The key to my Mom's good meatballs is the grinding of the meat. She made some at Thanksgiving without grinding finely, and they were nothing like normal. She bought her meat at a butcher shop, asked for 1 lb ground beef, 1/4 lb veal (optional), and 1/2 lb pork, ground finely twice.
To this meatloaf mixture she added:
1 egg, lightly beaten
1/4 c milk
1/2 c grated onion
1/2 c bread crumbs (italian seasoned ones are ok)
salt and pepper to taste.
I haven't tried grinding just beef finely twice. I usually add a little more to the mixture, like ketchup and garlic. For Swedish meatballs, the nutmeg and other spices may be added.
Place meatballs on cookie pan. Bake 375 F 20-30 min or until browned. freeze until needed. Use with spaghetti sauce or canned gravy (or other sauce of choice). Heat meatballs in sauce 1/2 hour on simmer (finishes the cooking).