Bullet
Bullet designs have to solve two main problems. A bullet must fit tightly in the gun's bore (inside of the barrel). If it does not fit tightly, gas from the burning gunpowder leaks past the bullet and does not help push it out. The bullet must also fit tightly against the rifling (spiral grooves cut into the barrel), but not so tightly that it causes damage. Bullets must have this tight fit without causing too much friction.  Bullets must be made to a high standard. Poorly made bullets may not be accurate. Poorly made bullets can damage the gun or hurt the person using the gun. The way the bullet behaves in the barrel is called 'internal ballistics'.