Shotgun "Break In" Lets Take a Quick Look

I would believe that this is true for semi-automatic shotguns and falls in the same category as “break in" myths for most firearms. If the fit and finish of your firearm isn't great, you might indeed need a “break in” period during which all the parts that move “wear in" and the action becomes smoother. Most decent firearms these days shouldn't need a “break in” period, but a lot of people take the “better safe than sorry" approach and do it anyway along with advising others to do it). The recommendation for “high brass" on a semi-automatic shotgun would mean the action is being run more forcefully and so any “breaking in" will be done faster.

If you're using a pump-action, then it doesn't really matter what you shoot to “break it in" because all of the motion comes from the shooter. You could do it faster, more cheaply, and in your own home by just holding the action release an racking the slide constantly for a few minutes (maybe even run it “dry" so the parts have more friction, then lube it up when you're done).