The US Army adopted the 300Win Mag as an upgrade from long action 308Win to total refit to 300Win Mag. This was done following US Navy Seal extremely successful deployment of 300Win Mag Mk 13 Mod 0/1/2. The cartridge used is a Federal ammo 220 SMK at 2850fps and is considered as effective as a 0.338 250 SMK.
It's an interim solution to increase sniper effective range from 800m to 1200m while using a proven, cost-effective solution. One of the many lessons of Afganistan is that weapon weight, range, mobility and maneuvrability is crucial in combat and that troops would rather carry a 50 pound anti-tank/anti-bunker missile that a huge .50 sniper rifle on patrol.
As for the legality of using large caliber weapons in anti-personel role, different army have different viewpoint depending on the treaties the have signed and their historic experience. The British Army is againt using open tip ammo for general use while the US Army bought 8 millions rounds of Trophy Bound Tip ammo...
We only need one good international treaty lawyer and one good forensic doctor to find out what is legal and what is illegal,
Alex