When you need this gun things will be bad. Military rifles can be used for hunting, but their main purpose in life is to kill people. I hate the thought of having to do that, but I hate the thought of losing my family even more.
Modern military rifles (by this I mean WWII vintage and newer) are generally designed to be simple to operate, to be reliable under adverse conditions, to have man killing knock down power, and to hold plenty of ammo. Nearly all standard issue military rifles manufactured since WWII are at least semi-automatic, and some have full auto capability. Full auto models are illegal for civilian ownership unless you buy a special license. Most manufacturers of military rifles make semi-auto civilian versions of the same rifles. Personally I think semi-auto is enough. Full auto burns a lot of ammo, and most people can't control the point of aim after the first few rounds. I believe that the U.S. military now advocates the use of three round burst rather than full auto. The volume of fire doctrines of Vietnam have pretty well been discredited; and accurate, aimed fire at a slower rate has been shown to be more effective in combat.
There are quite a few semi-auto versions of military rifles that available one the civilian market. Probably the three most popular in the U.S. are the AR-15 (civilian version of the M-16), different variants of the Soviet SKS, and different variants of the Soviet AK-47. The Ruger Mini-14 is also very popular. It is a carbine size rifle that fires the .223 military round. All of these are good solid weapons.
The AR-15 is chambered for the .223 cartridge. It is well made, with good finish and close tolerances. Thirty round magazines are readily available and the ammo is moderate to cheap in price. Some feel that the close tolerances of the AR-15 make it subject to jamming, and it is probably not a good idea to use cheap ammo in it. The .223 round is considered by some critics to be too light for a military round. The biggest drawback to the AR-15 is it's hefty price tag (in the $1000 range). Pictured below: AR-15
The SKS and its variants are a WWII vintage Soviet design. Many of the rifles were manufactured in China, Yugoslavia, and other Soviet block countries. The ones that I have seen and fired have been pretty well made. They are very utilitarian, nothing fancy, not the best finish, but the seem to be well machined and the one's I fired were surprisingly accurate. The SKS holds 10 rounds of 7.62 x 39 ammo that is loaded via a stripper clip. This low ammo capacity and the method of loading are probably the biggest draw-backs to the SKS. You can buy after market 30 round magazines for the SKS. I have not personally tried any of these, so I can't comment on them. I have had a couple of reports of them being somewhat clumsy to use and not feeding all that well, but like I say I haven't used one myself. One of the biggest pluses for the SKS is the price tag. At one time you could buy them for $89. Even in today's market you can still find them for around $200. Pictured below: Soviet SKS.
The Soviet AK-47 and its many variants is the most widely manufactured firearm in history. The AK-47 fires the 7.62 x 39 round. It is commonly available with a 30 round magazine. The quality of the AK varies widely. The finish is nothing to right home about. Many of these rifles have laminated (read plywood) stocks and had grips. The tolerances on the AK are very loose. Some of them actually rattle when fired. But it is because of the tolerances that the AK seems so impervious to jamming. I have seen a demonstration where a loaded AK is buried in the sand, pulled up out of the sand, and fired; without even blowing it off. AK's vary in price along with the quality. You can pick up an AK variant for around $450. I have one of the Rumanian manufactured variants which is totally plain Jane. It is surprisingly accurate, and I have never had a jam. Pictured below: Soviet AK-47.
The Ruger Mini-14 is a civilian weapon. The Mini-14 is manufactured by Strum Ruger and is a scaled down version of the M14 military rifle. Mini-14's are chambered for the .223 U.S. military round and 5.56 x 45 NATO round. The Mini-14 is used by civilians and many police and security forces, but it is not in used by any of the world's major military establishments. It is a good quality gun, as are all Ruger products, with good fit and finish. High capacity magazines and lots of after market accessories are available for the Mini-14. The price tag on the Mini falls somewhere in between the AK and the AR-15. Pictured below: Ruger Mini-14.
My recommendation for a homestead military rifle is the AK-47 or one of its variants. These guns are ugly but, in my experience, accurate and reliable. Ammo is plentiful and inexpensive ( $5 for 20 rounds at this time). One of the best things about the AK is that you can buy two of them for the price of one AR-15 and have some change left over.
So that's my picks for the five firearms that a homestead needs. You may disagree, and you may have good reasons; but I don't think you would go too far wrong with the guns that I have recommended.