The legality of 12.7mm Multi-Purpose ammo
The St.Petersburg Declaration
Declaration Renouncing the Use, in Time of War, of Explosive Projectiles Under 400 Grammes Weight. Saint Petersburg, 29 November / 11 December 1868.
The Contracting Parties engage mutually to renounce, in case of war among themselves, the employment by their military or naval troops of any projectile of a weight below 400 grammes, which is either explosive or charged with fulminating or inflammable substances.
The Hague Conference
Final Act Of the International Peace Conference. The Hague, 29 July 1899.
To prohibit the use of bullets which expand or flatten easily in the human body, such as bullets with a hard envelope, of which the envelope does not entirely cover the core or is pierced with incisions.
At 47g, the 12.7mm MP is clearly below 400g, and is affected by St.Petersburg. Whether it
expands or flattens easily in the human body is less clear.
Both conventions prohibit the use of such ammo against other states that have signed. Iraq hasn't signed, nor has the U.S., so Norway may freely use the MP round against them, as well as for any civilian purpose.
That doesn't imply that selling the MPs to civilians and non-signing states is very wise or ethical.
Clear treaty text vs military needs
No matter how clear the treaty text is, when the military and political needs are strong enough, the treaty will have to go.
With Blitzkrieg technology, we got very tempting targets (aircraft, vehicles) for explosive projectiles in the 12.7-40mm range, as well as good guns to deliver those rounds.
As a result, the commonly accepted interpretation of St.Petersburg is now that it only applies to individual targeting of humans. I.e., the use of 12.7 MP in itself is legal, with each gunner responsible for not targeting humans. See Moosberg: Swedish Use of 12.7 MP.
What's in a name:
As the 12.7 MP is a reasonable compromise round for both material and human targets,
the name seems to imply that one of its
purposes is human targets.
Given that targeting humans is illegal, the name is unfortunate.
Comment from a reader who has not signed the above conventions: Man this is some sweet ammo, you should see what it does to an 88 Volkswagen Passat