HopUp is the term used to describe the backspin effect on airsoft BBs (i.e. the projectiles fired by airsoft guns). to increase their range through the Magnus effect.

Hop-up devices give the projectile a backspin, or backward reactionn. by reducing the air pressure on the top of the projectile. This causes the plastic pellet to fall less distance over a given distance than it would without the spin, increasing the effective range of the weapon without increasing velocity.

In airsoft guns, this is usually achieved by a protrusion ("bucking") in the trajectory of the bullet at the top of the barrel. This protrusion momentarily obstructs the top of the bullet, giving it a backspin that can exceed 1,000 rpm. With many weapons, the strength of the hop-up can be adjusted - that is, adapted to the player's skills and demands.

The main cause of the hop-up lift is Magnus effect. This effect can also be observed in various ball sports, such as soccer or handball. In soccer, skilled shooters achieve a curved trajectory of the ball by cutting it. The effect can often be observed in crosses or free kicks. In table tennis, the effect is seen with "cut" balls.

HopUp allows high-quality precision airsoft weapons to fire accurate shots up to a hundred meters away, which adds a very realistic component to the game.