The top turret on U.S. planes used quite a more complex mechanism than was on a Lancaster. You can see images and a discussion of the electrical device here:
http://warbirdinformationexchange.or...c.php?p=513302
The device maintenance (in particular the alignment) had to be done by ground personnel but the turret operator was suppose the check the alignment before missions.
There is a section in this next link on the test procedure that was "suppose" to be done by the top turret gunner. Search for the phrase:
Testing the Fire Cutoffs
http://www.ibiblio.org/hyperwar/USN/...y/TURRETS.html
As Bill has mentioned, if the device "drifted" and was not tested by the operator, bad things could happen.
One advantage of this device over the Lancaster arrangement was that it did allow the top turret to shoot between the twin rudders of a B-24, for example. (See the diagram in the test procedure link). It looks to me that the mechanism used on the Lancaster would not allow that. I don't see twin bumps on the cam ring to start and stop the guns as they swing across the rear of the plane.