41 Commando or No. 41 (Royal Marine) Commando was a unit of the Royal Marines trained as Commandos during the Second World War.
 They were part of the all Royal Marine 4th Special Service Brigade that took part in the Normandy landings in June 1944 and later that served in World War II, the Korean War, and in Northern Ireland.
 They were disbanded in 1981.
Eric Anthony Sykes (5 February 1883–12 May 1945), born Eric Anthony Schwabe in Barton-upon-Irwell, Eccles, Greater Manchester, England, was a soldier and firearms expert.
 He is most famous for his work with William E. Fairbairn in the development of the eponymous Fairbairn-Sykes Fighting Knife and modern British Close Quarters Battle (CQB) martial arts during World War II.
 Originally working for an import/export company selling weapons in East Asia, he claimed he volunteered for and served in the British Army as a sharpshooter on the Western Front during World War I.
 Returning to China in 1917, he joined the volunteer branch of the Shanghai Municipal Police (SMP) Specials with the rank of Inspector in 1926.
Lieutenant-General Gerald Robert Poole CB CMG DSO (1868-1937) was a heavy artillery commander during the First World War with the Royal Marine Artillery and the Royal Garrison Artillery.
 He was Colonel Commandant of the Royal Marine Artillery in 1921-22, and during this period also served as Marine ADC to King George V.
Julian Sommerville Hatcher (June 26, 1888 – December 4, 1963), was a U.S. Army major general, noted firearms expert and author of the early twentieth century.
 He is credited with several technical books and articles relating to military firearms, ballistics, and autoloading weapons. His premier works are "Hatcher's Notebook" and "Book of the Garand", along with "Pistols and Revolvers and Their Use"s and "Textbook of Pistols and Revolvers".
 In the latter work he introduced the Hatcher Scale, probably the first attempt to determine the stopping power of a handgun round by a formula.
 He was also a pioneer in the forensic identification of firearms and their ammunition.
 Hatcher retired from the United States Army as a Major General.
 Afterward, he served as Technical Editor of the National Rifle Association's "American Rifleman" magazine.
The Royal Marine Depot, Deal (also called the Royal Marine Depot, Walmer) was a military installation occupied by the Royal Marines and located at in South Deal on the road to Walmer.
The modern technique (abbreviation of modern technique of the pistol) is a method for using a handgun for self-defense, originated by firearms expert Jeff Cooper.
 The modern technique uses a two-handed grip on the pistol and brings the weapon to eye level so that the sights may be used to aim at the target.
 This method was developed by Cooper into a teachable system beginning in the 1950s, based on the techniques of shooters like Jack Weaver and others, after experiments with older techniques such as point shooting.
 The method was codified in book form in 1991 in "The Modern Technique of the Pistol" by Gregory B. Morrison and Cooper.
Robert Jobson is a newspaper correspondent, broadcaster and author.
 He is co-author of "" written with Princess Diana’s police protection officer Inspector Ken Wharfe MVO.
 He is also the co-author of "Bulletproof", the life story of Royal Marine and George Cross recipient Matt Croucher GC.
 He has written several non-fiction books on royalty, including "The Future Royal Family" (2015).
 He has reported on the British Royal Family since 1991 as royal correspondent for UK national newspapers including the "Daily Express", "The Sun" and "London Evening Standard".
 He was the recipient of the London Press Club "Scoop of the Year" award for his world exclusive that Prince Charles and Camilla Parker-Bowles were to wed in 2005.
 Jobson is Royal Editor for Australia's 7 network top-rated breakfast show Sunrise.
 He is also royal commentator for US network ABC News.
 He is Royal Consultant for Mark Schwahn's originally scripted series "The Royals" for E! starring British actresses Elizabeth Hurley and Dame Joan Collins and played cameo roles as himself in series 1, 2 and 3.
William Ewart Fairbairn ( ; 28 February 1885 – 20 June 1960) was a British Royal Marine and police officer.
 He developed hand-to-hand combat methods for the Shanghai Police during the interwar period, as well as for the allied special forces during World War II.
 He created his own fighting system known as Defendu.
 Notably, this included innovative pistol shooting techniques and the development of the Fairbairn-Sykes Fighting Knife.
John "Jac" Weller (January 6, 1913 – August 18, 1994) was an American college football player, firearms expert and military historian.
 He was a consensus All-American in 1935 at the guard position.
 He played for Fritz Crisler's Princeton Tigers football team at Princeton University that went 25-1 during Weller's three years on the team.
 Weller later became known as a firearms expert and published several works on munitions and military history.
Major Francis John William Harvey, VC (29 April 1873 – 31 May 1916) was an officer of the British Royal Marine Light Infantry during the First World War.
 Harvey was posthumously awarded the Victoria Cross, the highest military award for gallantry in the face of the enemy given to British and Commonwealth forces, for his actions at the height of the Battle of Jutland.
 A long serving Royal Marine officer descended of a military family, during his career Harvey became a specialist in naval artillery, serving on many large warships as gunnery training officer and gun commander.
 Specially requested for HMS "Lion" , the flagship of the British battlecruiser fleet, Harvey fought at the battles of Heligoland Bight, Dogger Bank and Jutland.

Question: What type of weapon did a British Royal Marine help a soldier and firearms expert create?
Answer: Fairbairn-Sykes Fighting Knife