[See Part 1 for receiver options, Part 2 for a documentation of the fiddly bits, and Part 3 for a discussion on bolts.]
Everyone knows that if you take a football and just huck it in the air, it will spin out of control and come down Lord-knows-where. So what do you do? You impart a bit of spin to it as it leaves your hand, and the nose stays forward and it goes more-or-less where you want (assuming you have any skill at throwing footballs, which I do not).
The rifling in your gun’s barrel provides much the same effect. Modern bullets are not round balls – in fact, they more resemble either teardrops with the rounded bottom truncated (in the case of most rifle cartridges these days) or elongated hemispheres camped out on top of cylinders (in the case of pistol cartridges). Without some way of stabilizing those shapes, the wind velocities they encounter – sometimes beyond even the speed of sound – would be sufficient to knock the bullet entirely off target, off profile, and off everything else, which kind of defeats the purpose of having a firearm to begin with. Fins are right out, given the scale, so we moved on to another alternative: gyroscopic stabilization.
As a bullet is spinning through the air, that spin causes the mass to resist any movement off the axis perpendicular to the direction of rotation (said axis also being coincidental to its axis of movement, at least to start with). In other words, the spinning causes the bullet to resist tumbling.
And the rifling imparts that spin. Rifling comes in two major flavors – "land and groove" rifling, which basically amounts to a series of grooves cut helically down the length of your barrel, or "polygonal" rifling, in which the cross-section of your barrel actually resembles a hexagon or other polygon – but the point is that it twists its way down your barrel, and due to its absurdly tight fit to your bullet, and the fact that the bullet expands to fill in what little space there is, it drags the bullet along with it.
But just spinning the bullet is not the whole story; you have to spin the bullet enough. "Enough" is a concept relative to the specific bullet and its diameter, length, and specific gravity, but the only thing you really need to know is how that spin is expressed. As you know, rifling twists in the barrel, and that helical design is expressed as "X twists in Y inches". X is almost invariably "1", and Y depends on the bullet used, but the not-so-obvious detail is that the lower the Y, the faster the spin – a 1 in 12 barrel needs 12 full inches to spin a bullet all the way around, but a 1 in 9 only needs 9 inches, resulting in a faster-spinning bullet (in fact, assuming two bullets with equal muzzle velocities, the 1 in 9 bullet would be spinning 33% faster, as you might expect).
Finally, barrels come in a variety of lengths, starting at the 16" federal minimum necessary to avoid having to pay a stupid-assed $200 tax stamp to a corrupt federal agency, all the way out to 30" if you really think size matters. Does it? Less than you might think.
Still with me? Great.
When it comes to .22LR rifles, here are the only things you really need to know: If you plan on shooting standard .22LR ammunition, your barrel should have a twist of 1:16. If you plan on shooting subsonic .22LR ammunition (because you own a suppressor or just want the quieter round), your barrel should have a twist of 1:9 (due to the greater length of the round). Finally, standard .22LR ammunition reaches its maximum velocity after about 13"-14" of barrel, but does not experience significant decreases in speed (if any) until after 18" of barrel.
One last detail: 10/22 barrels come in two distinct flavors – "factory taper" and "bull / heavyweight / 0.920" / etc.". Basically, what this means to you is "does the barrel body get narrower towards the tip, or is it the same diameter from stem to stern?" If the former, "factory taper", if the latter, "bull barrel". Why does it matter? Some stocks are only set up for factory taper barrels and you will not be able to drop a bull barrel into those without some serious sanding or shaping. Likewise, only factory taper barrels can have iron sights mounted on them (at least from what I have seen), so you will have to keep that in mind when you chose how you want to aim your rifle. Aside from that, though, the added mass of the bull barrel supposedly reduces vibrations, recoil, and barrel whip, resulting in an arguably more-accurate, heavier rifle; I have never sat down with otherwise-identical-rifles-with-bull-and-factory-barrels and made the determination for myself, but that is the story.
So where does this leave you? With way too damned many choices for me to even bother going through them all. Here are some important takeaways, though:
1. Barrels are very much a "you get what you pay for" market. In general (there are exceptions), a cheaper barrel will perform less well than a more-expensive barrel. Does it matter? In other words, do you, personally, shoot well enough to see the difference between the two? Do you want to pay for the ammo necessary to see the difference? Those are questions you are going to have to answer yourself.
2. Pay attention to how the barrel is chambered. Yes, .22LR is generally .22LR, but some chamberings specifically recommend against CCI Stingers or hyper-velocity rounds in general (some barrels also do not like high-velocity rounds, so really pay attention). What do you plan on shooting through it?
3. Likewise, pay attention to the break-in instructions for the barrel. Honestly, I have no idea if it makes a difference or not, but people in the know think it does, while other people vociferously disagree. Me, I tend to listen to the manufacturer.
4. What do you plan on using the rifle for? The barrel may be the single heaviest piece of the rifle, but if you are benchrest-only shooting it, that hardly matters. If you plan on humping it across hill and dale, however, you might want to start paying attention to mass.
5. Read all the reviews you can. The people writing them may not have a damned clue what they are talking about, but you are bound to discover something you were otherwise unaware of (this is actually how I found my eventual barrel).
6. "Tensioned barrels" are the new (at least to me) competitors/replacements for bull barrels, and if you are considering one of the latter, you should really look at the former. The concept is pretty simple – you take a narrow-profile barrel, you thread it at both ends, you slide a lightweight, high-compression-strength sleeve (typically carbon fiber or aluminum) over the barrel, and then you use a nut at either end to clamp down on the sleeve. The sleeve resists compression, pulling the barrel inside of it taught, and theoretically the entire rig results in the same reduced vibrations, reduced barrel whip, and reduced deformation you see in full-weight bull barrels, but at a fraction the mass.
7. If you are planning on a suppressor, get an appropriately-threaded barrel (duh).
8. In the end, especially when it comes to .22LR, the majority of barrel "features" are for purely aesthetic purposes, and you will probably never notice the effect, if any, they have on your shooting. Find a good name, then decide what you want it to look like.
What did I end up buying? A WhistlePig GunBarrel Company 20" Matte Black Ported Octagon Barrel. Why? Because octagon barrels are cool. No, seriously; Better Half actually asked me on the way back from a shoot if anyone made the classic octagon barrels for 10/22s and I replied that I did not think so. A few days later, I was browsing the intertubes and stumbled across a link to WPGBC, then discovered they actually did make Octagon barrels, and after that, it was all over. It does not hurt that WPGBC gets really good reviews, and my own shooting shows why.
(Tensioned barrel image borrowed from Brownells. Octagon barrel image borrowed from WPGBC.)