I really enjoy a good .410 shotgun, but the ammunition costs have always been double (or more) the cost of good 12 or 20-gauge. Other than levels of demand I have never been able to figure that out.
Old friend has a beautiful 16-gauge Winchester that is absolutely deadly for anything with wings. I am constantly on the alert for 16-gauge ammo to help keep him supplied. Even during "normal" times it can be difficult finding 16-gauge ammo regularly.
I down-sized to 12-gauge only about 20 years ago. Anywhere there is shotgun ammo there will be 12-gauge. A 12-gauge target load (1-oz. shot) will do anything that a 1-oz. load will do in 20 or 16-gauge. Decent quality is available in "promo" shells by Federal, Winchester, and Remington in both target and field varieties.
Over the years I have accumulated hundreds of rounds of 12-gauge ammo. Target loads (#8 and #7-1/2 shot), field loads (#7-1/2, #6, #4, some steel shot loads, and an old box of lead BB-shot), buckshot (#00, #1). My shotgun has 3" chamber but I have never fired a 3" shell in it, never felt the need or desire.
I've been reloading rifle and pistol calibers for 49 years, but never got around to shotgun shells. Always meant to do so, but in retrospect I recognize that I never shot the shotguns enough to justify doing it.
I recently saw a post somewhere on-line that described the efficiencies of 12-gauge shotgun ammo; the box that comes with 25 rounds is just about the right size to hold the two doves I am able to hit with 25 shots!
I'm over 70 and good to go for all the dove, quail, grouse, pheasant, and waterfowl seasons I have left to enjoy.