Which shell would fit sniping role the best?

I am assembling a team of my friends for Marathon server slam, and there is one who really like using sniper rifles. I really am not a fan of those in games like Apex Legends and AR, since by the time you down someone they are already being resurrected under some random rock/cover and getting ready to fight you. However, having seen a video of Drewski, it seems like snipers can oneshot quite reliably, so I am wondering what shells should my friend try for the sniping role. I am thinking he will probably need something with good movement to reposition after shots or come back into a fight. I and my other friendo will probably go into fights heads on.