Since you seem interested in lighter calibers, I would suggest 22 Mag (WMR) as an option. During the last ammo shortage, there were almost always 22 Mag ammo available at Walmart, but this time it has been different.
The other common caliber is obviously 223 but ammunition availablity today is sketchy and when you find it, it tends to run close to $0.50 per round. It is certainly fun to shoot in a semi-auto rifle but the ammo count can rise rather quickly for an afternoon's outing.
If you are thinking long term about your needs, you might consider a handgun in 9mm, 40 S&W (semi-auto), or a revolver in 357 mag/38spl for home defense needs. Shooting 38spls are a lot of fun and the recoil is not significant in most normal sized (vs pocket sized) revolvers.
Get what appeals to you and your needs, but try to stock up a bit after ammunition inventories return back to normal so you have some around the house if you want to do some shooting.