Anatomy of a High-BC Bullet — The New 183gr 7mm MatchKing
In case you haven’t heard, Sierra has a great new 7mm match bullet, the 183 grain MatchKing (item #1983). This bullet boasts an impressive 0.707 G1 Ballistic Coefficient (BC) at 2300+ fps. These bullets also have very consistent bullet-to-bullet BC, thanks to the tips being “pointed” at the factory. Sierra explains: “A final meplat-reducing operation (pointing) provides an increased ballistic coefficient for optimal wind resistance and velocity retention.” Top F-Open shooters have told us that these bullets shoot exceptionally well, with minimal vertical dispersion at 1000. “Holding waterline” at long range is a reliable indicator that the BC is very uniform from bullet to bullet.
The design of this bullet represents a radical departure for Sierra. The 7mm 183gr MK employs a new bullet shape with a special geometry: “To ensure precise bullet-to-bore alignment, a unique bearing surface to ogive junction uses the same 1.5 degree angle commonly found in match rifle chamber throats.” Mark Walker, Sierra Product Manager, explains: “We thought instead of using the typical ogive radius that can allow slight misalignment, why not use the same straight angle that is used in the chamber leade on the bullet ogive to force itself to always align with the bore? Once past that area, we use a traditional high caliber ogive radius[.]”.
Insight Into Sierra’s New 7mm MatchKing®
by Sierra Product Development Manager Mark Walker
In late 2015, Sierra introduced a new 7mm MatchKing® bullet with a different type of ogive. As part of the introduction, I had the opportunity to use them at the F-Class Nationals held in Phoenix with very good results. While at the match, several people had questions about what exactly was different about the ogive on this bullet as opposed to our tried and true blended tangent ogive. So with that in mind, hopefully this blog will answer those questions.
In the past, Sierra has typically used a tangent radius ogive design on our MatchKing® bullets. This is one of the most forgiving ogive designs due to its ability to shoot extremely accurately when jumped, as well as, jammed into the rifling. On rare occasions, some of our MatchKing® bullets have used a secant ogive due to design constraints. However, this ogive is much more sensitive to changes in seating depth than the tangent ogive so we tend to shy away from it. When we decided to work on this new bullet, we wanted to see if we could improve on the accuracy of even our best shooting tangent ogive bullets.
One of the main factors of what makes an accurate shooting bullet is how it aligns itself with the bore when fired. If a bullet is slightly crooked when entering the bore, it will cause inaccuracy on the target. We set out trying to think of ways to make sure that the bullet has no choice but to align itself with the bore perfectly.
The first part of the barrel to encounter the bullet is the leade in the chamber. The leade is an angle that is cut into the leading edge of the rifling which helps to guide the bullet into the bore. To illustrate how current bullets fit into the leade, picture a cone (leade) with a ball (bullet) sitting inside it. The ball can be rotated in all directions and the cone cannot force the ball to orientate itself in any particular direction. When a bullet with a radius encounters the leade, it behaves in a similar way. Now this of course is a very simple example and of course advanced shooters use tight necks and brass that is perfectly formed to the chamber to make sure the bullet is aligned as perfect as possible. However, there is always a small element of misalignment that is possible even with all this precise preparation.
This brings us to the ogive on the new 7mm MatchKing®. We thought instead of using the typical ogive radius that can allow slight misalignment, why not use the same straight angle that is used in the chamber leade on the bullet ogive to force itself to always align with the bore? Imagine the same cone as above (leade) with an identical cone (bullet) sitting inside of it. The cone inside has no choice but to align itself perfectly with the cone that it is sitting in every time. With that in mind, we designed the area of the bullet which contacts the leade in front of the bearing surface using a straight 1 1/2 degree angle instead of the typical radius. Once past that area, we use a traditional high caliber ogive radius to provide a very sleek, high BC bullet.
I hope that explains our thought process behind this new bullet. In our testing, it is one of the most forgiving high-BC bullets we have ever made.
