I am not a huge fan of the M16 family, and its civilian AR-15 counterparts. I think that it's a dirty system, and the locking lugs (the infamous "star chamber") are annoyingly hard to clean. On the positive side, ergonomics are superb, accuracy is usually very good to excellent, and perhaps best of all, the system is modular. Modular usually means it's easy to exchange one part for a somewhat different part- say, a longer barrel for a shorter one- but in the case of the AR-15, this goes even further. These rifles are separated into upper and lower receiver assemblies.
This becomes especially useful when changing calibers. If you want to switch to another caliber (which has the correct dimensions to fit inside your existing receiver, obviously), it's as simple as purchasing the upper receiver of your choice, pushing out a couple of pins with your finger or a writing pen, dropping the new receiver on, and (if necessary) using a different magazine. A literal minute of time is required to change. Once the lower receiver or receiver is purchased (the piece that the ATF defines as the firearm), the upper receiver assembly can be purchased directly, in person, or through an online retailer. So, if you have an AR-15-type rifle, to switch calibers, you just need the purchase another upper receiver, and possibly, a different caliber magazine.
The price of metals has gone up dramatically in the past few years. This, along with U.S. inflation in general, has led to substantial increases in the cost of ammunition. The least expensive centerfire ammunition now available is surplus imported ammunition, and the least expensive rifle ammunition is 5.45x39mm, available from companies such as Aim Surplus, JG Sales, and Ammoman at no more than $.15/rd, delivered. The 5.45x39mm was the Soviet attempt to duplicate the U.S. 5.56x45mm, and the Soviet round is just slightly less powerful than the .223, with the 7N6 cartridge firing a 53-grain bullet at about 3000 feet per second.
I have long been of the opinion that any centerfire cartridge more powerful than .22 Magnum is sufficient for military use. Smaller rounds mean more ammunition can be carried. More ammunition equals more potential threats addressed. Putting a hole through a hostile target is more important than the size of the hole. If the threat is at closer ranges, put more holes in the target. Simple. In any case, I personally believe that 5.45x39mm is just fine for defending your home or your homeland. (Though if you're defending your home, I believe in using good expanding rounds.)
Smith and Wesson now make AR-15s. They made a 5.45x39mm AR-15 upper receiver and rifle, for a while, to capitalize on the cheap ammunition available for the cartridge. CDNN has the upper receiver assemblies available for $500,
with one magazine included. Additional 5.45mm-specific magazines from CDNN are $15, though standard AR-15/M16 magazines seem to work fine for folks who've tried them. This is an extremely good deal on a complete upper receiver assembly, with cartridge ballistics very close to standard 5.56mm rifles, and the cheapest rifle ammunition you can find. I suggest you get one while you can.