204Ruger

While we can, on line ordering is a good way to go, when stores are limited in our area, with supply. Another one that usually has items and good prices is Powder Valley. Bill K

I second Powder Valley, they are close to Colorado and give great service and pricing
Spud

I can't really address overall popularity, but I can say I did a lot of research for a 20 caliber and when it came to readily available Ar barrel, ammo, and brass without a forming hassle, the 204 won easily.
I bought a box of ammo at my local gun shop and he said that was the first box he sold! (He has been open for several years.)

Somebody here pointed me to bangitammo.com, and I got 50 rounds of Hornady 32 or 40 grain for $25. Considering that 20 rounds are just under $20, that's a pretty good deal!