How far off can the cylinder be out of index with the barrel and still be safe to shoot? How does one fix that problem?
As jethunter mentioned, the lockup should be tight when the trigger is pressed. Best is to check the revolver in DA, the whole process should be tight since the trigger is the first motion in DA, right? If not, you have to build up metal on worn parts. If the indexing is slightly off and not tight, it will still shoot up to a certain point, most Webley bulldogs don't tighten up, just use soft lead bullets or if off too much, have work done on cylinder stop and/or hand. But as I mentioned, that problem usually doesn't occur on Webley Mark revolvers.