The English saying, full bore, means to the fullest extent possible, as in "To win the war, each soldier had to fight full bore." It also means to do something completely or thoroughly, such as "It was a full bore investigation that utilized every detective in the police force." Additionally, it can mean to move at the greatest possible speed or with the most power, as in an automobile traveling at top speed. Great force or effectiveness are usually implied.
The expression can be used as an adjective, an adverb, or a noun. In each case, its meaning reflects full capacity or power. It has traditionally been a British idiom, but it is now more widely used in English throughout the world. It is defined by most dictionaries.
The etymology of full bore can be traced to its use as a measurement of the diameter of a cylinder. This sense comes from the word bore, as in to bore, auger, or drill a hole. There has been some speculation as to whether this use as a measurement of a cylinder can first be attributed to engines or rifles, but consensus points in the direction of rifles.
In its reference to firearms, bore is a synonym for caliber; thus, a full-bore weapon is either larger, or loaded to its full capacity with gun powder and ammunition. Such a weapon might have a wider barrel, larger bullets, and be more powerful than a small-bore weapon. Here, small bore is the opposite of full bore; however, the evolution of small bore as an expression has taken a different route and it has come to mean trivial, trifling, or provincial. The two expressions are only antonyms when they specifically reference weapons.
In the sense of measuring the cylinder of an engine, full bore refers to a completely open cylinder operating with maximum fuel and moving at top speed, as with an unchoked carburetor. It may also refer to piston diameter and the boring of an engine for more power. Both roots for the idiomatic expression lead to its modern meaning and usage, pointing to the greatest capacity of force, power, or execution.
Many of the synonyms for full bore are also idiomatic expressions. With reference to moving at top speed, these idioms include the following: full throttle, full speed, flat out, and all out. When referring to the fullest extent possible, or operating at full capacity, the idiom "no holds barred," which means without restriction or limitation, is another synonym.