Caliber and Actual Measurements

[rwm]

I understand that the AR-15 .223 and the NATO 5.56 round are equivalent and that the AR-10 308 and the 7.62 round are equivalent; sort of. Before you go ballistic on me (Ha) , I understand that they have different chamber pressures and one can safely fire the other but not vice versa.
My question is more basic. 0.223" does not equal 5.56mm and .308" does not equal 7.62mm. I have measured these projectiles with a caliper and the Imperial measurements are correct. How are these numbers generated and why don't they match up? What is the diameter tolerance of a bullet? Are they actually different in diameter? Please school me!

 

[Winegrower]

Huh, how timely. I just got an AR15, takes both flavors. I bought a couple hundred .223 and a bulk can of 5.56, similarly trying to reconcile what I measure. Standing by...

 

[RJSakowski]

.30 caliber is .300" or 7.62 mm. >308 is tha bullet diameter, larger than the caliber for an interference fit necessary create s gas seal and to build the high pressure .

Cartridge nomenclature doesn't follow any standard convention. The 30-30 Winchester was named so because it was .30 caliber and 30 grains of powder were used in a load. The 30-06 was named so because it was .30 caliber and it was standardized in 1906. The 308 Winchester because the bullet diameter is .308..

My Hornady Reloading Handbook is old enough that it doesnt show the .223 cartridge but 22 caliber shoots a ,224" diameter bullet

 

[Jim F]

The idea that .223 and 5.56 cannot be interchanged is a myth.
They have different pressure because the use different units.
One is PSI and the other is CUP.
They are the same round.

 

[rwm]

I measured a 556 round today. It does indeeded measure about .223 or .224.
7.62 mm equals .300". So are the NATO rounds the actual barrel inside diameter?

 

[GeneT45]

The idea that .223 and 5.56 cannot be interchanged is a myth.
They have different pressure because the use different units.
One is PSI and the other is CUP.
They are the same round.

This is not (entirely) true. They have a very slightly differing spec with regard to headspace, which is usually over-emphasized in the literature. Their pressure difference is largely irrelevant (and for the most part everything loaded for one, works for the other). The same is true of .308 and 7.62 Nato. In theory a .308 minimum chamber *could* have a throat shallow enough that a 7.62 round would be constricted at the neck and generate excessive pressure. In practice, every .223 chamber I've seen is throated so deeply that either round can be fired with impunity. Those desiring extra confidence in a chamber to fire both can use the .223 Wylde. I can attest that this chambering has negligible deleterious effect on accuracy. My Wylde chambered competition gun shoots sub-0.75" off the bench and could probably do better if I wished to make the trigger more bench friendly.

That said, "caliber" is used in a fairly nonsensical fashion. It seems to be, or pretends to be a precise measurement, but it's not...

.308 and .300 caliber are the same (.308" diameter bullet)
yet .303 (caliber) is somehow .311 diameter...

It's all history and convention, so don't, for a minute, think that "caliber" (or calibre, if thou preferest) has a definitive meaning.

GsT

 

[Jim F]

This is not (entirely) true. They have a very slightly differing spec with regard to headspace, which is usually over-emphasized in the literature. Their pressure difference is largely irrelevant (and for the most part everything loaded for one, works for the other). The same is true of .308 and 7.62 Nato. In theory a .308 minimum chamber *could* have a throat shallow enough that a 7.62 round would be constricted at the neck and generate excessive pressure. In practice, every .223 chamber I've seen is throated so deeply that either round can be fired with impunity. Those desiring extra confidence in a chamber to fire both can use the .223 Wylde. I can attest that this chambering has negligible deleterious effect on accuracy. My Wylde chambered competition gun shoots sub-0.75" off the bench and could probably do better if I wished to make the trigger more bench friendly.

That said, "caliber" is used in a fairly nonsensical fashion. It seems to be, or pretends to be a precise measurement, but it's not...

.308 and .300 caliber are the same (.308" diameter bullet)
yet .303 (caliber) is somehow .311 diameter...

It's all history and convention, so don't, for a minute, think that "caliber" (or calibre, if thou preferest) has a definitive meaning.

GsT

7.64X54R and 7.62X39 usually are .311, also.

 

[pdentrem]

I thought that the bullet size might be referring to the bore diameter from land to land instead of groove to groove, usually? .308 bullet that is engraved by the lands to .300? Maybe?
Pierre

 

[rwm]

And I always thought a thirty ought six was a 0.3006 OD round! Thanks all for the education. Feel free to keep going.

 

[Jim F]

And I always thought a thirty ought six was a 0.3006 OD round! Thanks all for the education. Feel free to keep going.

.38 and .357 use the same size boolits.........
9MM is .355......
If you really want stupid speed, with lower accuracy, you can get Sabots that put .224 boolits in a .30 cal case......
.308 up to ..300WM and beyond.