With the firearm pointed is a safe direction, and safety on, perform your test using real ammo.
Snap cap are designed to “snap” the hammer or action allowing the hammer or firing pin to fall and spring pressure to be relieved. The snap cap's shape and dimensions may, or may not, cycle through the action of the firearm as live ammo would. There are “dummy rounds” available which replicate the dimensions and weight of real ammo for testing cycling functions.