Is this kooky or do I not understand physics?

[SaxonPig]

Put a Weaver K3 on a CZ550 in 30-06. Plan to start at 25 yards to get scope dialed in and then move out in stages. At 25 yards 1st shot is low to the left so I make adjustments. The 2nd shot is perfect on windage but still low. Make another adjustment. Third shot cuts the X in a nearly perfect pinwheel. Fire 3 shot group at 25 that obliterates the X and goes no more than 3/4".

Move to 50 yard target. Fire another 3 shot group to see how much it drops going from 25 to 50 before correcting for longer range. Expected maybe 1-2" of drop. Imagine surprise when group is centered but 3" above X. Had to pack it in at that point so never got to shoot at 100.

Now, it was shooting dead on at 25 and it seems to me that moving to a greater distance should make the bullet drop a bit. Am I wrong or am I missing something? The bullet can't rise above the line of the bore, but is it possible that at the closer range the bore is angled upward so the bullet rises at 50? Does that make sense? Why would the barrel be angled upward?

 

[rbernie]

The scope mount is naturally angled down so that the barrel always has a slight upward cant when the scope is level.

 

[RecoilRob]

The scope is something like 1.5" above the bore. Aligning it to hit dead on @ 25yds means the bullet is rising that 1.5" and is still going up by the time you get to 50yds. Might still be high by 100 but will start to come back down sooner or later...most likely by 175 or so.

Perfectly normal.

 

[rangerruck]

just tippin up the other end of the see-saw. remember a 2 inch rise up , at only 25 yards, is a pretty sharp incline, so with a 30.06 you could easily be over 5 inches high at 100. start about 1 inch low at 25, then take it out to 100.

 

[Cannonball888]

SaxonPig, you may also be surprised to know that a rifle has two zeros due to the bullet's trajectory. In this case the gun has a zero at 50 and 200 yards.

 

[iamkris]

You don't understand physics (insert smiley here...not meant to be a denigrating remark). Cannonball's post is a perfect explanation of the effect of trajectory. All guns work in exactly the same way. The line of the bore is angled upward in relation to the line of sight of the front and rear sights.

That's why you hear so many people talking about a "25 yard zero" to emulate a 200 or 250 yard zero on their military rifles.

 

[Richard.Howe]

But...everyone should be clear...contrary to conventional "wisdom," the bullet does not fly upwards -- it's pointed upwards on most rifles due to the non-parallel alignment of the bore axis and the scope's line-of-sight.

If your bore axis and scope center were both perfectly parallel, the you would hit about 1.5" low at the bore, and that number would steadily increase with yardage.

Instead, we tend to point the rifle up a bit relative to the scope, in order to get the point-of-impact and the line-of-sight to coincide at some convenient yardage (say 100 yds.). This necessarily means that the bullet will move in the direction it's pointed -- i.e. up -- until gravity starts to win and bullet drop begins.

I've heard several otherwise reasonable people swear that, even if shot perfectly horizontally, the bullet will rise due to some mysterious aerodynamic effect. Not true.

 

[Richard.Howe]

Funny aside -- I learned this lesson when I was 15 behind a 15" .223 Encore and a 4x Bushnell. Gun was sighted-in (and of course dreadfully accurate ) at 15 yds, then off the top of the paper at 100. Couldn't figure out what was happening until I sat down with pencil and paper!

 

[Titan6]

A 15 yard zero on .223 would be something.


Most Army units zero at 25 meters but some units are shifting to a 125 meter zero. This appears to make the bullet shoot a little more flat. Also most engagements are at that range or less.

 

[Chipperman]

the bullet does not fly upwards -- it's pointed upwards

+100

A lot of people simply do not understand that.

They also fail to realize that the offset makes a big difference. If you have high scope rings, you will have a bigger offset between that scope and barrel. This means that the points where line of sight and trajectory intersect will be farther apart than a scope mounted close to the barrel.

