There are several terms we could use but in general ordnance is a widely used term in the military. Other words could be , bombs, shells, ammo and UXB to name a few.
A quick search on the internet on the definition of ordnance ,
mounted guns; artillery.
"the gun was a brand new piece of ordnance"
synonyms: guns, cannon, artillery, weapons, arms, munitions, military supplies, materiel
a branch of government service dealing especially with military stores and materials.
"the ordnance corps"
If the term ' bomb' was used then as can be seen this doesn't cover everything we deal with
A bomb is an explosive weapon that uses the exothermic reaction of an explosive material to provide an extremely sudden and violent release of energy.
The term bomb is not usually applied to explosive devices used for civilian purposes such as construction or mining, although the people using the devices may sometimes refer to them as a "bomb". The military use of the term "bomb", or more specifically aerial bomb action, typically refers to airdropped, unpowered explosive weapons most commonly used by air forces and naval aviation. Other military explosive weapons not classified as "bombs" include grenades, shells, depth charges (used in water), warheads when in missiles.
The Navy , Army and RAF have their own teams to deal with ordnance and are referred as EOD teams - Explosive Ordnance Disposal
Anyway , the important message is, if you find something and not sure what it might be , then call the Coastguard / Police.
This was found some years ago in "little sea" Studland by some children.
A type of mine in this case , Studland was a test area for the D Day landings and although 80'000 items were cleared after the war, ordnance is still turning up despite the area being regularly checked.