As others have said, must be purchased as a complete rifle from the factory and be unmodified except for the allowed modifications outlined in the
rule book. You want section 2.3.2, which includes the following text:
"a rifle is defined as the
original manufacturers configuration of a complete firearm which will be comprised of at least but not limited to the following: stock with bottom metal or chassis, a complete action, a barrel and a trigger mechanism."
For those curious, you can now have a factory barrel threaded by a gunsmith, assuming no other modifications are made to the action (no truing, etc), you can "safely tune" factory trigger but not replace it, and you can have your action rebarreled by the original manufacturer with an identical replacement barrel only - no aftermarket, prefit, etc.