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.257 Roberts
The .257 Roberts, also known as .257 Bob, is a medium-powered .25 caliber rifle cartridge. It has been described as the best compromise between the low recoil and flat trajectory of smaller calibers such as the 5 mm (.22") and 6 mm (.24"), and the higher energy but harder recoil of larger popular hunting calibers, such as the 7 mm (.28") family and the popular 7.62 mm (.30").[3]
The bullet diameter of the .257 Roberts is .257 inches (6.5 mm), not to be confused with the more well known 6.5 mm caliber (like 6.5×55 mm, 6.5 Grendel or 6.5 Creedmoor) which uses 6.7 mm (.264") bullets, or the .25 ACP pistol caliber which is 6.35 mm (.251"). The .257 Roberts uses the same caliber bullets as .250 Savage or the more powerful .25-06 Remington.