https://www.gamespot.com/articles/another-red-dead-redemption-2-weapon-discovered-in/1100-6460822/

Red Dead Redemption 2 is still a few months away, but Rockstar is already giving players a chance to earn certain items for it through another one of its games: Grand Theft Auto V. Last year, the developer announced a crossover mission in GTA Online that would unlock a weapon in the upcoming title, and it appears another similar tie-in has now been discovered in GTA 5's code.

The Guru Team, which was responsible for uncovering the first tie-in mission in GTA Online before it was formally announced by Rockstar, have found a second Red Dead Redemption 2 weapon in the game through datamining: a stone hatchet (via PC Gamer). According to the group, the hatchet can be acquired through a treasure hunt, which involves traveling to five different locations on the map and either capturing or killing a target.