Two different discussions going on here. The upper can be reassembled to the lower with the hammer in the fired position by setting the upper on and sliding it back slightly to make sure the hammer hasn't caught on the back of the FP. I almost never put the hinge pin in and lever the upper closed FWIW.
The BCG can't be assembled into the upper with the bolt "retracted". The cam pin will prevent the assembly from entering the upper.
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