Another noun form is "chambering," which refers to the cartridge which fits a particular gun. For example, if a gun is intended for use with the 30-06 cartridge, its chambering is said to be 30-06.
When used as a verb, "chamber" may mean to place a round of ammunition into the chamber. This may be done by inserting a cartridge into the chamber directly from your hand, or by working the gun's action to do so.
Chamber may also be used as a verb to refer to the act of cutting a chamber into a firearm. To chamber a new barrel means to use a special reamer to cut a cavity, or chamber, of specific dimensions into its rear end. The dimensions of the barrel's chamber and bore dictate what type of ammunition may be safely used in it.