A couple of points:
This thread is called "semi-auto 7.62 recommendations" and the poster was asking about a SHTF scenario rifle. Therefore, any rifle that is neither semi-auto nor 7.62 is not germane to the discussion.
I think most of us have assumed the poster meant 7.62x51 since he referred to the FN-AR and FAL.
It is true that no rifle is perfect for all situations. However, my understanding is most of us conceive a SHFT situation to mean a rifle that needs to do it all, without the presence of support weapons or even another person.
As a former infantryman and infantry officer, I have no issue with the M16 variants as a military rifle. It performs its role very well. But it is used in the context of fire teams and larger units, with support weapons available on call - including 7.62x51 and larger weapon. This is not the SHTF situation most people visualize.
Granted, probably the most versatile weapon in a real STHF collapse situation is a 22 rifle, let's stick with the poster's original query.
Military or combat experience may or may not be relevant to the scenario, so we might as well dispense with that requirement right now and try and keep things civil.
It seems that the best place to start is to review operational requirements for the SHTF rifle.
It must be durable. We will assume that repair and parts will be limited to what is on hand. The user should be able to perform all repairs.
It must be reasonably accurate. The lone survivor will need to be able to engage targets as far as possible to minimize danger to himself.
As a follow on, it should be able to mount an optical sight to maximize utility as a long range weapon and hunting rifle.
It must be lethal. The rifle must be able to drop game and neutralize threats. Since we are not limited to military ball ammunition, there is no arguing the lethality of 7.62x51 over 5.56x45, 7.62x39, etc.
It must have enough firepower to neutralize multiple targets, or to provide enough covering fire to unass the area as required.
It must be light enough and compact enough for constant carry and use in restricted spaces. In a SHTF scenario, you shouldn't be house clearing, but you may find yourself in a MOUT situation. Basically, you shouldn't be house clearing if you don't have a team. Solo house clearing is a recipe for getting killed. As a lone survivor you want to avoid conflict, and barring that, keep it as far away as possible.
Given the above, I can really only see two possibilities: the M14 clone or the FAL. Both are reliable, accurate enough and durable. Which the Saiga and the G3 clones are also accurate and reliable, there is a lack of available spare parts. The FN-AR is too new and proprietary to even be considered.
Some will argue you won't ever need parts for the AK rifles, or the G3 types, but that is not my experience. Even AK's fail, however rare that may be. You can get parts for the FAL or M14 type rifle fairly easily and keep replacements handy.
In the final analysis between the two rifles - the M14 clone and the FAL, it is going to depend on what is most important to you: accuracy or reliability. I give the FAL a slight edge in reliability but the M14 wins in accuracy, mostly because the is a whole industry devoted to making the M14 type rifle shoot well for matches while the FAL is fairly new to American shores.
Note that the match grade M14 type rifle is very costly and much heavier than the standard grade version.
If cost is a big factor, the FAL can be had for at least $500 less than a comparable M14 type rifle. When you start adding things like scope mounts, trigger work, match barrels, etc, the M14 gets far more expensive.
In either case, I would look at one of the 18 inch variants. These are generally short enough for urban work, but don't have unreasonable muzzle blast or give up too much velocity.
I didn't include the AR-10 rifles in this list, but they are certainly viable alternatives. There is still some variation between makers that you have to be aware of when buying parts and magazines. When it comes to accuracy, the AR-10 is king, with examples of MOA or better guns being relatively common. If you are already a good marksman, I might lean towards one of these rifles but I would build one as a dedicated DMR type rifle, rather than a general purpose SHTF rifle. The AR-10 is just less rugged and durable than the FAL and M14.
Finally, keep in mind that the most likely SHTF scenario is some sort of natural disaster that won't end civilization. In such a case, wandering around with a rifle may be counterproductive. Something you can easily conceal on your person is probably more useful for the type of disasters we've seen in the past.
Whatever rifle you do select, make sure it is something you like so that you will shoot it enough to be proficient. The most important part of the system if the person behind the trigger.
