However, I believe that the term originated from artillery and in that context it definitely is equivalent to "ready to fire" and therefore also indicates that a live round is chambered.
The term definitely originated with artillery, and like some other artillery terms, changes slightly when applied to small arms.
watch the guys shoot cannon. The gun fires and recoils back several feet (to several yards, depending) is then reloaded, and pushed back "into battery" meaning, the original firing position, and ready to fire again.
You could push the gun back into battery position, then load it, but considering the nature of the positions that usually meant your gun crew was more exposed to enemy fire, and so most of the time, the gun would be reloaded THEN returned "to battery", so over time, it became the accepted usage that "in battery" meant gun both in position and loaded to fire.
later, when artillery developed recoil mechanisms, "in battery" meant that the gun had finished recoiling, and had returned to its firing position on the gun carriage.
When the term is used for small arms, it means that the gun breech is shut (and locked if the design allows for that). It does not mean the gun is loaded or ready to fire, but that the breech mechanism is completely shut. Further action may be required to cock the gun in order to fire it (like revolvers) but the gun is mechanically "in battery" when the action is fully and correctly shut.
"In battery" means the breech mechanism is in the proper configuration for firing. It does not mean the gun is loaded ready to fire, all it refers to is the breech mechanism's physical position (shut and locked or not)
Some folks expand the term to mean loaded and ready to fire, but that's not the proper usage.
Another interesting difference in artillery terms is the difference in the use of the word "caliber". In small arms usage, the term "caliber" only refers to the size of the bore. In artillery usage, caliber not only refers to the size of the bore, but ALSO to the length of the barrel, in units of bore diameter.
Those big guns on
Missouri class battleships are 16" 50 caliber Naval Rifles. Meaning the bore is 16" and the barrel is 50
caliber meaning 50x16" for a barrel length of 66 feet (rounded off)
Same word, different meaning, in context.