.22 long rifle.
Buy a Ruger 10-22. There literally thousands of aftermarket customization parts. You can buy one that is a take down model, so it stores in a very compact space. Good for camping, travel etc.
Ammo is cheaper now than it has been in years as well. And - coincidentally, AR-15s are cheaper than I have ever seen them. One effect int eh change of governance via election I suppose. so while you are at it...consider a .223/5.56mm chambered black rifle. They are among the cheapest ( if not THE cheapest ) rounds you can buy and are quite potent for such a small caliber. Great for fox, coyote, ground hog etc....