It is OK to dry fire a slingshot, but it may decrease the life span of your bands.
Always match your bandsets to your ammo. Ideally, you want to make sure that the projectile absorbs as much of the energy from the bandset as possible, then there's no excess energy in the bands after you release which can cause painful handslap or adding extra wear to the bands which will shorten band life.
The video also addressed this secondary question:
Why would light ammo not have the same effect as pulling back on a slingshot band without ammo?
- When you dry fire a slingshot (i.e., release the bands without any ammo), the bands crash into the frame of the slingshot without anything behind them, which can "bruise" the rubber.
- If you're using bands that are matched to the projectile (in this case, the clay ammo), then the bands deliver all their energy into the projectile and don't retract away from the slingshot, preventing them from smashing back into the frame.
- If you're shooting over the top and experiencing hand slap, it indicates an inefficient combo. You might need to lengthen your bands or add more weight to the projectile.
- While dry firing or using light ammo with heavier bands might lead to reduced band life, it's not a significant concern given the low cost of the bands. The main point is to have fun and learn from the experience.
Understanding the nature of latex, its limits, the strengths, and the weaknesses is crucial in the sport of slingshots.