As stated in my post, it really doesn't matter. Any caliber is acceptable.
But, different cartridges do different things. An AR in 5.56 will reach out 4X further than a pistol.
First thing to decide is exactly how far out you expect this "rifle" to reach, and what kind of target. Plinking is fine, but a live target - deer or human - needs to reach at least 250 yards. No pistol cartridge will do that ethically for game, which needs to deliver enough power downrange to stop them so they can't run off wounded and get lost.
Basically, 5.56 at the minimum which will do. Next, some way to aim accurately that far, which will lead to either extensive open range practice out past 200m or, an optic.The latter works faster more accurately, the upper will need a rail to mount it. That makes it a flat top upper. While a lot of folks suggest a railed handguard is a good thing, nope, it's mostly looks, as there is no way to bridge an optic from upper to handguard that will prevent damaging the optic, and if it's out there completely, then any barrel movement would move the optic. If you free float it - a different handguard not attached to the barrel, then any pressure from the sling attached will move the point of aim, and if you attach the sling to the barrel, you move the point of impact.
So, get a comfortable handguard and don't bother with a long rail on top, it causes more trouble than its worth. A no rail handguard which can take a short rail for a ;bipod or light is ok - in MO, not lights are allowed during hunting season. YMMV. That leaves a flat top with rail on it and smoother handguard of your choice with the option later to add something on.
Not forgetting the barrel, a standard military barrel is 2MOA and they are tested for acceptance. If more accuracy is wanted then a precision barrel should be researched, those can shoot as small as .50 MOA, which is a half inch group at 100m. For the most part, hunting or human, you would be aiming at an 18 inch circle which is center of mass, and most shots in that region will usually stop as internal organs are hit which cause enough bleeding to ut them down.
We all pay for the most accuracy we can afford, but with game or humans, it doesn't take as much as it would to win a competitive match. The point of competition is to make it harder, thus separating those with skills into tiers of expertise. The reality is that some shooters buy all the accuracy they can, yet, they really aren't that good. They can't shoot to the level of the gun. As an upper tier barrel for an AR will run over $400 and some past $700. They are also short lived when used in the rarefied upper ranks, after a few thousand shots it can be seen to slowly lose accuracy, what was a .25 barrel on a good day slips toward .50 and gets replaced. I wish more could come my way, they would be great hunting barrels as long as I accept that I am the only reason I missed.
Note we haven't even got to the lower yet, as ammo is the next thing which controlls accuracy. What's nice about 5.56 is that you can shoot cheap steel cased ammo 7.99 a box and a lot of it in an afternoon banging away at some larger targets, or you can buy $50 a box target ammo expecting to see your groups improving after a year's practice. But - the barrel has to be capable. Commodity barrels rarely are better than 1 MOA.
The lower? For the most part you buy a rollmark. All a lower does is hold the trigger parts, house the magazine, lets your hand rest on the grip, and hold it to your shoulder to sight in better and steady it. Recoil isn't even a thing with 5.56, soldiers in Basic were shown how to trainers would shoot them with a metal buttstock against their chin. It rarely leaves any mark.
There is a lot of brouhaha about lowers, features, options, quality, etc. If the mag goes in and stays until you push the button, then it drops out, its working. If you rotate your safety and it fires when its supposed to, and won't when it shouldn't, it's working. If the stock stays attached and the grip doesn't fall off, it's working, More than that and somebody is trying to sell you something, in this market it's usually style or some kind of ambidextrous control that only the ambidextrous find useful. If you aren't, don't bother.
Slings are an accessory which help you carry your rifle, or sight it will less wobble, but they are also snagmasters in heavy brush and will bend the point of aim away from the point of impact when improperly used. Buyer be aware. The Infantry School in the 1980's just had us take them off to avoid all the problems.
So, choose how far you want to shoot, what kind of target, then what cartridge, how good a barrel, what sighting aid you prefer - the rest is a lot of eyewash, which most of the AR fan club will argue to kingdom come. But that is what the upper tier shooters will say, and I listen to them. I did a bit of that in high school, then went Infantry, and the precision target discipline really has nothing to do with combat or hunting at all. Be careful of who tries to insist it's all the same thing when it's defintely not.
What range, what target? Tell us that, we can tell you what works for what you want, and save you some money in the process.
