Range Report How to calculate bullet nose ogive
- Thread starter Heman
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after pointing the bullet or something do you mean?
If you mean; specific to your chamber/lead to lands. acquire a modified case with a fairly loose neck and place your projectile in the case a little longer than you think your lead is. close the bolt slowly and remove the case. you should be able to see where the lands touched the projectile. You may need to put a little magic marker ink, etc to better see the contact points. Once you determine the contact point, utilize a comparator gauge to get the measurement on the "in battery" cartridge length. You may want to do this excersize several times to assure a good average measurement. And, I'm assuming this is what your are asking about. If you want a "soft seated" round then you want just a slight longer seating so as to push the bullet softly in the case as you cam the bolt down. Start with reduced load from max if you attempt this as it will compact your powder somewhat.
Given an arc of a circle representing the exact profile of the ogive, construct a cord with end points coincident to the end points of the arc. The length of this chord is the width of the arc.
Measure of the length of the sagitta formed by the chord and the arc. The length of the sagitta is the height of the arc.
Calculate the radius of the circle described by the curvature of the ogive with the formula:
Radius = (H/2)+(W^2/8H)
...where H is the height of the arc and W is its width.
Dividing ogive radius by bullet caliber yields the ogive number.
Measure of the length of the sagitta formed by the chord and the arc. The length of the sagitta is the height of the arc.
Calculate the radius of the circle described by the curvature of the ogive with the formula:
Radius = (H/2)+(W^2/8H)
...where H is the height of the arc and W is its width.
Dividing ogive radius by bullet caliber yields the ogive number.
Ok I tried that formula for example on the 175gr SMK which is a 7ogive if I take the nose length of 0.710 (width) and use the height of 0.1205 (.308/2 - Meplat/2) you get an ogive radius in calibers of 1.89 and it is a 7ogive bullet. I saw this formula and it seems it should work but not sure what I'm doing wrong.
Nowhere did I use the terms "nose length" or "meplat." The reason being, neither plays any role in calculating the radius of the ogive. In geometry, terms like arc and chord and sagitta have exact meanings which are not interchangeable with whatever unrelated terms Sierra conveniently has provided on their bullet diagram. If any of those terms should be unfamiliar to you, Wikipedia is your friend.
Here I have marked a Berger bullet diagram with different colored line segments indicating the approximate locations of the chord (or width, in blue) and the sagitta (or height, in green) of the arc represented by the ogive.
If you do not have an alternate source for these measurements, you have to take them yourself. You cannot calculate the radius of the ogive without them. There are no substitutes.
The simplest way to take these measurements would be to transfer the 3-D profile of your bullet onto a 2-D rendering so it more easily lends itself to being measured. I suppose you could use a digital camera, or scan it. But that leaves open the possibility of errors induced by perspective or parallax. I cheated and used a 2-D scale model somebody else already had created. I calculated the radius of the ogive as per my previous instructions using measurements I took from this drawing and came up with 2.326, versus Berger's 2.489, a 6.5% error.
Once you have the 2-D rendering, construct a chord to the arc with a straight edge by drawing a line segment connecting any two points along the arc. This chord forms the base of the circular segment, and its length equals the arc's width. Accuracy increases the further apart those two points are located.
The sagitta is an interval of a line that perpendicularly bisects the chord that forms the circular segment's base. One end point of the sagitta lies on the chord and the other end point lies on the arc.
Easy to follow instructions on constructing the perpendicular bisector to a line segment are here. It requires the use of a (draftsman's) compass. If you don't have a compass, you're not properly equipped for geometric constructions. Mine cost like a dollar and I got it in the kid's art supplies at Wal-Mart.
Without measuring the sagitta, you've got nothing telling you how much the arc curves, which is the key to then calculating the radius of the circle. In theory you could accomplish the same thing by measuring the circular segment at its widest point and perpendicular to the chord. But as you can see, I still had a 6% error even though I was strictly adhering to the guidelines for geometric constructions, measuring to the 1/1000th of an inch and using a 6x hand lens to make my measurements as precise as possible.
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I looked this up just to research a bit. (As an engineer...well you know). The proof is slightly different from the formula stated above, but they are equal. I validated this through some simple algebraic manipulation. The proof is very clear on how to derive at the equation if anyone is interested.
Thanks for the post. Before this I wondered what the hell ogive is and I have a minor in math. Go figure. I got it now.
I looked this up just to research a bit. (As an engineer...well you know). The proof is slightly different from the formula stated above, but they are equal. I validated this through some simple algebraic manipulation. The proof is very clear on how to derive at the equation if anyone is interested.
Thanks for the post. Before this I wondered what the hell ogive is and I have a minor in math. Go figure. I got it now.
It's a function of bullet diameter, CALIBERS OF OGIVE.
The Ogive refers to the number of "calibers" the radius of an arc is, in order to copy the frontal body form (pointy-ness) of the projectile. I have a copy of E. I. Dupont's worksheets, showing how this is done. Unfortunately, they were copywrited, and I don't think I can legally upload them here. Still, the sheets are really cool, full of all kinds of data.
The Ogive refers to the number of "calibers" the radius of an arc is, in order to copy the frontal body form (pointy-ness) of the projectile. I have a copy of E. I. Dupont's worksheets, showing how this is done. Unfortunately, they were copywrited, and I don't think I can legally upload them here. Still, the sheets are really cool, full of all kinds of data.
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