If I can convince the owner that I am not a TOTAL jerk, I am a firm believer in the pop a pencil out the barrel trick. This proves that the action/firing pin will actually bust a cap.
Pull hammer back, hold 38/357 revolver pointing upward, drop pencil with eraser down barrel, pull trigger. Should launch the pencil out and up a short distance. This proves firing pin will bust a cap.
Note that lightweight snubbies often will not bust harder caps ((CCI) reliably, but should bust softer caps like Federal. Just a fact of life.
I would not trust a gun/ammo combo without at least 100 rounds no/fail firing test.
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