My Webley Mk VI has been "shaved" and works only with S&W half moon clips. AR brass has too thick a rim to work in my gun. Full moon clips I have tried are also too thick for my gun. My gun will still fire .455 ammo, but ignition is undependable.
Cast bullets are ok, but hard cast are not needed. The loads you should use are in the 600-700fps range, depending on bullet weight. The Webley is sighted for a 265gr bullet, so expect a differnt point of impact with lighter slugs.
Standard GI ball level .45acp loads are a proof level load for the old Webley, do not use them.
Check the cylinder on your gun. There are numbers around the back edge. If these numbers have their bottoms cut off, your gun has been converted to use .45acp BRASS, and Auto Rim brass might work. IF only the very bottom of the numbers is gone, AR brass might be too thick. If about the bottom 1/4 of the numbers is gone, AR brass should work.