Artillery (cannons), helmets, military emblems or decoration, rifles, swords, etc. are usually found on the tombs of soldiers, who either have died in a battle or who were veterans from a war.
The flag is a common symbol for veterans, as a sign of their patriotism.
Depending on its shape, the type of helmet may also help to identify in which War (and on which side or for which country) the buried person has fought.
While the sword may also be part of a coat of arms, the rifle is sometimes also found on the graves of hunters, though this would be more common in North America.