The timing issue that several have mentioned is that very often Colt revolvers (1917 and others) will not be quite locked up (cylinder) as the hammer is slowly pulled to full cock.
The "test" is to slowly pull the hammer to full cock and then try to move the cylinder into lock. If it moves just a little bit (rotation) and you hear a faint click, it is a little bit out of time. If it is locked up properly it will not move any more. This is not as bad as it sounds since the Colt lockwork pushes the cylinder into lock as the hammer falls, so it is not dangerous to shoot a slightly-out-of-time Colt.
However, "in time" is better, so if you have several Colt 1917's to choose from, do the timing test and pick the best if the other factors are equal.
(Seems odd to be discussing Colts on this forum.)
Joel