Some cycle everything fine and some don't due to differences in strength of the action spring from gun to gun. And some which don't initially, get better as the spring ages and compresses from use. As a general rule most seem to cycle anything 1 1/4 oz of shot or more. Mossberg just doesn't make any promises regarding any 2.75" shells because the long action and spring aren't tuned for it. Just expect some possable failure to ejects and your good to use 2.75" for sighting in. You can always check your cocking indicator after each shot to see if the action fully returned and reset the hammer.
Easiest thing to do while sighting is just don't put any shells in the magazine. Single load each by pulling the action back, dropping a shell onto the lifter thru the ejection port and dropping the action with the release button.
