ANATOMY OF THE BOAT-TAILS IN RIFLE PROJECTILES

  

ANATOMY OF THE BOAT-TAILS IN RIFLE PROJECTILES

BOAT-TAILS:

          A properly designed boat-tail can increase the BC of a bullet by decreasing base drag. There is a low-pressure area at the base of the bullet as it travels thru the air at high speed. That low pressure region (vacuum) behind the bullet is applied to the area on the base of the bullet and results in base drag. The goal of a boat tail is to reduce the effective base area to which this vacuum applies.

          Tapering the back of a bullet allows the air to flow along the boat-tail before separating off the reduced diameter base. The boat-tail produces a smaller base area for the low pressure to act on, thereby reducing base drag.

          There are some limitations to what you can do with boat-tails. One common instinct is to make the boat-tail longer, the idea being to further reduce the base area and base drag. The reason why extra long boat-tails don’t work is because they compromise the dynamic stability of the bullet. A boat tail length of around 0.8 calibers is the practical maximum length for boat-tails on spin stabilized projectiles. Another common thought is to increase the angle of the boat-tail so that the base area is decreased for the same length boat-tail. However, as noted above, air flow separation limits how steep the boat-tail angle can be. Imagine if you make the boat-tail very steep, 45 degrees for example. The air flow will not follow the boat-tail and separate off the base. Instead, it will separate at the body/boat-tail junction. In that case, the bullet acts just like a flat base with separated flow and the suction is applied to the entire cross-sectional area of the bullet. Tests have shown that the optimal angle for boat-tails is around 7 – 8 degrees.

AIR FLOW SEPARATION AND BASE DRAG REDUCTION:

A properly angled boat tail allows the airflow to remain attached to the surface until it reaches the reduced diameter base. As a result, the low pressure (suction) will apply to a smaller area at the base of the bullet and result in less base drag.

On flat base bullet, the air flow simply separates from the bullet body and the low pressure is applied to the entire base of the bullet resulting in maximum base drag.

If the boat tail is too steep, the air flow will bot remain attached and will separate off the bullet body just as if it were a flat base. No base drag reduction is accomplished with a boat tail that is too steep (more than about 20 degrees).

DRAG REDUCTION CURVE OF THE BOAT TAIL:

The above figure shows the drag reduction for a 0.7 caliber long boat tail for the range of angles between 0 ang 21 degrees. Notice the drag is minimized for a boat tail angle of about 7 to 8 degrees. This is true for any length of boat tail at supersonic speeds.

Boat tails help out more at lower speeds (1500 fps to 2500 fps) that at higher speeds (3000 fps +). That is because at higher speeds, the strong shock wave generated at the nose of the bullet is responsible for most of the overall drag. As the bullet slows down, the shock wave weakens, and base drag becomes a more significant component of overall drag. For this reason, the benefits of boat-tail bullets are more pronounced at longer ranges that short range. Sometimes this fact is misinterpreted as “bot tails only help at low speed”. The truth is, boat tails reduce base drag and increase BC at all speeds, but their effect is proportionally greater at lower speed.

BOAT-TAILS AND PRECISION:

            Boat-tail bullets are inherently less precise than flat base bullets for several reasons. First of all, the added steps in making boat-tail bullets allow more potential for imperfections like making the boat tail off center. Modern manufacturing practices minimize the imperfections, but the bottom line is that forming a boat-tail is an extra step that can potentially introduce misalignment and mass imbalance.

            Furthermore, muzzle blast effects are more detrimental to boat-tail bullets than flat bases because the boat-tail offers a surface that is nearly parallel to the bullet axis for the escaping high pressure turbulent gases to buffet, which can cause the bullet to pitch and yar early in flight.

            Finally, boat-tails affect the mass properties and aerodynamics of a bullet in a way that causes them to require higher twist rates to stabilize. Since many components of dispersion are proportional to spin rate in some way, increasing spin rate has a negative impact on precision potential. The difference in accuracy potential between boat-tail bullets and flat bases is very small. The reason why we use boat-tails is because at extended ranges, the aerodynamic benefits overcome atmospheric variations better than flat bases, and make up for the marginally compromised precision potential. The net result is smaller groups at longer ranges. Over short ranges, the ballistic performance advantage of a bot tail is not enough to make up for it being less inherently precise.

REBATED BOAT-TAILS:

            Rebated boat-tails are boat tails that have a step down in diameter from the bullet body to the start of the boat-tail, and taper to the base from there. There are many claims about the advantages of rebated boat tails related to accuracy and BC. As for as drag reduction, there is no way a rebated boat tail can reduce base drag any more than a conventional boat tail of equal length and angle. In fact, there is a potential for the rebated boat-tail to compromise the boat-tail effect by causing flow separation if the step is too big. How big is too big depends on the caliber among other things. Makers of rebated boat-tail bullets claim 0.015” step is not too bad. This step size happens to be the smallest stop that can be reliably made with typical custom dies. Most claims about rebated boat-tail bullets are unsubstantiated. In testing no drag reduction compared to conventional boat tail bullets of similar length and angle.

Proponents argue that the sharp 90-degree transition helps prevent muzzle gases from interfering with the bullet's base upon exit, while minimizing base drag in flight, Instead of the angled boattail tapering smoothly down to the absolute edge of the bullet, it steps down into a small, vertical, 90-degree cylindrical section before ending.

Unlike traditional boat tails that channel escaping muzzle gases into a turbulent ball right behind the bullet, the 90-degree rebate acts as a spoiler. It forces high-pressure gas to disperse in a clean, concentric ring outward from the edge. The sharp notch pushes the shockwave outward, away from the bullet. The bullet flies through clean, calm air. Because the gas expands outward instantly, the bullet passes through clear, stable air rather than being pushed and buffeted by trapped gas.  In mid-to-long-range flight, the tapered tail minimizes base drag, helping the bullet retain velocity and resist crosswinds.

 As a bullet flies, it punches a hole in the air. A flat-bottom bullet leaves a giant vacuum behind it. This vacuum sucks the bullet backward. This is called base drag. The sloped part of the rebated boat tail gently channels air inward. This makes the vacuum much smaller. The bullet loses less speed and flies flatter.

Key benefits include, The sudden drop in diameter at the tail allows propellant gases to escape efficiently, which many shooters credit with improving short-range and long-range accuracy.

The initial "rebate" section seals tightly against the bore for a fraction of a millisecond longer, reducing gas blow-by which can erode a rifle's barrel.

Like traditional boat tails, the tapering reduces the vacuum effect created behind the bullet as it cuts through the air.