While the receiver may legally be the "firearm" part of the gun, it is arguably the bolt that does all of the actual "work" in the device.
It is the bolt that strips a new round from the magazine, seats it in the firing chamber, seals the firing chamber (inasmuch as .22 rifles do so), strikes the rim of the cartridge with its firing pin, extracts the spent cartridge, and then does it all over again. Unsurprisingly, you can spend as much – or as little – as you like on this particular part.
To begin with, you can go with the standard Ruger bolt which comes in every 10/22 you find on the rack in a gun store these days, and while that is the choice I personally decided on, it is a little… scary. Apparently these new bolts are cast instead of machined, and there are visible marks and arguable defects on pretty much every surface. The bolt seems smooth and time will tell whether or not it does what it needs to, but it definitely looks the part of "cheapest of the lot". Additionally, as you can see in the pictures at the Brownells listing, there is a strange square protrusion sticking up on the top of the bolt at the back by the ramp; I actually had to grind that entire protrusion off the bolt* and make the entire top surface level in order to get it to cycle in my receiver. Is that the fault of the bolt or the fault of the Tactical Machining receiver? Well, while the bolt was quite snug, it would barely cycle in Better Half’s Magnum Research receiver, so it is hard to say. In either case, I hope non-Ruger bolts would not have the same problem.
Speaking of aftermarket bolts, it just becomes a matter of how much you want to spend versus what kind of features or materials you want. Arguably one of the companies that sets the standard for 10/22s, Kidd offers a bolt machined out of tool steel, hardened, and polished to ensure perfect looks, function, and durability. On the flip side, certain receivers (specifically, the one I bought) do have to be slightly fitted to be able to accept the bolt. Another industry leader, Volquartsen builds their own entirely-custom bolt with attached operating handle and specialized firing pin, though not many parts off it will be compatible with anything else you have (I assume). Cost is the primary reason I went with the standard Ruger bolt, though; it runs about $30, aftermarkets start at $100 and go up fast.
Additionally, there is no shortage of folks who offer to do all manner of things to your bolts or bolts you buy through them; it all depends on what you want or what you think works, because I honestly have no idea what, if any of it, makes a difference. Lord knows I would love to try to polish the bolt (and I ended up polishing a lot more than I thought I would in order to get rid of that peculiar hump), if only for aesthetic reasons, but something tells me the best way to do that is to put a few thousand rounds through it.
One thing I do know that makes a difference, however, is the bits on the bolt itself. If you, like me, go the bargain basement route on your bolt, I would strongly recommend replacing the extractor with something that will actually continue to work properly for many, many rounds in the future. The stock Ruger extractor seems to have a marked tendency of losing its tip over time, which can result in Failures To Eject the casing from the breech, Failures to Extract the casing from the chamber, and a host of other problems. Go ahead and spend the money to save yourself the aggravation in the future (especially if you are attending an Appleseed shoot as well, where FTEs of either sort can kill your qualifying shoot).
Likewise, some folks recommend upgrading the firing pin while you are at it. I chose not to go this route, simply because I have heard of significantly fewer problems with firing pins than I have with extractors, but it is something else on the table for your customization dreams.
Finally, there comes the small matter of actually operating the bolt in question – after all, it is not going to chamber the first round out of a magazine by itself. This choice is actually a bit more important than it superficially might look, simply because the operating handle assembly also includes the bolt spring and spring guide responsible for returning the bolt to battery after every shot. Your choices include decisions about handle shape and material, spring characteristics, whether or not it is a captive spring and, if not, what kind of method is used to hold the spring in place (crimping, c-clips, etc.), and so forth, and, really, all of that comes down to personal preference.
I ended up going with one that included a Wolff Extra Power Spring just because I know that brand works well in almost everything else it is put into, but, in retrospect, I probably would have preferred a captive spring of some type. The guide rod for the non-captive spring is predictably shorter than it would be if it had to catch the handle and spring, which makes removing the whole assembly easier in one regard, but then you have to be careful to catch the spring before it launches various bits everywhere, which makes removing it more complicated.
It is kind of impressive how many options there are for such a small piece, but, like I said, it does tend to do most of the work in the gun. Next up is the tube that gave rifles their name.
(* – Yes, I used a Dremel, or at least a Black and Decker knock-off of one. Yes, I know you should not use Dremels on firearms, but, damnit, it worked, and I really did not want to get a gunsmith involved on the build. Realistically, I should have used a bench grinder to keep everything straight and level, but that is yet another garage tool I lack.)
(Images of bolt, extractor, and operating handle borrowed from Brownells.)