Bullet Design–Secant vs. Tangent Ogive
Many shooters and hand-loaders lack a clear understanding of certain elements of bullet design. While most serious shooters know that Secant and Tangent Ogive bullets are different, many of us would have trouble explaining the precise nature of the difference. Thankfully, Dan Lilja clarifies this and many other bullet design matters in his excellent article, Calculating Bullet Weights. Here’s the straight scoop on bullet ogive differences, in Dan’s words:
“Spitzer bullets all fall into one of several different design types. A bullet’s geometry can be broken down into symmetrical sections. The body (or shank) of the bullet is that part of the bullet that comes into full contact with the barrel rifling. It is the cylindrical portion of the bullet of nominal groove diameter for that caliber. It is also referred to as the Bearing Surface.
The term “Ogive” is commonly used to describe that specific point where the curving part of the bullet nose begins–i.e. where the bearing surface ends and the curved part begins. When we talk about “bearing surface length”, this is the distance from the top of the boattail to the point where the bullet starts to curve.
However, the terms “ogive” or “ogive profile” are also used to describe the entire forward curved section of the bullet, ahead of the bearing surface. The Ogive Profile, or nose of a bullet, can be one of three varieties, two being common*. The ogive can have a tangent radius; that is, the intersection of the body of the bullet and the radius of the ogive blend together at a tangent point. This is the most common type and is typical of most bullets handloaders use. The other common type is the secant ogive. In this case the intersection of the ogive and bullet body does not flow together smoothly. The point of intersection is not tangent, the ogive radius being a secant of the arc circle of the ogive.
The radius of the ogive is generally given as a multiple of the bullet diameter. For example, a 30-caliber bullet might have a 7-caliber tangent ogive. That is to say that the radius, in inches, is seven times .308 or 2.156 inches. When a bullet is described as “7 Ogive” this is what is meant–the radius of the bullet’s nose section as a multiple of the bullet diameter. (A “7 Ogive” can be any caliber, not just 30 caliber–it is the ratio of the curve radius to diameter that counts).
The point or tip of the ogive is called the Meplat. This can vary in diameter from about .050″ to over 0.125″ for conventional spitzer-type bullets.
The base of the bullet can be either a boattail shape or flat base. If a boattail, its shape is determined by the axial length of the boattail and the angle at which the boattail leaves the body of the bullet.”
*The third bullet nose type is a cone. Cone-shaped bullets are not common with handloaders, but some experimental work is done with them.
Similar Posts:
- Tangent vs. Secant vs. Hybrid Ogive — Bryan Litz Explains
- Tangent vs. Secant vs. Hybrid — Bullet Ogive Geometry Explained
- Ogive No Jive — Litz Explains Tangent, Secant, and Hybrid Ogives
- Bullet Geometry Basics: Tangent, Secant, and Hybrid Ogives
- Tangent, Secant, Hybrid — Bullet Geometry Explained by Litz
Excellent info, thanks for the education! It’s much appreciated.
Thank you for the information. I have often wondered about secant, tangent ogives. now I know.
I believe that there is something wrong in this paragraph:
“The point or tip of the ogive is called the Meplat. This can vary in diameter from about .050″ to over .0125″ for conventional spitzer-type bullets.”
I believe that it should read “0.125”
EDITOR: Thanks for picking that up. Correction made.
Thanks again
Dave
Does the increased bearing surface area of the tangent ogive, have a +/- effect on accuracy vs. secant ogive?
My 6.5 A-Maxs(secant) seem to shoot better than Rem.PSP 140gr in my Swede.
Hi, after reading the articles on Dan Liljas website including “Calculating Bullet Weights” I had to try out the code.
After a week of programming I have compiled his work in an exefile. If you want to try it out the link is: http://www.weatherbyrifler.com/index.php?ind=downloads&op=section_view&idev=1
The filename is “BC kalkulator”
Best regards
Lars-K
There is another type of profile called a “Hydrid Ogive”. It is a combination of Secant and Tangent Ogives to blend the best of both to allow more forgiving entrance alignment through the throat of the chamber into the bore/rifling of the Tangent and still keep the aerodynamic advantage of the Secant.
Tangent Ogive, though less aerodynamic than Secant, is the most common for factory and handloading because of its forgiving tolerance for chamber throat length differences and aligns with bore/rifling more easily.
Secant Ogive is more aerodynamic and retains more velocity at longer distances like used in 1,000m or yard target,hunting,military uses, BUT Secant Ogive is more sensative to OAL (Overall Cartridge Length) and therefore more time consuming and critical to find “the OAL sweet spot” for each chamber throat length its used in as just a few thousanths of an inch difference in OAL can significantly change group size.
Also there is a difference in muzzle velocity between Tanget, Secant and Hybrid Ofive bullets due to the length if Bearing Surface on each.
Tangent Ogive has a longer bearing surface and more drag because more of the bullet surface is in contact with the bore/rifling.
Secant Ogive has less bearing surface and less drag because less of the bullet is in contact with bore/rifling.
Hybrid Ogive has less bearing surface than Tangent but more bearing surface than Secant so drag is in between both, but its primary advantage is in combining the better aerodynamics of Secant and more forgiving throat entrance of Tangent.
Secant and tangent ogives affect different parts of the ballistic travel – secant the external ballistics, and tangent the internal ballistics.
The problem with the secant profile is that because there is a smaller immediate bullet circumference forward of the front margin of the bullet’s body than is the case with a tangent profile, the tapered cone of the barrel Leade engages the bullet body later in the firing process. As the bullet moves and the neck of the case expands, the alignment of the bullet is no longer controlled, which can lead to loss of bullet longitudinal axis alignment with the longitudinal axis of the barrel as the bullet moves over the necessary jump distance required to avoid the overpressure spikes that would occur were the bullet to be seated in contact with the lands.
The tangent ogive functions like a synchromesh in that it provides a more immediate contact and alignment force with the leade, and provides a less abrupt start to the engraving of the bullet rifling, which in turn helps maintain the coaxial relationship between the bullet and the barrel. That alignment force makes it less sensitive to seating depth variation; It is thus a superior leading edge for the start of a bullet’s travel.
The secant ogive, on the other hand, excels in external ballistics because of its superior aerodynamic shape, leading to lower drag and superior downrange performance.
Hybrid bullets combine the benefits of the tangent’s alignment superiority with the secant’s superior drag coefficient. Also, the lesser sensitivity to bullet seating depth of a tangent ogive bullet is retained with a hybrid bullet, and provides higher consistency of performance (accuracy) as the rifle throat erodes from firing, adding to the retention of precision of the weapon’s performance as it wears.