When Max knocked Noodles out, he went away for his score but due to information given to the police by Noodles, they weren't successful (as we know upto that scene) and were shot dead by the police:
Minutes after calling the police, Max knocks Noodles unconscious during an argument. Regaining consciousness, Noodles finds out that Max, Patsy, and Cockeye have been killed by the police, and is consumed with guilt over making the phone call.
But it was all Max's trick, he explained it to Noodles later in the movie:
Max explains that corrupt police officers helped him fake his own death, so that he could steal the gang's money and make Deborah his mistress in order to begin a new life as Bailey, a man with connections to the Teamsters' union, connections that have now gone sour.
How much time elapses between the body-bags scene and the opium den, where Noodles already has a newspaper with the latest news?
If you are referring to the last scene of the movie, then it was just after Noodles found the dead bodies of his friends. The opening and the ending scene are sequential:
The end returns to the opening scene in 1933, with Noodles entering the opium den after his friends' deaths, taking the drug and broadly grinning.