By Daniel Lilja
After ordering a new set of bullet dies recently, I began to wonder how the new ogive shape that I had decided on would compare to the throat angle of my chamber reamer. Typically the reamer makers will grind a one and a half degree angle on the throat (or leade, as it is sometimes referred to) for cartridges intended for target shooting.
The SAAMI specifications for some cartridges designed for hunting call for a steeper throat angle, often up to 3 degrees or so. For example, the 222 Remington has an angle of 3 degrees, 10 minutes and 36 seconds. The 7mm Remington Magnum has a throat angle of 3 degrees, and the 308 Winchester has a standard leade of 1.75 degrees. A few cartridges, but not many, have a more shallow angle. For example, the 270 Winchester has an angle of 47 minutes and 33 seconds. The NRA book Handloading by William C. Davis has a reference section that has SAAMI chamber drawings for many of the popular commercial cartridges, including throat angles.
Why there are so many angles, I am not sure. As mentioned above though, target chambers often have a 1.5 degree throat angle. That brings up the question, why 1.5 degrees? I asked one of our long established and knowledgeable reamer makers that question. His reply was that angle was most often requested and just seemed to work. We can't argue with success and as the saying goes, if it works don't fix it.
The bullet swage die that I ordered, though, was not typical of common 7 caliber radius, tangent ogive bullets used in benchrest shooting. It was a 13 caliber secant ogive.
It seemed as though the "best" throat angle, if there is such a thing, would be one that is tangent to the bullet ogive radius at that point on the bullet which would be engraved by the rifling. Put another way, if we opened a pair of calipers to rifle barrel bore diameter (nominally .237" for a 6mm) and slid it along the nose of the bullet until both jaws of the caliper touched the bullet nose, we would have found that point on the ogive where the lands of the barrel would begin contact with the bullet. If we were to then draw an imaginary line from that point to the point on the bullet where the ogive and the bullet bearing surface meet, we would have the throat angle as described above. In the case of a 6mm bullet, the angle would be measured between the diameters of .243" and .237". The axial distance between these two diameters will vary with changes in the ogive radius. (See illustration.)
There are several ways in which this angle could be found for a specific bullet. The bullet could be physically measured on an optical comparator or a tool maker's microscope. There are also machines called coordinate measuring machines that are capable of making such measurements and computing angles and radii. Another method is to determine the throat angle mathematically, and this is the technique I used.
To use the mathematics method, certain physical dimensions of the bullet must be known, such as the diameter of the bullet and the rifle barrel bore diameter. The type of ogive, tangent or secant, must be known, and its radius. In the case of a secant bullet, the meplat diameter also must be known, as well as the ogive length. The ogive length is determined mathematically with a tangent radius. Its length depends on the radius of the ogive and the meplat diameter.
From this information, the location of the ogive radius center can be determined. Then, using calculus the axial distance can be determined between the point where the ogive and bearing surface meet and the point on the ogive where the diameter is bore diameter. Knowing this distance and the barrel land height, then it is a simple trigonometry calculation to determine the throat angle.