for a quick chamber cast use candle wax. put a paper plug into the barrel, maybe 2 cm after the chamber ends. oil the chamber with gun oil and heat the candle wax. don't need cocking just enough heat the wax is fluid. mould the wax into the chamber, first only a little bit so the paper plug can soak it up. waiting two minutes.
than mould the wax into the chamber. be careful, the wax is shrinking when it becomes cold, wait and mould again if nessecary. the chamber cast will not hold 0,1 +/- mm but its enough to see the desig of the case and measure the length.