Why is ammunition/artillery called "ordinance"?

Answer 1

It's actually "ordnance" in military use.

Ordnance has been used to describe ammunition and artillery since the 17th Century and is a clipped version of "ordinance" which was used before then - Ordinance goes back until the 12th Century as meaning "military provisions".

Answer 2

Ammunition, Small-arms, Side-arms, Artillery in fact just about any thing that is obtained under the Military Procurement Ordinance's is nominally referred to as being, collectively, "Ordnance". (note the spelling)
"Ordinances" are the rules and regulations which govern the standardization of the dimensions, materials, structural component, physical make- up and all other intrinsic aspects of the equipment being supplied under contract, or tender, for purchase by the Armed Forces.
The term itself dates back to before the 'Battle of Agincourt' (Henry V) when the King's "Ordinance Artificer" would go around the country buying weapons for the army.
During the American Civil War President Abraham Lincoln had to suppress the trade in unsuitable equipment being bought for, and distributed to the Federal Troops. Shoes made out of cardboard, Rifles not chambered for the ammunition that was being issued. Uniforms that fell apart due to "shoddy" workmanship.
A classic example of imprecise ordnance would be the American artillery at the "Battle of Breed's (Bunker's) Hill". Both the Powder and the Shot supplied to the guns were of the wrong caliber and had it be altered (reduced in size) prior to loading into the cannons. A factor that had a marked effect on the outcome of the battle.