I would know better if I knew exactly what the MP-153 was actually. Just by your description of the problem I was guessing that's what it could be. But for some types of pistols, rifles, and shotguns with conventional firing pins/hammers, the trigger sear engages in a notch on the hammer. If the sear won't fall in the notch on the hammer, the hammer may ride forward with the bolt instead of hitting the firing pin. Like I say, guessing at your description. I'll see if Brownell's might happen to have diagrams.