Question: Of the common calibers , what is the metric caliber of the cartridge that was the second highest velocity commercially produced ammunition when it came out ?

text: Caliber
Metric and US customary
The following table lists some of the commonly used calibers where both metric and US customary are used as equivalents . Due to variations in naming conventions , and the whims of the cartridge manufacturers , bullet diameters can vary widely from the diameter implied by the name . For example , a difference of 0.045 in ( 1.15 mm ) occurs between the smallest and largest of the several cartridges designated as `` .38 caliber '' .
In guns, particularly firearms, caliber (or calibre in British English) is the specified nominal internal diameter of the gun barrel bore regardless of how or where the bore is measured and whether or not the finished bore matches that specification. It is measured in inches to an accuracy of hundredths or thousandths of an inch or in millimeters. For example, a .45 caliber firearm has a barrel diameter of roughly 0.45 inches (11 mm). Barrel diameters can also be expressed using metric dimensions. For example, a 9 mm pistol has a barrel diameter of about 9 millimeters. Due to the inaccuracy and imprecision of imperial dimensions converted to metric units, metric designations are typically far out of specifications published in decimal inches. True caliber specifications require imperial measure, and even when cartridge designations (often mistakenly referred to as caliber) only specify caliber to even tenths or hundredths of an inch, actual barrel/chamber/projectile dimensions are published to at least thousandths of an inch and frequently tolerances extend into ten-thousandths of an inch. In a rifled barrel, the distance is measured between opposing lands or between opposing grooves; groove measurements are common in cartridge designations originating in the United States, while land measurements are more common elsewhere in the world. Measurements across the grooves are used for maximum precision because rifling and the specific caliber so-measured is the result of final machining process which cuts grooves into the rough bore leaving the lands behind. Good performance requires a concentric, straight bore that accurately centers the projectile within, in preference to a tight fit which can be achieved even with off-center, crooked bores that cause excessive friction, fouling and an out-of-balance, wobbling projectile in flight. While modern firearms are generally referred to by the name of the cartridge the gun is chambered for, they are still categorized together based on bore diameter. For example, a firearm might be described as a 30 caliber rifle, which could be any of a wide range of cartridges using a roughly 0.30 inches (7.6 mm) projectile; or a 22 rimfire, referring to any rimfire firearms firing cartridges with a .22 caliber projectile.
Inch caliber Metric caliber Typical bullet diameter Common cartridges
.172 4 mm 0.172 in .17 HMR , .17 Hornet , .17 Ackley Hornet , .17 Winchester Super Magnum , .17-32 Magnum , .17 VHA , .17 Remington , .17/222 , .17 Mach III-IV , .17 Ackley Improved Bee , .17-357 RG , .17 Remington Fireball , .17 Incinerator , 4.39×39Rmm SPS
.20 , .204 5 mm 0.204 in .204 Ruger , 5mm Remington Rimfire Magnum
.221 5.45 mm 0.221 in 5.45×39mm Russian family
.22 5.6 mm 0.223 in .22 Short , .22 Long , .22 Long Rifle , .22 Stinger , 22 Extra Long , .22 WMR ( magnum ) , .297/230 Morris Extra Long , .22 Hornet , .22 Rem Automatic , 5.66 x39 MPS , .22 Rem Jet
.224 5.7 mm 0.224 in .218 Bee , .219 Zipper , .22 Hornet-K , .220 Swift , .222 Remington , .222 Remington Magnum , .223 Remington , 5.56×45mm NATO , 5.7×28mm , .22 TCM , 5.8 × 42 Chinese , .224 Weatherby Magnum , .225 Winchester , .223 Winchester Super Short Magnum ( Obsolete ) .223 Ackley Improved , .219 Donaldson Wasp , .221 Remington Fireball , .22-250 Remington and many more
.243 6 mm 0.243 in .243 Winchester , .244 Remington , 6mm Remington , 6mm Whisper , 6mm PPC , 6mm Bench Rest Remington , 6×45mm , 6×47mm , 6mm Cheetah , .240 Weatherby , 6×62 Freres , 6mm Norma BR , 6XC Tubb , 6mm JDJ , 6mm SAW , 6-250 Walker , 6.17 Spitfire , 6.17 Flash , 6mm Lee Navy , and more ,
.25 6.35 mm 0.257 in , 6.35 mm .25 ACP ( 0.251 ) , 250/3000 Savage , 257 Roberts , .25-06 ( 0.257 ) ,
.26 6.5 mm 0.264 in , 6.7 mm 6.5×55mm Swedish , .260 Remington , .26 Nosler , 6.5mm Creedmoor , 6.5×47mm Lapua
.27 6.8 mm 0.277 in , 7.035 mm .270 Winchester , 6.8 SPC
.284 7 mm 0.284 in , 7.213 mm .280 Remington , 7mm-08 Remington , 7mm Weatherby Magnum , 7mm Remington Magnum , 7×57mm Mauser , 7×64mm
.308 7.62 mm 0.308 in , 7.82 mm .30 Luger ( 7.65 × 21mm Luger ) , .30-30 Win , 30 Herrett , .300 Whisper , .30-378 Weatherby , 7.63 Mannlicher-Schoenauer , 7.63 Mauser , .30 USA Rimless , .308 Corbon , .3-9 Savage , .30 Kurz , 300 AAC Blackout ( 7.62×35mm ) , 7.5mm Schmidt-Rubin , .300 Winchester Magnum , .30 Carbine , 309 JDJ , .30-03 , .30-06 Springfield , .30-06 JDJ , .307 GNR , .308 Winchester ( 7.62×51mm NATO ) , .300 Weatherby Magnum , .30 Army ( .30-40 Krag ) , 7.82mm Lazzeroni , and dozens more
.311 7.9 mm 0.311 in , 7.92 mm .303 British , 7.62×39mm Soviet , 7.62×54mmR , 7.62×25mm , 7.7×58mm
.312 7.94 mm 0.312 in , 7.94 mm .32 ACP
.323 8 mm 0.323 in , 8.20 mm 8×57mm IS , .325 WSM , 8mm Remington Magnum , 8 mm plastic ( airsoft ) BBs
.338 8.6 mm 0.338 in .338 Lapua , .338 Norma Magnum , .338 Winchester Magnum , .338-378 Weatherby Magnum
.355 9 mm 0.355 in 9mm Luger , 9×19mm Parabellum , 9mm Ultra , 9mm Bayard Long , 9mm Browning Long , 9mm Mauser , 9mm Winchester Magnum , 9mm Glisenti , 9 × 21mm , 9 × 23mm Winchester , 9mm Mi-Bullet , 9mm Steyr , .356 Team Smith & Wesson , 9mm Federal , 9mm × 25mm Dillon , 9mm Action Express
.356 9 mm 0.356 in .380 ACP ( 9mm Short ) , 9×56mm Mannlicher-Schoenauer , 9mm × 57mm Mauser
.357 9 mm 0.357 in .38 Super , .38 Special , .357 Magnum , .357 SIG , .35 Remington
.363 9 mm 0.363 in 9×18mm Makarov
.365 9.3 mm 0.365 in 9.3×62mm , 9.3×64mm Brenneke , 9.3×72mmR , 9.3×74mmR