Do Bullets Go To Sleep?

[Kennibear]

MikeCr

P.S. to last post. First adjustable objective scope I bought was the original Weaver T 16. Back when they only made a T 16 and no other target scopes. There are no range marks on the objective turret, only a vernier scale. You have to adjust it until it eliminates the parallax.

The procedure is: set the objective with the rifle sandbagged well. Without contact with the rifle, move your eye side to side and see if the cross hairs move relative to the target. Adjust the side focus/ objective turret as necessary until side to side movement of your eyes does not change the cross hairs position on the target. It remains fixed relative to the target. If you must adjust the ocular turret to refocus the scope then do so because the setting for parallax is correct and the scope is unfocused.

Friend Ed's Vortex is a side focus and I was surprised the manufacturers assume the range settings are spot on. It was not but we penciled a table with the correct settings out to 300 yards.

Not that I know Diddley or his brother, Squat....

 

[Mikecr]

My scopes are side focus.

I know that bad bullets and bad release of them are bad for LR shooting. In fact, there is no good in it.
The common Connotation of bullets 'going to sleep' is that they will not shoot well UNTIL beyond xxx-distance. It's like shooters have mistaken the elliptical tip rotation for actual bullet path.

The truth is, bullets are not corkscrewing about expected path until dampening back to right where we're aiming,, and they won't shoot better beyond xxx-distance,, only worse.

 

[Kennibear]

Thanks for the info.

The elliptical eccentric up close may explain the weird performance on game we see on the really close shots.

Adjustable objective or side focus doesn't matter IME. The scope is optically corrected for parallax when the the reticule appears fixed on the target no matter the eye position behind the scope.

Think of it as the scope focusing on both ends. When you look backwards through the scope things appear far away. The ocular adjusts the scope's focus for your eye. The parallax focuses the scope at the target. Hence the reticule does not wander across the target.

Your input is very helpful. It always helps to visualize what happens to the bullet in flight.

KB

 

[Michael Courtney]

2) Does the change in Cd affect the BC of our bullets enough to increase the uncertainty (or create a variable) BC that compromises the predictability of the bullets path? Are our computer ballistics programs going to have a margin of error large enough to limit their accuracy for our purpose because of this?

Our experimental work has shown that bullets can often exhibit as much as 10-15% shot to shot variations in BC. Both our work and Bryan Litz work has shown that hollow point match bullets tend to be some of the worst offenders when it comes to shot to shot BC variations. We choose a plastic-tipped bullet to study yaw dependent drag variations because this bullet has about the smallest shot to shot drag variations that we have seen. There are a number of articles around that discuss meplat pointing and trimming as an approach to reduce shot to shot variations in BC. See:

http://www.appliedballisticsllc.com/index_files/Lapua_Scenar.pdf

http://www.appliedballisticsllc.com/index_files/7mmNumberOne.pdf

Bullet Pointing « Daily Bulletin

It is simple enough to run numbers for your own favorite bullets and ranges at JBM to see how much a +5% and a -5% BC variations will effect your trajectory at the relevant ranges.

3) Why not Cd instead of BC? Is the tide of Ballistics Research so ingrained in Ingall's original mathematical theories that we cannot change? Aerodynamics uses Cd and not a coefficient that adjusts the airfoil performance to match some arbitrary standard (G1/G7). The wind tunnel data generates a data curve (a data cloud really) across a range of Reynolds Numbers (think different velocities at different altitudes) and performance predictions are based on that. Our mathematics have always bugged me because we use BC's for maybe three ranges and none are accurate at correcting for form factor (to 1/10th of 1%) and so on and so forth. Cd seems a cleaner albeit mathematically more extensive solution. But we have computers today.

This would only make sense if you had the whole Cd vs. V curve for a given bullet from a given rifle. Rifle to rifle variations introduce bigger uncertainties than the errors introduced by a given BC drag model, especially now that we have the G7 drag model. Measuring a G7 BC with a specific rifle and attending to reductions in BC variations with the pointing or trimming techniques (or using a plastic tipped bullet) will do much more for long range accuracy and predictability than measuring the whole Cd vs. V curve.

