The firing pin can go farther into the channel without a cartridge or a snap cap.
The OD of the pin/shaft and the ID of the firing pin channel may be working to capture that firing pin at some point as it travels "too far" down the channel.
Might not be able to do it with a standard firing pin return spring. Combination of the longer spring, stiffer spring probably keeps the firing pin from traveling the same distance forward.
Coil bind is a thing on rocker arm springs (and most other coil wire springs). Could be damaging that reduced power spring, too, if the firing pin is going far enough forward to smack those coils into each other.