In the past installing a magazine tube extension on one of the 870/1100/11-87 series of shotguns was as simple as removing the mag cap prying up the magazine spring retainer and swapping out springs before screwing on the extension.  With the advent of the detent style magazine spring & mag cap retention system this is no longer possible.  Now one has to find a way to remove those detents so the follower can slide past them.