Post by Bullshop on May 5, 2016 19:07:19 GMT -7
I am pondering having a 45 cal barrel re bored to 50 cal so have been pondering a twist rate that will be correct for my application. My application will be extreme range shooting to the tune of Billy Dixon and his famous infamous 1500 yard shot on a mounted Indian.
From all that I have gathered from published information that was produced through the transition period from black powder to smokeless powder it was believed by the cutting edge accuracy advocates of the day that there was a potentially most accurate rate of twist for any projectile length for the longest range it would be fired at. This optimal twist rate was to deliver the projectile to its most distant possible target with the nose of the bullet following the trajectory arc and not maintaining its original elevated nose angle.
There are two reasons for this which have to do with drag or put another way that is easy to understand decreased BC and spin drift. The increased drag effect of a bullet that maintains a nose high attitude in flight has a tremendous effect in long range shooting but at the 40 rod range that was common to the shooters of the day it had little effect because the original elevated attitude of the bullet was elevated only slightly over the target so the increased drag was minimal so caused little shot dispersion of itself.
The issue of optimal twist was considered by many to be quite important to top shelf grouping. It was considered by many at the time when bullet velocities were barely beyond the speed of sound that over stabilization was ruinous to best grouping potential. Today it is a largely disagreed point that higher rotational velocity has any negative effect on grouping but I have to add here that this is not an apples to apples comparison. The reasons I say so are that today the velocities are higher and the bullets are jacketed so can we really apply the things that were held as common knowledge of the day to the things that are held as common knowledge today?
Now I am getting to the meat of where I was intending to go with this diatribe and here is a question I am looking for an answer to. Was the reason for the belief of optimal twist rate for projectile length/weight and maximum range due to an increased spin drift effect causing more erratic in flight behavior from cross wind effect? The trend today in long range BPC shooting is for increased twist rates and longer projectiles.
Many twist rates for certain calibers have been established with terminal ballistics in mind so the bullet will stay on a straight course in its penetration of a game animal. For target shooting that much stability is not needed but what is absolutely needed for long range shooting is to minimize the effects of wind. We can calculate wind drift if we have a reasonably close BC as long as the BC is not changed by the bullet attitude at long range. A nose high attitude exposes a greater amount of bullet surface to push against. In the testing I have researched done by our own gov with the 45-70 trials with 500 gn RN bullets with a 1/20" twist the recovered bullets showed a point on attitude when dug from the ground fired at extreme ranges to 2000 yds. I wonder if the faster twist rates in use today of as tight as 16" in 45 caliber would do the same.
Also I wonder if increased rotational velocity will increase the effects of cross wind on spin drift. Will a bullet with higher spin rate fired from a right hand twist barrel clime a 3:00 wind more than a bullet with a slower rotational velocity? Will a bullet with higher rotational velocity fired from a barrel with right hand twist sink more from a 9:00 wind than one with a lower rate of spin? If the answer to these questions is yes and I think it is at least with the BC's of the bullets we are using at the velocities we are shooting to then that points to the fact that there is an optimal spin rate for any given bullet length at the maximum range it will be fired to.
OK now I have come full circle and back to where I started. I now have to decide what that optimal twist rate will be for a 50 cal projectile of 600 to 650gn starting at a velocity of about 1300 fps and intended to be used to 1500 yards. I am sure to nail it I will need to apply more science than I am in control of so I will rely on my best educated guess and input from anyone that didn't quit reading half way through this perhaps uninteresting adventure in obsolete ballistics.
From all that I have gathered from published information that was produced through the transition period from black powder to smokeless powder it was believed by the cutting edge accuracy advocates of the day that there was a potentially most accurate rate of twist for any projectile length for the longest range it would be fired at. This optimal twist rate was to deliver the projectile to its most distant possible target with the nose of the bullet following the trajectory arc and not maintaining its original elevated nose angle.
There are two reasons for this which have to do with drag or put another way that is easy to understand decreased BC and spin drift. The increased drag effect of a bullet that maintains a nose high attitude in flight has a tremendous effect in long range shooting but at the 40 rod range that was common to the shooters of the day it had little effect because the original elevated attitude of the bullet was elevated only slightly over the target so the increased drag was minimal so caused little shot dispersion of itself.
The issue of optimal twist was considered by many to be quite important to top shelf grouping. It was considered by many at the time when bullet velocities were barely beyond the speed of sound that over stabilization was ruinous to best grouping potential. Today it is a largely disagreed point that higher rotational velocity has any negative effect on grouping but I have to add here that this is not an apples to apples comparison. The reasons I say so are that today the velocities are higher and the bullets are jacketed so can we really apply the things that were held as common knowledge of the day to the things that are held as common knowledge today?
Now I am getting to the meat of where I was intending to go with this diatribe and here is a question I am looking for an answer to. Was the reason for the belief of optimal twist rate for projectile length/weight and maximum range due to an increased spin drift effect causing more erratic in flight behavior from cross wind effect? The trend today in long range BPC shooting is for increased twist rates and longer projectiles.
Many twist rates for certain calibers have been established with terminal ballistics in mind so the bullet will stay on a straight course in its penetration of a game animal. For target shooting that much stability is not needed but what is absolutely needed for long range shooting is to minimize the effects of wind. We can calculate wind drift if we have a reasonably close BC as long as the BC is not changed by the bullet attitude at long range. A nose high attitude exposes a greater amount of bullet surface to push against. In the testing I have researched done by our own gov with the 45-70 trials with 500 gn RN bullets with a 1/20" twist the recovered bullets showed a point on attitude when dug from the ground fired at extreme ranges to 2000 yds. I wonder if the faster twist rates in use today of as tight as 16" in 45 caliber would do the same.
Also I wonder if increased rotational velocity will increase the effects of cross wind on spin drift. Will a bullet with higher spin rate fired from a right hand twist barrel clime a 3:00 wind more than a bullet with a slower rotational velocity? Will a bullet with higher rotational velocity fired from a barrel with right hand twist sink more from a 9:00 wind than one with a lower rate of spin? If the answer to these questions is yes and I think it is at least with the BC's of the bullets we are using at the velocities we are shooting to then that points to the fact that there is an optimal spin rate for any given bullet length at the maximum range it will be fired to.
OK now I have come full circle and back to where I started. I now have to decide what that optimal twist rate will be for a 50 cal projectile of 600 to 650gn starting at a velocity of about 1300 fps and intended to be used to 1500 yards. I am sure to nail it I will need to apply more science than I am in control of so I will rely on my best educated guess and input from anyone that didn't quit reading half way through this perhaps uninteresting adventure in obsolete ballistics.
