If you liked the Shamo manga, the Anime-Planet community thinks you'd like:
Both mangas deal with the same theme - a fragile teenager needs to learn martial arts in order to survive (however Ryo from Shamo is an addult after the initial arc), but holyland takes a more lighthearted approach to it. If you don't like the brutality of shamo, maybe give Holyland a shot.
In 1955 era Japan, ten years after World War II caused countless to descend into poverty, six young men are transported to a special disciplinary school where countless horrors await them. Overseen by a sadistic, abusive guard and a perverted doctor who requires invasive "inspections," the school is the new home of these boys who committed petty and oft-necessary crimes. Alongside their cellmate, the wise and hardened Rokurouta, the young men will try their best to survive their incarceration and hold on to the fleeting rainbows in their hearts.
Shamo's first arc, which lasts a few vols, is very much like Rainbow's story, revolving around delinquents in very harsh situations. The difference being in Rainbow, the difficulties lie with the staff/conditions, while in Shamo, the difficulties lie with the other inmates. Both have disturbing themes, as well, though Shamo is far more explicit/graphic.
Ken lost his family as a child, and when he confessed to the girl he loved from afar, Yumin, he lost her too in a very different way! For Yumin made her own confession: she's actually Korean, and left for her homeland to become a police officer. One year later, Ken leaves Japan to find his lost love, unintentionally becoming the leader of a brutal Korean gang in the process! It's tough to stay alive in the underworld, but Ken will try his best to keep the peace and rise to power, even though all he wants is a date with the lovely Yumin.
Both these series are dark and take alot of readers out of their comfort zone. Sun-Ken Rock has more of a heroic lead than the Anti-hero in Shamo, but they both have equaly horrific tales. If you liked Shamo then you will most likely enjoy reading Sun-Ken, ecpecially after the 5th volume.