Building something?

So I'm thinking about starting up a little fun project. Since I don't want to burn through my ammo reserves and I'm not rich enough to add more to them, I'm considering building a new toy. Nothing fancy, just something AR pattern that wouldn't be pricey to have fun with if the rush ever dies down. I've got a Beretta cx4 so I'm planning on avoiding 9mm. I haven't looked into it much yet but it seems like 45 is really the only other big option out there?