April 27, 2020
In the shooting world, much has been said through the years about reloading. What load gives us the best performance, what bullet works the best for an application, and on and on. It’s not often we get an opportunity to visualize and verify or debunk what was an opinion or a hunch. Tom Beckstrand recently wrote about a test series that was done by Guns & Ammo TV at the New Mexico Tech-affiliated Energetic Materials Research and Testing Center (EMRTC). Beckstrand’s work centered on what happened when a bullet in flight struck a raindrop. As the resulting high-speed video showed, it blew away a lot of conjecture and opinion that has been around for a long time.
Another test that was done during this time was an investigation with the same high-speed Schlieren photography setup, and the difference between the muzzle-exit behavior of a boattail versus a flat-base projectile.
There has been a lot said over the years about the superiority of boattail projectiles, and that flat-base projectiles are “old school.” There are differences in the two. Anyone who has spent much time reloading different types of projectiles knows that there are peculiarities of different designs, and some can be harder to get good results with than others. To a great extent, these differences center on what happens in the barrel and at the muzzle, and how forgiving or sensitive a type of projectile is.
The test series done by Guns & Ammo TV at EMRTC illustrated significant differences in the behavior of a boattail and a flat-base projectile as they exited the barrel. Tests were conducted using a Thompson/Center Icon chambered in 6.5 Creedmoor. A Hornady 6.5mm 140-grain ELD-M was used for the boattail projectile, and a Swift 6.5mm 140-grain A-Frame was used for the flat base. Both projectiles were shot under the same conditions at as close to the same muzzle velocity as standard deviation allowed. The two were shot with the muzzle of the barrel at the edge of the Schlieren photography system’s field of view so that the projectile could be seen exiting the muzzle, as well as several projectiles lengths of initial free flight. The muzzle was then moved slightly outside the field of view so that late-time interaction between the projectiles and the muzzleblast cloud could be observed.
The images from the high-speed camera of the two projectiles were paired to show what was happening at the same point in time (PIT). This was done by matching up the images based on the PIT that the projectiles first appeared at the muzzle.
What Was Observed
Several things were apparent in the Schlieren images of the two different projectiles. The first observation in both bullet videos was of the gases from the barrel ahead of the projectile. This is primarily attributed to the air in the barrel being compressed and pushed out by the projectile. The gasses are observed long before the projectile appears.
The compressed air is seen much sooner with the boattail than with the flat base. This indicates that there were likely some propellant gases getting past the boattail before the projectile seals the bore. In each of the images seen, the boattail is on the left and the flat base on the right. The first emergence of the air and propellant gases is six frames sooner with the boattail than with the flat base.
Further indication that propellant gases got past the boattail before it sealed the bore were also apparent at the point each projectile exited the bore. As you can see in these images, the muzzleblast cloud for the boattail is noticeably bigger and further ahead for the boattail than the flat base. Since the volume of air in the barrel is the same for each bullet, the extra gas observed in the boattail images can only be propellant gases.
Before we discuss the next observations, we have to review what happens in the first few inches of projectile flight after it exits the barrel. The propellant gases behind the projectile are still at quite a high pressure, approximately 10,000 pounds per square inch (psi), depending on the projectile weight, specific propellant and barrel length. These gases, when initially released at the muzzle, are travelling for a very short period of time and distance — between 8,000 and 10,000 fps. These gases travel past the projectile to differing degrees depending on many factors, creating what is called a “reverse flow” on the projectile. This reverse flow occurs for a very short distance of travel — 6 to 12 inches.
This reverse flow can have detrimental effects on the projectile and is the primary reason for “tip off,” or disturbance of the projectile attitude at the muzzle. It is largely this phenomena that leads to what we refer to as the time for the projectile to “go to sleep.”
In the images taken during the Guns & Ammo TV test, the boattail projectile is entirely overtaken and surrounded by the propellant gases. This is not the case with the flat base. It is never completely overtaken by the propellant gases. The flat base on the right still has its nose protruding from the propellant gases and the shapes of the blast clouds are completely different. This suggests that the boattail bullet allows more high velocity gases to flow around it in the reverse flow stage than the flat-base projectile.
An additional test was done with the barrel approximately 5 inches out of the field of view of the Schlieren system in order to look at when the two projectiles cleared the influence of the muzzleblast. These images show the flat base clearing the muzzleblast quite a bit sooner than the boattail. In fact, the video shows it takes five frames longer for the boattail projectile to clear the muzzleblast. The flat-base projectile will clearly be influenced less by the muzzleblast than the boattail projectile.
What Does This All Mean?
The Schlieren images show that some propellant gases blow past the boattail projectile before it seals the bore. This can have adverse effects on the projectile. The gases that travel past the projectile are very hot and travelling very fast.
Despite how brief this propellant gas blowby is, it can result in non-uniform deformation of the projectile, or flame cutting, of the jacket. This can result in erratic flight of the projectile and is the cause of the troubling fliers we have all seen in our shooting groups from time to time. I’m not saying these things always happen, but it does create the potential for them to happen.
Perhaps the most important takeaway from this test is the response of the boattail projectile versus the flat-base at the muzzle’s exit. The larger and longer-lasting reverse flow on the boattail projectile can have a significant effect on its performance. With the sloping boattail, the reverse flow can produce a lift force on the boattail causing yaw and essentially making the projectile wobble. The duration of this lift force is very short, but it can have a significant effect on the projectile’s flight performance. This can cause larger groups and contribute significantly to the time it takes a projectile to go to sleep. Again, I’m not saying this will always occur, but the potential is definitely there. This dynamic condition does not exist with the flat-base projectile.
The Bottom Line
As you can see from the images and the analysis, the boattail projectile has much greater potential for undesirable things to happen. The boattail projectile has a greater sensitivity to factors not being perfect, either in the barrel or at the muzzle. The boattail projectile would be more sensitive to a large throat, throat wear, a damaged crown or worn rifling at the muzzle. If you have a rifle that won’t shoot very good groups with a boattail projectile, try a flat-base bullet and see if it results in better groups.
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