If you choose an older Military surplus caliber you should be well stocked for that caliber. Also, remember one of the great lessons learned by the south in the War Between the States... “Live off the land, resupply off the enemy”. I don't think our enemies will be shooting .300 Winchester Magnum.
Sidearm - If you choose a secondary weapon it should be of a major caliber: 9mm, .40S&W, or .45acp for semi-autos, and .38spl/.357mag for revolvers are going to be the most common. Be sure to choose a reliable handgun with a well established reputation. A 1911, 92FS, Glock, or S&W would serve you well (as would many others).
Spare Rifle/Pistol Parts - The loudest sound on the battlefield is not a bomb going off or the belch of a machinegun, but the unmistakable "click" of a hammer dropping on a firing pin without the expected crack of the bullet breaking the sound barrier. You should know your weapon, it's weaknesses, and what parts are prone to failure, and have those parts on hand.
Ammunition - You should carry as much ammunition as you can comfortably bear. Obviously, weight will be a concern, and while .308 win has more punch than .223/5.56, you will not be able to carry as much of it. Most folks will carry about 200-300 rounds on their combat load. If you opt for a sidearm, you should have at least 3-5 magazines (or speed-loaders) for your sidearm in your Level three/72 hour kit.
Ammo Cache - 1,000 rounds for your primary weapon. You should have as much as you can afford, and keep adding to it whenever your budget permits. If you opt for a sidearm, you should keep plenty of ammo on hand for that, too. 200 rounds is a good start.
Weapons Cleaning Gear - For obvious reasons. If you don't keep it clean, it ain't gonna work.