Thanks for the replies. Because of the shape of the bolster, I did not think that I could get all of the bolt behind the cock
actually I mean in the flatten area cast in the plate behind the cock. I did not like the idea of the drill hole being thru the edge of the flattened area
as I thought the drill would wonder.
I ended up drilling between the fence and the edge of the flatten area, This brought the bolt quite close to the back end of the barrel.
I would say now that I should have for seen this in the planning stage. I seen how important it is to look at the geometry of the lock being used.