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By Grant Woo
The simple truth is Bore-Up kits do not work. I'm not saying that the idea of a larger bore being beneficial is wrong, but the shape of the piston heads are all wrong. The convex head leads to turbulence and lower fluid flow rates. I’ll explain why.
Friction is everywhere. Even air has friction. Friction leads to deceleration in moving objects. As air moves down a tube the air molecules experience friction against the sides of the tube. This friction causes the air near the walls of the tube to slow down. The air in the center (away from the walls) moves faster because it does not run into as much friction. This results in a convex flow of air as it travels down the tube. (see diagram below)
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The convex flow of air will cause turbulence because, air molecules will flow faster near the center of the tube, and slower near the outer perimeter. When you have an object moving at a faster speed on one side than on the other you will experience rotation. This is similar to how tanks steer; by moving one track faster than the other the tank turns. Also, when an object spins it wants to change its trajectory. Imagine your gun with too much hop-up. The bb's end up curving rather than shooting straight. This is exactly what happens to the air molecules; they curve out towards the walls of the tube instead of moving straight. (see diagram below)
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The greater the curve of the airflows convex shape determines the amount of turbulence. This is because of the difference in velocities on opposite sides of the air molecules is greater. The greater difference causes more spin, which results in more turbulence. Using a convex piston head increases this curve and causes even more turbulence. This results in very inefficient airflow, which in turn results in a lower muzzle velocity. This is because the air is not pushing directly on the bb, thus wasting energy that would other wise accelerate it. For example, if you were to push a box on a smooth surface, it would go farther if you push it straight than if you were to push it at an angle. |
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Why do they make convex piston heads? Well, tapered cylinder heads help reduce turbulence. Because of this, people believe that the convex piston head will help use all of the air in the cylinder, instead of leaving a small portion of unused air in the cylinder. The truth is that trapping air in the cylinder can act as a buffer to make the gun quiet. Look at the VSR-10. The air brake may leave air in the cylinder and lower the velocity slightly, but it makes the gun quieter. Basically, the extra energy gained by using a larger cylinder is mostly lost to inefficient airflow.
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