His county record continued to improve: he took 150 wickets for 20.46 each in 1953, and bettered that in 1955 with 159 and 1958 with 165. In 1955, Shackleton had the outstanding match analysis of 14 for 29 against Somerset at Weston-super-Mare, taking 8–4 in the first innings, and 6–25 in the second. In taking eight wickets for four runs, Shackleton posted the most remarkable bowling figures in county cricket in the 20th century. He was a Wisden Cricketer of the Year in 1959, and recorded his best innings figures, 9 for 30, against Warwickshire at Portsmouth in 1960. His captain, Colin Ingleby-Mackenzie, called on him to bowl with Warwickshire on 196–4, playing for a draw with 45 minutes left to play. As ordered by his captain, he conceded a four to secure the new ball, and then took 6 wickets in 26 deliveries without the batsmen taking another run. He took nine wickets in an innings on three further occasions, and also took five wickets in nine balls against Leicestershire in 1950, but never managed to take a hat-trick.