Starting cast off through the lock sounds a really bad idea Goo. It was never done in the day.
I know we say never say never, but I say never this time and mean it.
There is plenty of time for cast off in the wrist.
More English and Continental than American, check GBG 1740-90, as it was more common than not on British guns.
On a Longrifle, the same effect can be brought about by a shade more drop.