September 19, 2019
Unless you’ve been living under a rock these last 15 years, you’ve had to have noticed that there’s been a lot of new cartridges introduced. They all seem to be similar in shape with no belt, minimum body taper and a sharp shoulder. A lot of them have moderate cartridge case capacity. Good examples are the .224 Valkyrie, 6mm Creedmoor, 6.5mm Creedmoor, 6.5 PRC, .300 PRC, the entire Nosler line and the Winchester Short Magnum (WSM) cartridges, just to name a few.
When was the last time a belted magnum was introduced? When was the last time we saw a cartridge with taper on the body or a shoulder angle of 20 degrees or less? It’s been a long time. What has changed in cartridge design philosophy? And why? I think I can sum up the change in one sentence: The demands of modern maximum-effective range have driven designers to wring every bit of accuracy out of a cartridge. Current maximum-effective range requirements have pushed designers to consider every aspect of cartridge and chamber design, and how they affect the uniformity of accuracy, pressure and velocity performance.
For a long time, a “long shot” in hunting was considered 400 yards. Anything successful beyond that was thought to have a lot of luck involved and was, at the very least, unethical. Those days are over. Today, a number of manufacturers offer rifles chambered in a number of the cartridges mentioned, which are considered effective past 1,000 yards. The advancement of long-range hunting and equipment demands performance to 1,000 yards.
Consider U.S. military sniping. It wasn’t but a few years ago that the outer envelope for a sniper was considered to be around 1,000 yards. If he had a rifle chambered in .300 Winchester Magnum, maybe 1,400 yards. The demands of the War on Terror and where it continues to be fought means that snipers are being asked to be capable of effectively engaging targets out to 2,000 yards — an incredible range with a shoulder-fired rifle. That’s in the range of what is considered mortar fire.
Many of us read Tom Beckstrand’s article on the .300 PRC in Guns & Ammo’s February issue. His reporting on that cartridge indicates that it is indeed effective to ranges in the order of 2,000 yards. I was intimately involved in the development of that cartridge, and there were a lot of tiny things that went into the design of it. The attention to detail and quality control in the .300 PRC is at an incredibly high level.
The level of accuracy and performance consistency required to achieve these types of ranges drives the designer of the cartridge to consider every aspect to achieve it. Designers now look closely at the projectiles they plan to use, along with twist rate, propellant, case geometry, how the cartridge headspaces, and throat and chamber design to minimize projectile Principle Axis Tilt (PAT). The advancements in the understanding of how all these things play together is what has led us to where we are today.
Perhaps the biggest change in philosophy of ammunition performance is the advancement in projectile design. For a long time, the answer to high performance was to make the bullet go fast. Put a huge propellant capacity case behind a lightweight bullet, then make it go fast and shoot flat. They didn’t pay a whole lot of attention to the projectile design. The problem with that is it works well for 400 yards, but a lightweight bullet loses velocity rapidly due to its high drag, and wind drift is overwhelming at long ranges. If you want to be effective at extreme ranges, it is far more important to have a heavy, very low-drag (VLD) projectile that takes longer to lose velocity, (just like a freight train). This is why the trend in many of these new cartridges are heavy bullets at moderate velocities. The heavy, low-drag bullet more than makes up for the muzzle velocity difference as compared to a high-drag, high-muzzle velocity and lightweight projectile at long ranges.
In the last five years, huge advancements have been made in small-arms bullet design and performance largely because of the use of radar. Designers can now use radar drag data to tweak the shape of a number of influences on the projectile to get the absolute lowest drag out of a design. Let me give you an example: The .300 PRC fires a 225-grain bullet at about 2,860 feet per second (fps). That’s not slow, but it’s also not a screaming fast velocity with a gigantic case of propellant as we’ve had in the past. Because of the aerodynamics of the bullet, .777 G1 BC, it stays supersonic to ridiculous ranges. I have shot this ammunition with radar and the bullet stays supersonic beyond 2,000 yards. Furthermore, because of the fast twist rates, long bullets and improved aerodynamics, these new bullets behave well as they transition through Mach 1. I have watched U.S. Marine sniper instructors fire this round and bullet to 2,300 meters and hit a target just slightly larger than a person. That’s 1.43 miles or 7,545 feet! Ten years ago, you’d have been laughed out of the room if you said you could do that.