 

[SaxonPig]

I knew that a bullet doesn't rise above the line of the bore but I was sort of assuming that the scope and the bore were pretty much aligned but apparently not and this causes the bore to be angled upward (as I was guessing in my first post).

So I wonder where it will shoot at 100 yards if dead on at 25 and 3" high at 50?

Thanks for the help and no, I am lousy at math. When the counselor saw my math placement score when I entered college he had campus security escort me to the Humanities Department.

 

[Titan6]

So I wonder where it will shoot at 100 yards if dead on at 25 and 3" high at 50?

That really depends upon the round and the gun. Remington used to have some good charts and programs for free in the ammo section on their web site. I think they sell it as a program now. They still have a lot of useful info over there.

 

[AndyC]

There are trajectory tables/software available online for the various calibers - do a google for your specific one

 

[Chipperman]

I was sort of assuming that the scope and the bore were pretty much aligned

If they are perfectly aligned then you will NEVER have a point where Point of Aim intersects Point of Impact. The scope HAS to point slightly down relative to the barrel to have the two intersect.

 

[USSR]

So I wonder where it will shoot at 100 yards if dead on at 25 and 3" high at 50?

In all likelyhood, even higher that it is at 50 yards. Sight it in several inches low at 25 yards before moving back to 100 yards.

Don

 

[Medusa]

I've heard several otherwise reasonable people swear that, even if shot perfectly horizontally, the bullet will rise due to some mysterious aerodynamic effect. Not true.

There is an effect - Magnus effect - that creates lift because of the bullet's forward motion, spin and wind. This lift can be positive or negative, depending on the direction of spin and wind. When the bullet spins the boundary layer is a vortex, spinning the same direction, say clockwise for example. If the wind is from right the relative speed over the boundary layer is higher on the topside (two airflows in opposite direction, thus summing the speed) than on the bottomside (two airflows going in the same direction, total relative being one minus the another), giving the effect like on a wing of the aircraft, creating lifting force due the dyn.pressure difference. I'm sure this effect is pretty small, being felt only when you start to shoot really long ranges, but it's there. There was a testing ship made that used the Magnus effect (can't cite the name, but I know where to look for it) for propulsion and it made 4 knots on that alone.

All kinds of funny stuff start to happen if you move through media at high enough speed.

[/off topic]

 

[Chipperman]

The magnus effect will not reliably make a bullet rise or fall, however. Partly this is due to the variability of winds, but also due to the fact that the spin of a bullet is PERPENDICULAR to the direction of travel. When English is put on a ball, the spin is PARALLEL to the direction of travel, producing a more reliable effect.

 

[jacobhh]

An extreme example of what your encountering, as an illustration,
is the "reverse slope" artillery solution where you fire over an obstruction
or hill and hit a target which, while on the other side and not in your
line of sight, due to the hill, is at a higher or lower elevation than yours.
The only limitation being the range/power of your gun.

 

[Richard.Howe]

Mangus effect is a complete red herring in this discussion.

It only applies to spinning cylinders whose axis is moving perpindicularly to the primary gas flow. Think throwing a paper towel cardboard core and putting back spin on it. It rises sightly as it moves downrange.

When fired, a bullet's axis moves parallel -- not perpindicular -- to the primary gas flow. Even if there was a crosswind, its magnitude would be so small compared to the bullet's velocity, it would be completely irrelevant unless you're taking a shot measured in miles.

Then, depending on crosswind direction, your round will move fractions of an inch up or down, depending on whether the crosswind is left-to-right or right-to-left, and which direction rifling your barrel has.

Think about it. My rifle fires handloads at about 2100 miles/hr with a twist rate of 1:12. To have appreciable Mangus effects, you need about 40% of that velocity manifested in the perpindicular direction.

If you're on the range in 840 mph crosswinds, you need a different job.

 

[Medusa]

That was my thought, actually, that the effect is minuscule and applied in the ranges of 1+ mile. (thinking .50BMG and .408 Cheytac). Sorry for being out of topic, but there should be a thread on ballistics.