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M4 Piano-Wire Made Powerful Recoil Spring
• Material: Tempered high-carbon steel.
• Compatible with GHK M4 GBBR, give users a powerful blowback recoil experience.
Piano wire, or music wire, is a specialized type of wire made to become springs or (as the name implies) to be used as piano strings.
It is made from tempered high-carbon steel, also known as spring steel, which replaced iron as the material starting in 1834.
Piano wire has a very high tensile strength to cope with the heavy demands placed upon piano strings; accordingly, piano wire is also used for a number of other purposes, including springs, surgical uses, and in special effects. (Resource from Wikipedia)