Recent radar work with twist rates and projectile long-range performance have led to a new understanding of projectile performance and twist rates. It’s no longer good enough to say that we only care how a projectile performs from 1,800 to 3,000 fps. We now have to seriously consider how a projectile behaves at transonic and subsonic velocities. Studies with Doppler radar have shown that, particularly for long, heavy projectiles, these bullets fly better at long distances when spun faster. Look at the drag coefficient plots versus twist rate of the Hornady .30-caliber, 225-grain ELD-M projectile in Figure 2. As the twist rate and stability of the long, heavy 225-grain projectile increases, its drag coefficient function gets lower, particularly in the transonic Mach numbers — below 1,200 fps. We also see that the drag curves flatten and does not climb at transonic Mach numbers. In short, the projectile is transitioning through transonic Mach numbers much better as it is spun faster. This leads to significant improvements in extreme-range accuracy and performance repeatability. This is why we are seeing twist rates that are faster than we have been used to seeing in the past.
Chamber design has evolved in recent years and settled on some basic design criteria. Perhaps the biggest change in chamber design is the throat design, sometimes called “freebore.” This is the section of the barrel immediately ahead of the chamber and is the transition to the bore. It can be a straight taper from the case mouth or a straight section followed by a taper to the bore. In the past, there seemed to be little rhyme or reason to the design of the throat. Until the last 15 years, the diameter of the throat of many chambers could run anywhere from .003-inch to .010-inch greater than the projectile diameter. For example, the .308 Winchester maximum throat diameter is .312 inch. The .300 Winchester Magnum has a maximum throat diameter of .317 inch!
The problem with this type of throat design is that it can allow the projectile to become yawed in the throat while engaging the rifling. This is called Principle Axis Tilt, or PAT. The angle of the projectile centerline to the centerline of the bore is not controlled. It is allowed to yaw to a lesser or greater degree. When this occurs, the projectile will maintain this yaw down the barrel and at the muzzle. This is a primary cause of inaccuracy and why, for a very long time, the first thing that was done in handloading to attempt to improve accuracy and control PAT was to seat the bullet in the case so that it was touching the rifling when chambered.
Recent cartridge designs are using throat diameters of .0005-inch greater than nominal projectile diameter. Some examples are the .204 Ruger, 6mm and 6.5mm Creedmoor, and the .224 Valkyrie. These cartridges all have throat diameters of .0005-inch over nominal projectile diameter. They have a straight section ahead of the chamber prior to the taper into the rifling. These cartridges are all known for very high levels of accuracy. When a throat is designed this way, the amount of straight section (i.e., freebore) prior to engaging the rifling really doesn’t matter. This allows longer throats so that the projectile can be seated out of the propellant chamber. It also allows more room for propellant in the case and reduces the chances of projectile distortion or bending because it is unsupported while engaging the rifling, and improves accuracy. The Creedmoor cartridges and the new .300 PRC are examples of this. The only part of the projectile extending below the neck on these cartridges is the boattail.
Lead angle has also been pretty much standardized at 1.5 degrees. This allows a very gradual easing of the projectile into the rifling. In the past, lead angles were random, anywhere from 1.5 to 15 degrees. With the quest for extreme range, these details have become very important.
Current cartridge design seems to have evolved into minimum body taper and sharp shoulders. It’s hard to say that this is new, as former Guns & Ammo writer P.O. Ackley was designing Ackley Improved (AI) cartridges decades ago with the same philosophy. Ackley’s primary reason for doing this was to increase cartridge case propellant capacity to gain velocity. Although there were some increases in performance, when these cartridges were tested with legitimate pressure, measuring equipment improvements were small. At this point, the reason for cartridges with minimum taper and sharp shoulder is to improve the cartridge alignment with the centerline of the bore, and to decrease the PAT of the projectile. With a nearly straight wall body and sharp shoulder, the cartridge is not allowed to lay in the bottom of the chamber at any kind of a significant angle to the bore.
Belted magnums seem to be gone, too. Designers have abandoned belted magnums and gone to the sharper shoulder unbelted designs in order to get tighter control over the headspace between the cartridge and chamber. This resulted in decreased movement of the unfired cartridge in the chamber and provided the potential for performance uniformity improvements. The more consistent the space the cartridge has to move forward when the primer is struck, the more consistent the output of the primer.
Have all of these design features actually achieved inherently accurate cartridges? I would argue yes. Perhaps the best argument for this is the 6.5 Creedmoor. This cartridge has an enviable reputation for being extremely accurate no matter whose rifle or what platform it’s fired from. A cartridge as small as this has the capability to reliably hit targets in excess of 1,000 yards. It’s supersonic at sea level to nearly 1,500 yards with the right bullet. This used to be the domain of big, large-caliber magnums throwing heavy projectiles. We get the same performance from a moderately sized cartridge case firing a moderate weight bullet. Hitting long-range targets now depends on our ability to read wind.
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