Hammer falls on half cock.

[Pistol City]

I have an 1858 New Army Remington BP. I sent it off to have conversion cylinder timed. Got it back timed perfectly. Now it will not hold in half cock. Put in half cock and pull trigger and hammer drops. It also cocks in about 4 different positions. Talked to gunsmith and he swears he never broke gun down just changed out cylinder cap.

 

[AlexanderA]

Debris may have gotten into the half cock notch on the hammer. The first thing I would do is take the gun apart and clean out all the internal workings.