A surplus 6.5 mm Carcano rifle.
A 6.5 x 52 mm Italian Carcano M91/38 bolt-action rifle.
6.5 mm Mannlicher-Carcano Italian made rifle
6.5 mm, or more specifically 6,5x52 Carcano
President John F. Kennedy was the only president to be assassinated by a rifle. The weapon was a 6.5x52 mm Italian Carcano M91/38 bolt-action rifle.
Your rifle is an Italian military rifle known as a Carcano. It was manufactured by Fabricca Armi Terni, the Italian military arsenal, in 1942. There are several versions, yours is probably the "fucile" or long rifle version. It should be chambered to fire the 6.5x52 mm (or "6.5 Carcano" or "6.5 Italian") round, but some fired the 7.35mm Italian round, or the 7.9 mm (8mm) German Mauser round. You can find more information at: http://personal.stevens.edu/~gliberat/carcano/
He got a mail order 6.5 mm Carcano Model 91/38.
2 bullets from a 1940 Italian Carcano M91/38 6.5 mm rifle
While conspiracy fans still argue, the official report said that JFK was killed with a 6.5 mm Carcano, a surplus Italian military rifle.
A 6.5 x 52 mm Italian Carcano (sometimes improperly called a Mannlicher-Carcano) M91/38 bolt-action rifle was found on the 6th Floor of the Texas Book Depository by Deputy Constable Seymour Weitzman and Deputy Sheriff Eugene Boone soon after the assassination of President Kennedy.
To get an answer you will need to state the model and the condition of the rifle.
JFK was shot from multiple directions with multiple rifles. It's just physics. Are you asking what caliber rifle Lee Harvey Oswald supposedly shot Kennedy with? That would be a 6.5 mm Carcano Model 91/38 rifle.