bullet drop and scope leveling

[B Jordan]

i have 3 rifles that i shoot long range with and on any still day staring at about 400 yds i start shooting to the right and progressivly gets worse until i am shooting 8-10 right at 800 this has happened on several occations could this be because my crosshairs are not level? i dont dial i just hold over. i have scope levels on all the rifles and i do my best with getting them set straight on the gun and help would be greatly appreciated thanks,Brian

 

[Black Diamond 408]

This may sound wierd, but the rotation of the earth will cause the bullet to drift to the right, in our hemisphere.
What say you all?

 

[1doug]

black diamiond 408

you are correct, however i dont think that is whats causing his problems.

i would bet on the canted reticle first, secondly if it started life as a custom barrrel in the "white" it could not have been turned between centers.

d-a

 

[Dave King]

Doing hold-overs and having drift to the side is as mentioned probably due to a canted reticle.

Check by putting up a white backer at 100 yards and then a plumb-line of dark string in front of the backer. View the backer with the rifle indicated level with your bubble (scope level) and see if your reticle tracks with the vertical plumb-line. If it's not plumb you can back calculate your induced drift by dividing your 800 yard drop (in inches) by 8, this will give you ~ MOA for 100 yards. Go to your 100 yard backer and measure down that many inches (MOA) from the center mark (bullseye) and make a mark (small dot) on the backer. Return to the firing line, line up the crosshair on the bullseye and then without moving the rifle begin to "click" the windage on the scope until the vertical reticle line touches the mark... figure the number of MOA you cranked in and you'll know... should be about a 1" (~1 MOA) offset error to equal 8" at 800 yards..



Coriolis can cause some drift and to check that fire in a due east or due west direction then a due north or due south direction. If the drift (on a windless day -- this is theory and not real life) is the same (or does not account for the entire value) when east west and north south targets are compared it's not (completely or partially) due to Coriolis. Coriolis is most significant in a due north and due south direction for "windage" errors and is possibly responsible for "elevation" errors in the due east and due west directions. Vary the direction off these Cardinal directions and you'll mix "elevation" and "windage" offset to some degree (no pun intended).


Disclaimer: This information is of no value to man nor beast and is offered only as speculation and conjecture on a theoretical problem or situation. Don't try this at home or offer this information to any other person (or sentient being) living or dead. If I made a gross (or even minor) error in anything posted here it is due to some medical condition I suffer and I'm not responsible (nor are any of my kin, friends, neighbors or anyone else I know (or don't know)). Your mileage may vary.... I have to go now, its time for my medication and bed check.

 

[Bart B]

[ QUOTE ]
This may sound wierd, but the rotation of the earth will cause the bullet to drift to the right, in our hemisphere.
What say you all?

[/ QUOTE ]
Having worn out several barrels shooting long range competition in all directions using both aperture and scope sights with the same zero for all ranges through 1000 yards, I can attest to the fact that the coriolis effect of the earth spinning at different speeds (about 500 yards per second at the equator) depending on latitude is zero. If there was an effect, I and all the other master class long range competitors would use a different windage zero for each range. No correction is needed; our windage zero didn't change for range. Most of the time aperture sights were used. The front sight has a spirit level that was zeroed with the elevation axis of the rear sight. After zeroing at 100 yards in a zero wind condition then coming up in elevation what's needed to zero at 1000 yards, windage didn't change. Same thing with scopes.

Large caliber long range military projectiles do need a correction. The gun aiming computers I worked on aboard Navy ships had an input for latitude that ended up making corrections to gun orders to correct for the coriolis earth rotating effect. Corrections at ranges less than 2000 yards were negligible. As range increased to 45,000 yards the correction got bigger. Corrections were made up to 70 degrees of latitude; above that the trig functions got too cumbersome to make mechanical computing cams for.

This guy probably has a canted scope reticule which can easily be corrected.

 

[JBM]

I tend to agree with the others that it's probably cant. Coriolis forces would only give you and inch or two at 1000 yards. It could however be drift (due to bullet yaw angle). This can be up to a foot or more at 1000 yards which would agree with what your are seeing. I "think" I've seen it with my .338 at long ranges. With a 500 yard zero, it shoots left at ranges less than 500 yards and right at ranges greater than 500 yards. The deflections I'm seeing agree with the models I've run.

 

[Michael Eichele]

Dont forget spin drift. While the rotation of the earth with a hand carried rifle is barley noticable spin drift can have a very noticable affect. Esspecially if you have a very fast twist for bullet combo and a long TOF. Even when the crosshairs are perfect and the rifle is not canted you WILL notice spin drift.

 

[Bart B]

[ QUOTE ]
Dont forget spin drift.

[/ QUOTE ]
It's not yaw, but precession caused by the bullet's tip continuously pointing down as its spin axis stays parallel to the trajectory path. As the bullet noses over, its right-hand twist causes it to change its direction to, or drift to the right. But with less than a 3 degree spin axis change in elevation for a rifle bullet going 1000 yards, there's not enough precession effect to cause any noticeable drift. I'm convinced it's much less than 1/4th MOA at 1000 yards.

