Bryan,
Some of the PRS shooters are noticing that they have less drop with MI rifling vs. conventional or R5 rifling when the 105 hybrids are launched at the same velocity. With the same velocity (3120 FPS) on the 105 berger hybrids, a G7 BC of .278 works well for the conventional rifling, but a G7 BC of .309 is required to match the trajectory with the MI rifling. Their elevations (in mils) start to diverge at 600 yards by 0.1mil, and are a full mil less at 1300 yards with the MI rifling. They were shooting with several rifles that have conventional rifling and two that had MI rifling at the same time, under the same conditions. I don't have all of the specifics of the test, and I know your feelings on deriving BCs from drops, but the fact that there were multiple samples shooting under the same conditions and showing a clear trend piqued my interest. The rifling is the Caudle 3 Land Polygonal type.
I suspect that if this rifling is indeed producing better ballistics from the same muzzle velocity that the reduction in drag isn't seen in the longitudinal direction of travel so much as by the axial rotation of the bullet. There is probably a smaller loss of RPMs during flight since the bullet surface is smooth vs. having groves from the rifling.
I was wondering what your thoughts were on this. If you have tested these barrels and seen similar results and what you attributed the better ballistics to.
Kris
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Kris,
I think it’s possible that the different riflings could produce lower drag but there’s another mechanism which I believe is more likely in this case which you eluded to.
I’ve measured the RPM decay of different rifling types and found that there can be a noticeable difference between just a new vs. worn barrel of the same rifling type. Something like the MI rifling is likely to allow the bullet to retain RPM’s much better than other rifling types.
Retaining better RPM’s means the bullet goes into transonic flight with much greater stability (as though it was fired from a faster twist rate to begin with). Higher stability thru transonic has a measurable effect on drag and effective BC. I think this is likely the major effect of the different rifling type, and the reason for the observed better flight performance.
Much of my experimental work on these subjects is published in ‘Modern Advancements in Long Range Shooting – Vol 1’.
The Ballistic calibration tool in AB Mobile is geared to handle scenario’s like this. Calibrate MV based on observed drop out thru supersonic range, then input points of drop thru transonic and the app will apply a correction factor to manage all the subtle variables that are currently not able to be modeled and accounted for such as stability level thru transonic.
I’m not clear on the use of chronographs in this testing; it’s possible that the MI rifling is producing a higher MV than the other barrels and that may account for at least some of the difference.
Another point, just for completeness, is to consider if the MI rifles have the same scopes as the others, and if all turrets are verified to track properly all the way thru the range of adjustment.
-Bryan
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Bryan,
They did measure the MV with a MagnetoSpeed at the beginning of the test, however they were not measuring it during firing. Two of the rifles were identical builds with identical scopes. The question of the turrets was brought up, but not addressed specifically.
I will copy your response into the thread, if you don't mind and send the responses, or maybe you could pop in as the discussion will be focused on this specific issue?
Kris
