According to a rather detailed article written by a veteran armorer of the era the wandering zero was found to be a problem with some No.5 carbines that had suffered spreading or springing of the rear receiver walls. These could be spotted by the key pin of the rear sight pivot being nearly sheared through, which would take some serious springing of the rear walls.
Due to lack of No.5 action bodies for repairs these were sometimes fitted with No.4 action bodies in order to get them off the books and back in service.
It was believed this sort of springing was due to long rapid fire strings (mad minutes) that heated up the action.
Near as I can tell wandering zero is not a problem common to the No.5 nor exclusive to that rifle. It showed up more with the No.5 possibly due to the shorter barrel and relatively coarse sights.
I have fired a No.4 that seemed to have a wandering zero, but when I saw that there was sideplay in the rear sight and corrected it the problem went away.
Some No.4 rifles have a tiny shim to take up any side play of the rear sight, if this is missing the sight may move a hair to either side.
Whether the wandering is entirely due to loose sights, or due to some shift in the bearing of the lugs caused by the springing/spreading I couldn't say.
If you have a No.5 that holds its zero well now, and don't abuse the rifle with hot loads or excessive rapid fire excercises it should continue to maintain its zero just fine.
With handloads taylored to the short barrel a friend acheived consistent sub MOA accuracy out to 600 yards with the issue sights. He's one of the best shots I've ever met, and he had glass bedded his carbine fore end so its probably not typical results, but shows what can be done if you put some work in it.
The fore end was very oil rotted so drawing out all the old oil and glass bedding was the only way to salvage it.
Last edited by Rainbow Demon; January 28, 2013 at 10:17 PM.