I was told years ago that drift from bullet precession was a concern when the rear sight for the M1 Garand was designed. But the ballistic engineers at Aberdeen Proving Grounds said to forget it; not enough to correct for regardless of what bullet was used.

Back to the big military guns, they are corrected for precessional drift. With trajectory axes changing 90 degrees or more, there's enough precession to cause a bunch of drift. The computers I worked on essentially had the output of their range controlled drift cam multiplied by the cosine of the target elevation angle as a correction to the gun firing line in the horizontal plane. Works great at 14,321 yards be the target a plane, ship, or tank atop a hill.

 

[JBM]

[ QUOTE ]

It's not yaw, but precession caused by the bullet's tip continuously pointing down as its spin axis stays parallel to the trajectory path. As the bullet noses over, its
right-hand twist causes it to change its direction to, or drift to the right. But with less than a 3 degree spin axis change in elevation for a rifle bullet going 1000 yards, there's not enough precession effect to cause any noticeable drift. I'm convinced it's much less than 1/4th MOA at 1000 yards.


[/ QUOTE ]

Precession is in addition to the steady state yaw of repose. The yaw of repose is caused by a small torque due to the interaction of axial spin and gravity. It causes a steady state change in yaw angle which causes the drift.

For a Sierra 168 grain International, with 12" twist at 2600 f/s, the drift is about 9" at 1000 yards. For a 10 twist, the drift is about 11" and for a 14" twist, about 8 inches. Much more than a 1/4 MOA. (Data taken from McCoy's book, "Modern Exterior Ballistics")

[ QUOTE ]

I was told years ago that drift from bullet precession was a concern when the rear sight for the M1 Garand was designed. But the ballistic engineers at Aberdeen Proving Grounds said to forget it; not enough to correct for regardless of what bullet was used.


[/ QUOTE ]

Probably because the effects of wind are much bigger, but who knows.

[ QUOTE ]

Back to the big military guns, they are corrected for precessional drift. With trajectory axes changing 90 degrees or more, there's enough precession to cause a bunch of drift. The computers I worked on essentially had the output of their range controlled drift cam multiplied by the cosine of the target elevation angle as a correction to the gun firing line in the horizontal plane. Works great at 14,321 yards be the target a plane, ship, or tank atop a hill.

[/ QUOTE ]

Interesting. Do you have a reference for "precessional drift"?

Thanks!

 

[B Jordan]

i set all my scopes up for level the same way so i am pretty sure now that i have canted crosshairs because it is happening in three rifles i was really wondering about the bullet rotation thing but i did not want to sound stupid
what is the best way to level the crosshairs on a scope? they always look crooked just a hair one way and then the next maybe my heads not screwed on straight my wife says its not all the time, anyway thanks for the help,Brian

 

[Michael Eichele]

Some of the tests I have done in absolute dead still air concluded that in the dense sea level -3 degree air I was shooting in and the tight 11.25 twist with 155 SCENAR bullets was that there was about 10-12" of drift at 1K. This was also with a perfectly level scope and rifle. Modern ballistics have proved (by other than my own tests) that "spin drift" does occur. The average is about 8-12" at 1K.

 

[sambo3006]

Huh? Could you repeat that? /ubbthreads/images/graemlins/confused.gif
Just kidding. I can kind of follow all that. This is why I frequent this site and hate to talk to the idiots that claim to kill deer at 500 yards with a dead on hold when sighted in at 100 yds. I think that I am the idiot here (relatively speaking, that is). Keep it comin', guys!

 

[Bart B]

[ QUOTE ]
...secondly if it started life as a custom barrrel in the "white" it could not have been turned between centers.

[/ QUOTE ]
Why?

 

[daveosok]

Bart

I was building a 300 Tomahawk for a buddy. He ordered the barrel himself from Lilja. He neglected to tell them to turn between centers after drilling the 34 in hole.

When it got here the muzzle end was off by over .125 thou.

When I called Lilja about it they said he should have asked for them to turn it between centers. I assumed my buddy whould have understood that but I made an *** out of myself.

The person whom I talked to at Lilja (I cannto remember the name) told me that drilling a hole 34 inches deep and being on center on both ends was near to impossible.

If you dont (Lilja) request that your barrel be turned from a 1.35 or 1.45 dia blank (cant remember what they said to request but it was larger than 1.25 dia) then on a 34 in barrel youre more than likly going to have a huge difference from muzzle to breach concentricity issue.

Needless to say they replaced the barrel with a new one for his mistake and turned the new one between centers. That is more than I can say for Pacnor which sent me a rockwell hardness tested barrel of 23 and claimed it was 28 but then said they hold it at 28 plus or minus 5 rc points. Thats another story I am not just over yet and still a bit peeved.

Dave