Although this is a wing shooting post the content about correct size loads for the Gauge of Gun applies to all.
This is a post by countryman Jonathan McGee, the reason I am sharing it, is because he shoots his 28g with a cartridge load (16gs) that was originally designed for this gauge Gun to use.
"Shooting Photograph 14 August at 19:05 ·
My first grouse of the season with my lovely Rizzini 28 gauge and Hull Imperial 16g 6s. What a day."
Putting anything heavier than a maximum of a 19 gram load through this gauge gun is completely unnecessary, prolonged use of loads such as 1.oz passes damaging energy (recoil) back on the shooter and onto the mechanics of the gun itself.
Its especially important that the correct load (16g to 19g max) be used for Juniors and Ladies, whether its a 28g O/U or Semi Auto.
Just one final point, do not rely on the shot speed from heavy loads to make you a better shooter, shot speed is no substitute for poor technique, learn to point the barrels in the right place consistently first, before worrying about other things!