He was also a long-time member of the English Eight Club, shooting, coaching and captaining England in the Elcho match for a total of more than sixty years.

He conducted research into ballistics alongside his mentor Henry St John Halford and the engineers William Ellis Metford and Arthur Mallock, and was regarded as an authority on the history and design of rifles, on which he published several books.

He was made an associate member of the Board of Ordnance and chairman of the War Office Small Arms Committee.

He donated several of the NRA's trophies and left the association £1,000 (equivalent to £31,526 in 2023) for the promotion of shooting competitions to support the development of long-range rifles.

He first competed in the English Eight, the match rifle team representing England in the international competition for the Elcho Shield, in 1885.

[18] In June 1899, he captained a Great Britain team in the International Rifle Match, held at The Hague in Holland.

Great Britain placed seventh out of eight teams: their poor performance was blamed on difficult range conditions, their choice of the Lee–Metford service rifle, and their decision to focus on shooting from the standing position.

[2] He was also captain of the British team, which included Arthur Fulton and P. W. Richardson, for the 1908 International Match, held at Bisley.

[21] From 1887, Fremantle began to conduct research into ballistics, together with the engineer William Ellis Metford and Henry St John Halford, another aristocratic rifleman who became his mentor.

[14] Halford built a 1,000-yard (910 m) rifle range on his family estate at Wistow in Leicestershire, including an iron target and a ballistic pendulum hut.

[24] In 1909 and 1911, working with the engineer Arthur Mallock, Fremantle devised a method to establish the maximum range of the Short Magazine Lee–Enfield when firing Mark VII ammunition.

[25] He also served as president of the Society for Army Historical Research and contributed to several editions of the Text Book of Small Arms, published by the War Office.

[26] He played an important role in the association's move from Wimbledon Common to Bisley Camp, first used for the 1890 Imperial Meeting,[18] which was overseen by Humphry.

[18] He was appointed by the association to the committee organising the programme for the shooting events at the 1908 Summer Olympics, which were held at Bisley.

[17] Several trophies awarded for NRA competitions, including a cup in memory of Henry Halford, were donated by Fremantle.