AKMS - are you sure your friend is not confusing the normal Winchester Silvertip hunting load, which has a separate tip to the bullet of silver-colored material, with a bullet that has a painted silver tip? Winchester loaded the 300 Win Mag cartridge with the 220 grain Silvertip bullet until 1997, the last catalog in which it appears. They simultaneously loaded a 180 Grain Ballistic Silvertip bullet in the Supreme Line of ammunition, and that is still in the 2007 catalog. That bullet is distinctive though, as it has the silver tip, but the rest of the bullet is black.
Just a thought. I am not into rifle much, but have never heard of an API .300 Mag round with tip lacquered or painted silver.
RUAG of Switzerland (and perhaps others) load military-type rounds for the .300 Win Mag due to military sniper work, including ball FMJ, Target (HPBT bullet), AP (not API. I don’t know the tip color, but it is probably black, like the 7.62 x 51 AP they have made), and what they call the “Styx Action” load, which has a RUAG-designed (I think - could be Lapua, which is part of RUAG), maximum expansion bullet). None of these has a silver tip.
The Swiss rounds have cases from Thun, with the usual “T” factory designator on the headstamp.