 

[Michael Courtney]

I know that bad bullets and bad release of them are bad for LR shooting. In fact, there is no good in it.
The common Connotation of bullets 'going to sleep' is that they will not shoot well UNTIL beyond xxx-distance. It's like shooters have mistaken the elliptical tip rotation for actual bullet path.

The truth is, bullets are not corkscrewing about expected path until dampening back to right where we're aiming,, and they won't shoot better beyond xxx-distance,, only worse.

This is basically right. The problem with pitch and yaw is not swerve, it is increased drag, and sometimes shot to shot variations in that increased drag which leads to unpredictable long range trajectories.

Other than active trajectory control (in-flight steering), the hottest topic in external ballistics right now that the military guys are pursuing is minimizing pitch and yaw (both reducing the initial tip off rate and quickly damping out pitch and yaw) because this is the biggest factor in predicting trajectories at REALLY long range.

 

[Kennibear]

Hopefully my last question as you are going to run out of patience:

Does it improve the bullet by making it longer or at least with a long bearing surface relative to weight to minimize this effect? That is, do long bodied bullets with long contact surfaces pitch and yaw less? Have you found a correlation between length of bearing surface as a function of sectional density that improves or reduces this effect?

Very informative article, written for laymen to understand. That alone is an accomplishment.

KB

 

[Mikecr]

the hottest topic in external ballistics right now that the military guys are pursuing is minimizing pitch and yaw (both reducing the initial tip off rate and quickly damping out pitch and yaw) because this is the biggest factor in predicting trajectories at REALLY long range.

The tactical folks need to go back to basics and mind their muzzle pressures. Their trend of ever shorter barrels regardless of cartridge capacity is counter productive for LR.
2-3" more barrel can go along way to reduce fireball slapping across the back of boat tails. And while this won't be seen with short range precision, a cleaner release helps at long ranges.

 

[Michael Courtney]

Does it improve the bullet by making it longer or at least with a long bearing surface relative to weight to minimize this effect? That is, do long bodied bullets with long contact surfaces pitch and yaw less? Have you found a correlation between length of bearing surface as a function of sectional density that improves or reduces this effect?

Very informative article, written for laymen to understand. That alone is an accomplishment.

I don't know. High speed video is the best way to quantify the maximum pitch and yaw angles of bullets in flight. We've produced and reviewed high speed videos to quantify pitch and yaw on a number of bullets, but we haven't noticed any length related trends, but we haven't carefully designed an experiment with bearing surface length as the only variable either.

I tend to think that controlling other factors is likely more important. You want a good concentric load with minimal opportunity for the bullet to get misaligned in the bore as it transitions from the case to the rifling, and you want a good crown and a clean release as it leaves the muzzle. As Mike said, you also want to keep the muzzle pressures down either with an appropriate barrel length or a good powder choice.

With a given level of maximum pitch and yaw (say 5 degrees), longer bullets will have a greater increase in drag than shorter bullets, because at a given angle, longer bullets have a greater increase in cross sectional surface area than shorter bullets.

 

[Pdvdh]

Interesting stuff. I was so involved in the discussion I ended up being disappointed when I got to the last Post. I was hoping for further discussion, but the discussion that's here shed a lot of light drag coefficients and BC values, the various factors that can influence these values, and what LR hunters and target shooters might do to improve on the consistency of these values. Thanks Michael. And Mikecr also.

 

[Michael Courtney]

Interesting stuff. I was so involved in the discussion I ended up being disappointed when I got to the last Post. I was hoping for further discussion, but the discussion that's here shed a lot of light drag coefficients and BC values, the various factors that can influence these values, and what LR hunters and target shooters might do to improve on the consistency of these values. Thanks Michael. And Mikecr also.

Litz has a new book coming out on July 25 that should shed more light on some of these issues.

 

[CB11WYO]

Hey...

http://www.longrangehunting.com/for...oot-thru-target-challenge-144359/#post1013982

Any takers?? ^^