There are many things to look at when choosing a rifle. There are the cartridge, the barrel, the action, the trigger, the furniture, and the capacity for accessories. I won't detail very much.
are hell to consider because you might not have the opportunity to try and compare before making your choice. You mention .308, which you should learn to call 7.62x51. I do not recommend it. It is almost unbearable in semi-auto (AR) and you will absolutely gain a nasty flinch when firing, making precise shooting impossible (sustained semi-auto isn't as badly affected). This can be curtailed by buying and getting used to a slightly weaker cartridge like 5.56x45, but at that point why even buy 7.62x51 in the first place? Unlike 7.62x51 or 5.56x45, 7.62x39 gets dicked by wind at and past 300 meters. Other options to consider are .450 Bushmaster, .50 Beowulf, .458 SOCOM, 6.5 Grendel, and 6.5 Creedmore. The calibre of the first three are in inches. Keep in mind that you shouldn't expect more than twenty-round magazines for 7.62x51 and the 6.5s, or more than ten-round magazines for the others. All of this is within the AR platform and I have seen bolt-action rifles in everything but 7.62x39. AKs are available in 5.56x45 (NPAP-85).
come in many different shapes and sizes. A longer barrel will give you greater accuracy and bullet-speed at the muzzle, or muzzle-velocity, while shorter barrels are less heavy. A thicker barrel's mass will better allocate the heat given off by the friction of the bullet when the cartridge is fired but can are heavier, whereas shorter barrels can not as easily allocate that heat and so may get too hot, losing their accuracy. Some barrels can come chrome-lined or nitride-lined on the inside, preventing a loss of accuracy, corrosion, or else, while having other benefits or drawbacks that I've no fucken clue about.
can determine the rifle's recoil, reliability, accuracy, and more. Roller-delayed blowback rifles (Cetme, PTR, MP5, Famas) have reputable reliability and less felt recoil, but none have floated barrels. A floated barrel has better accuracy than a non-floated barrel. Gas-operated rifles (AK, AR) have small tubes connected to the barrel partway through, and so never have floated barrels. But every rifle is different, with different lengths of its barrel, and different placement of that barrel's connection with the gas system, making some rifles' barrels closer or farther from floated and more or less accurate than others. Bolt-acion, break-action (Thompson/Center's Contender), and other rifles are easily free-floated and thus more accurate than others. Many non-bolt-action rifles have alot of free-space for dust, mud, dirt, debris, and so on, or space for errors in the design or manufacture of the rifle, making them less accurate (but probably more reliable depending where you take it).
is your most important interface with the rifle. If you want a rifle and want to eventually grind off more inaccuracy, you must only buy a rifle with a tweakable and replacable trigger. There are two types of triggers, the single-stage and two-stage triggers. The single-stage trigger reminds one of the keys set on a mechanical keyboard. Pressure is applied without any reaction by the trigger or key until it suddenly gives in, with all of the pressure you applied slamming it. Two-stage triggers are much different. Pressure is applied to a point where the trigger gives in, though with great resistance, until the second stage that requires greater pressure is met. If you use a two-stage trigger, you will easily learn to flinch. Using a variety of calibers, especially weaker ones, can help to confuse and unlearn the flinching.
are the things in your livingroom that you sit on and the things on your rifle that sit on you, on your forward, supporting hand, and in your shoulder. Don't touch the rifle to your bones. Do pull the rifle into you. The buttstock is that backward end which can look triagular. The handguard is that forward piece that separates your hand from the hot and unwieldy-when-fired barrel. These are the same and only called the stock on a famas, aug, mosin, etc..
The capacity for accessories
is driven by mounts at the forestock or at the side of the reciever, picatinny rails, cobra mounts, etc.. From these can be mounted a wide variety of scopes, lasers, flashlights, sights, and mounts for scopes. Other accessories to keep in mind might be muzzle devices, bayonets, ladle-holders, slings, and so on.
If you plan on buying a cheap, sub-$500 bolt-action rifle, I recommend that you buy an SKS or save up to buy an actually good bolt-action rifle. But this really hits because cheap scopes are garbage. It is said that you should spend just as much on your scope as you should spend on your rifle. If you now plan on buying an SKS, you might consider saving up to buy an Armalite from a reputable manufacturer (you will need to read up like hell), or an imported AK (Cugir, Zavasta, WBP), or any other decent semi-auto rifle (PTR 91, M1A, Mini-14, SU-16). Anything good, even a decent bolt-action rifle and scope combo, costs around $900 or more except for the SKS. If you buy an SKS, remember to fully disassemble it to clean out the firing pin, even if it's already been cleaned. If there is cosmoline around the firing pin, the firing pin will be stuck in place and it will not fire or it will fire all loaded ammo without stopping.