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A couple of unit converters gave me the same answer. I expected a different result because .223 ammo is smaller than 5.56. Does anyone know why a round labeled .223 is actually smaller than a round labeled 5.56?
 

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A couple of unit converters gave me the same answer. I expected a different result because .223 ammo is smaller than 5.56. Does anyone know why a round labeled .223 is actually smaller than a round labeled 5.56?
Hate to burst your bubble but they are the same. Only difference is the bullets used and powder charge weight used in the military loading which is designated 5.56. Cases are the same with differences only what one would expect from different manufacturers.

Chambers are different in the 5.56 to accommodate the longer CBTO military ammo uses but even that changes as hand loaders use longer and longer bullets in the .223 with mods to the .223 chamber (throating).

Much confusion has floated around regarding .223/5.56 due to the actual differences between .308 win and 7.62x51 Nato. THERE, there is a difference in rounds but not the .223/5.56. The M-15 was developed around the .233 Remington cartridge.
 

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It's a little dangerous to simply use a cartridge name as a guide to bullet size. The most obvious is .38 Special vs .357 Magnum, both of which use a .358 diameter bullet (or have a .357" bore.) Luckily, in this day and age, pretty good information is a web search away.


The following are the measurements for the two cartridges in question, from Wikipedia:

223 Remington:
Parent case .222 Remington
Case type Rimless, bottleneck
Bullet diameter 0.224 in (5.7 mm)
Neck diameter 0.253 in (6.4 mm)
Shoulder diameter 0.354 in (9.0 mm)
Base diameter 0.376 in (9.6 mm)
Rim diameter 0.378 in (9.6 mm)
Rim thickness 0.045 in (1.1 mm)
Case length 1.76 in (45 mm)
Overall length 2.26 in (57 mm)


5.56 x 45 NATO:
Parent case .223 Remington (M193)
Case type Rimless tapered, bottleneck
Bullet diameter 5.70 mm (0.224 in)
Neck diameter 6.43 mm (0.253 in)
Shoulder diameter 9.00 mm (0.354 in)
Base diameter 9.58 mm (0.377 in)
Rim diameter 9.60 mm (0.378 in)
Rim thickness 1.14 mm (0.045 in)
Case length 44.70 mm (1.760 in)
Overall length 57.40 mm (2.260 in)
 
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