Easy Van Horn to Blackwater Horseman 9 RDR2

Start a bounty mission (Lindsey Wofford is the best since is closer to Van Horn, don't take him alive, he talks to much), after you escape the gang trying to steal your bounty, go to Van Horn, start riding towards Blackwater, avoid water (even dry river beds counts as water). No bounty hunters will spawn in Tall Trees and Blackwater area, complete the challenge, you can go explore Great Plains and Tall Trees for points of interest, don't cross to New Austin, the sniper is still active and return to St Denis to complete the bounty mission.