Here some data provided by RSI, using their prototype multi-gage module. I think the gun was a bull barrel .223 with a jacketed bullet. I don't know any other details, and I don't know how accurately the strain gages work for pinpointing bullet position.
1) The bullet is confirmed to slow acceleration as pressures drop just prior to a secondary event, then accelerate as the pressure event occurs.
2) Exit time calculations are valid....for copper.
3) There is minimal reduction of secondary pressures read at the chamber in this fat bull barrel. Chamber readings were nearly identical to those down the barrel.