of RPMs between a 14 twist and an 18 twist is enough to mask a lot of bullet CG offset. If i was to stay in this Hunter game, i think that the way i would head with a barrel for a 30 BR would be like the yanks did in the early days and head off to 19, 20 or even 21 twist back when the 0.925" jackets were being used. Nowadays the bullets are most likely on the 1.000" jackets like the BIBs and 19 twist may be the limit. Which ever way you look at it, bullets shot with this much jacket eccentricity in any other caliber or same, and with more regular twist rates exhibit similar groups at 200 yards and are not worth a cracker. The way slower twist rate takes a lot of the bullet CG offset moment out of the equation and gives a very different outcome at the target. This has long been the theory and in this example seems to be the reality.
There has long been the two factions in long range. Those that go with minimal twist to get the job done and those that use more twist than required in the others view with the object of getting a SF of 1.5 or better. With a good batch of bullets, i doubt that an inch or less in twist rate choice that is within the parameters would have any great bearing either way. For me, the Juenke has confirmed known good shooting bullets in many calibers, and maybe shown why some other batches have not been so good. There has long been the question asked by many of what the machine actually measures and also the question of if in fact it is worthwhile. Put very simply, the machine measures bullet jacket concentricity. For short range use, just the Juenke alone seems to be able to deliver the results. For long range, i know for sure it will give a bullet that will group well when tested at shorter ranges, but also know that this is only one half of the the total equation and groups at the longer ranges are average at best when no other qualification is used. As i said earlier, this machine goes up for sale shortly, but before then my curiosity wants me to try and determine its value at 1K where bullets spun alone are shot, bullets just qualified as i have done for years are shot, and then finally a combination of both is used. There is no doubt that there is a direct correlation between bullets that are qualified as junkers by measurement with my methods, but there are also good measuring bullets that show big numbers of units on the Juenke. So a perfect measuring bullet using mechanical means in not always a perfect bullet when measured electronically using the Juenke. My guess is that the use of both methods will produce the best outcome.
I hope this little chapter has given a little insight on a machine that is not written about all that often. And possibly a little insight on the role twist rates play in accuracy.
Tony Z.
Now here comes the anomaly to this bullet spinning thing. For a number of years i have shot a 30BR with a well known brand of custom bullets. In my 18 twist barrel these bullets will wipeout the dot in a Hunter target with the right driver in good conditions. Groups below 0.250" are easily achieved at 100 yards with this combination. Then a little while ago i decided to spin these bullets on the Juenke, and guess what? They were not all that much better than the above mentioned commercial bullets. Now i checked the batches, and sure enough, they were the same ones that drill holes in the 18 twist 30 BR barrel. So they spin poorly, but still shoot well enough to net a 747.xx at the Innisfail 300 shot a few years back where they shot a 500.32 at 100 and 200 agg. So i know this combination shoots very very well, but the Juenke says they shouldn't. So yesterday i decided to shoot these bullets out of my Maddco 14 twist 3 groove 30 BR barrel. At 100 yards it shot a 249.6, which with the 6x scope class is a reasonably competitive score. Those bullets shot were no better than 10 units and averaged about 15 units. Some were as high as 25 but i did try and use these just for foulers. The one shot dropped was a very skinny 12 o'clock 9. The rest were skinny 10s and there was nothing convincing about this setup. I was surprised that 249 was the score as there were a lot of peripheral shots that just cut the ring. Shot again, i doubt i would have been so lucky.
At 200 yards, the 10 ring was very elusive and 9 points were dropped. No dots were hit, not even close and no single bullet hole was completely inside the ten ring. All 10s were line cutters which is not a good sign for Hunter. These same bullets shot out of the 18 twist just flatline across the target giving a wind weather report of the day. At 200 yards the one target was used and groups of five shots were fired for score on each roundel. All five of these groups were very poor with the same pattern at 100 being evident where the groups were very open and all had some shots that were well outside the 10 ring with one just cutting the 8 ring at 11 o'clock and heading for that seven ring. Lastly i shot a group of these same bullets that did spin 5 units or better and the rifle was completely different. That one group was half of the best of the Hunter targets score groups and more importantly was only a couple of calibers in height.
My conclusion with this is that the Juenke can determine a bullet both good and bad, but i am pretty convinced so far only in a short range application where bullets just spun on this machine will not necessarily be uniform for bearing length or ogive profile. Even using the pointer location as an indication of ogive position is deceiving as a full 15 units of scale is way less than half a thou of an inch of jacket thickness on a 30 cal bullet. Bullets that are less than 5 units in jacket concentricity, but may be 5 units apart on the scale, still shot through the same group when shot at short ranges. With a six mil or 22 that may be different.
As for long range, that is for another test i will be doing shortly for my last hoorah. From what i have seen so far bullets just spin tested will still give ordinary groups, mostly vertical, if shot at 1K with no other batching process. This is where i think the Juenke has become maligned as there have been many disillusioned with the machine as they just do not produce the desired result at 1K. But at shorter ranges, even to 500m they do look very impressive and deceptive at the same time. That much i already know for sure.
There is most definitely an accuracy advantage with twist rates as a bullet that measures poorly still measures poorly no matter what barrel it is shot in. But the difference of RPMs between a 14 twist and an 18 twist is enough to mask a lot of bullet CG offset. If i was to stay in this Hunter game, i think that the way i would head with a barrel for a 30 BR would be like the yanks did in the early days and head off to 19, 20 or even 21 twist back when the 0.925" jackets were being used. Nowadays the bullets are most likely on the 1.000" jackets like the BIBs and 19 twist may be the limit. Which ever way you look at it, bullets shot with this much jacket eccentricity in any other caliber or same, and with more regular twist rates exhibit similar groups at 200 yards and are not worth a cracker. The way slower twist rate takes a lot of the bullet CG offset moment out of the equation and gives a very different outcome at the target. This has long been the theory and in this example seems to be the reality.
There has long been the two factions in long range. Those that go with minimal twist to get the job done and those that use more twist than required in the others view with the object of getting a SF of 1.5 or better. With a good batch of bullets, i doubt that an inch or less in twist rate choice that is within the parameters would have any great bearing either way. For me, the Juenke has confirmed known good shooting bullets in many calibers, and maybe shown why some other batches have not been so good. There has long been the question asked by many of what the machine actually measures and also the question of if in fact it is worthwhile. Put very simply, the machine measures bullet jacket concentricity. For short range use, just the Juenke alone seems to be able to deliver the results. For long range, i know for sure it will give a bullet that will group well when tested at shorter ranges, but also know that this is only one half of the the total equation and groups at the longer ranges are average at best when no other qualification is used. As i said earlier, this machine goes up for sale shortly, but before then my curiosity wants me to try and determine its value at 1K where bullets spun alone are shot, bullets just qualified as i have done for years are shot, and then finally a combination of both is used. There is no doubt that there is a direct correlation between bullets that are qualified as junkers by measurement with my methods, but there are also good measuring bullets that show big numbers of units on the Juenke. So a perfect measuring bullet using mechanical means in not always a perfect bullet when measured electronically using the Juenke. My guess is that the use of both methods will produce the best outcome.
I hope this little chapter has given a little insight on a machine that is not written about all that often. And possibly a little insight on the role twist rates play in accuracy.
Tony Z.