The Ferguson rifle was one of the first breech loading

Breech-loading weapon

A breech-loading weapon is a firearm in which the cartridge or shell is inserted or loaded into a chamber integral to the rear portion of a barrel....

 rifles to be widely tested by the British military. Other breech loaders were experimented with in various commands, including earlier versions of the Ordnance rifle by Patrick Ferguson

Patrick Ferguson

Major Patrick Ferguson was a Scottish officer in the British Army, early advocate of light infantry and designer of the Ferguson rifle. He is best known for his service in the 1780 military campaign of Charles Cornwallis, in which he aggressively recruited Loyalists and harshly treated Patriot...

 when he was in the "Fever Islands" (Caribbean). It was often misreported by historians to be a .65 (.648 true) caliber

Caliber

In guns including firearms, caliber or calibre is the approximate internal diameter of the barrel in relation to the diameter of the projectile used in it....

 rifle. However, it actually used a standard British carbine ball of .615 caliber.