I know it is a bit long winded, but would you explain in step by step actions EXACTLY what bits you removed and in which order and then which bits and HOW you put them back together. I am not familiar with the 400, but have both a 391 and 303, which have been assembled/disassembled more times than I care to think about. If you have removed the bolt assembly from the receiver, are you sure you are getting the 'rat tail' at the base of the bolt assembly back into the right hole within the receiver (so to speak). Fitted back incorrectly will not allow the bolt to go back fully and lock nor will the barrel go fully home. If all else fails take the gun and the bits to Bisley, once again not a place that I have visited, but I feel certain that there will be someone there who will be able to assist you. You could also Goggle Beretta 400 dismantle and clean there will certainly be a blow by blow account on there, which could help.