The standard steel used for most rifle barrels is 4140 steel, which is a chrome-moly steel (chromium and molybdenum are added to the alloy of iron and carbon).  Mil-Spec guns like the M16 and M4 require 4150 steel, which has a higher carbon content, and retains its strength at higher temperatures (this is important for full-auto or rapid-fire semi-auto guns, but not an issue for guns with slower rates of fire).  4150 is a bit harder to machine and is more expensive.  Both 4140 and 4150 can be chrome-lined, which is when an industrial hard-chrome plating is applied to the chamber and/or bore of the completed barrel.

Most match barrels these days are made from stainless steel barrel blanks, usually 416 stainless.  It is easier to cut the rifling accurately on a stainless barrel, and that is one of the keys to a super-accurate barrel.  Obviously, a stainless barrel is also more resistant to corrosion than one made from plain chrome-moly steel.  The downside is that stainless barrels have a shorter lifespan compared to chrome-moly, and *especially* when compared to a chrome-lined chrome-moly barrel.

When choosing a rifle configuration, always start with figuring out what you want to do with the rifle (how will it be used?  Rapid-fire or slow, deliberate, accurate fire?  From a bench, or crawling around in the mud?  Etc.).  Once you can define how it will be used, you can choose the config you need.

Know that there is no configuration that's good at everything, and to get more of one characteristic, you usually have to sacrifice another.  TANSTAAFL!

-Troy

Stainless is indeed a very common choice for a varmint barrel, which needs to be very accurate and plenty long to impart a lot of velocity on the ammo.  For this type of gun, accuracy is more important than reliability, so a tighter chamber and slightly less headspace is okay.  You'd also want a 1:9 or slower headspace; 1:8 or faster will be less accurate with ligher varmint bullets.

-Troy