The AK-74M, service rifle of Russia.
The M4A1, service rifle of the United States.
A service rifle is the main issued rifle in a military, law enforcement agency, or private military corporation.
History
The service rifle has progressed as technology and ideas have evolved. At one point, a service rifle was the mainstay of militaries, and were often bolt-action rifles. Sometimes, a military had several different service rifles, often by branch (The U.S., for instance, had the U.S. Marine Corps, U.S. Army, and U.S. Navy all using different rifles of their choosing at one point).
Over time, this changed, and more nations began to standardize on one service rifle. This especially started happening as technology improved and more nations began using assault rifles and battle rifles as their service rifles.
Nowadays, there is often one main service rifle, and due to the modularity of some weapon systems, several variants of said rifles in service.