This article is going to address building a hunting style rifle, as custom builds for AR-15s and AK style rifles are very different. These types of rifle builds will be addressed later.
Almost everyone who has messed with guns even on a small scale dreams of building a custom rifle. However, most of us are lacking the tools to properly create and mount a barrel and set the head spacing, as well as perform the intricate work that can be involved when working with a trigger.
An easy alternative to building one completely from the ground up is to purchase a barreled action. These assemblies come with the action and barrel already attached. This takes care of the hard work with the barrel and the trigger, as all these components are already installed. This still allows you to add the stock, scope, and whatever other accessories you'd like to add to make your custom rifle. Most of these come with good, user adjustable triggers installed, but if you'd like, you can also drop in a new trigger pretty easily.
There are several of these available, depend on the cost, caliber, and type of action you'd like to use. One of these is the Howa barreled action, available from www.legacysports.com. These are fairly affordable actions, and have good reviews online. They are available in a wide variety of calibers from .204 to .375, as well as your choice of blue or stainless barrels.
Another option is the Montana Rile Company (www.montanarifleco.com). They offer a similar variety of calibers and options, and are a little more pricey than the Howa. You will find many people online, raving about how good these actions are.
Many people in the US love the German Mauser actions, and look for these when they want to build a custom rifle. If this is the action you'd like, check out Zastava barreled actions. These actions are made in Yugoslavia, and have been imported by everyone from Charles Daly to Remington. They are currently being imported by a company called European American Armory Corp (http://www.eaacorp.com/). These barreled actions are fairly inexpensive, and receive very good reviews. They are also available in a wide variety of calibers.
Stocks are available from many companies online. A quick Google search will bring up a plethora of companies that offer them. A few of the companies I've ordered from include www.brownells.com and www.midwayusa.com.
Stocks and actions may require some minor modification in order to have a snug fit, but most of these mods are pretty simple.
Some of these barreled actions will come pre-drilled for scope mounts, will others will require that you drill to install them.
There are other companies that offer barreled actions, and a quick check on a website like www.gunbroker.com will yield many results in lots of calibers.
The rifle is limited only by your imagination, so be as creative or simple as you want. Feel free to post any pictures of custom rifles in the comments section.