Almost all the o/u I have serviced over the years where the bottom barrel fires and the top does not is caused by :
1 human error , not releasing trigger 100%
2 Trying to shoot too light load or letting gun "free recoil "as to not allowing recoil to make rebound mechanism work.
3 broken firing pin
4worn firing pin and hammer
5 dirty firing pin tunnel or piled up firing pin return spring
6 last is usually an overall dirty gun
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