Why are metric designations so far off?

[ScottMcc]

I started wondering 30 years ago why the .223 is called the 5.56mm nato and the .308 is the 7.62 nato, when the actual conversion numbers are different. One inch equals exactly 25.40000 millimeters, so .224 = 5.689 mm, .243 = 6.172, .257 = 6.528, .264 = 6.706, .277 = 7.039, .284 = 7.214, .308 = 7.823.

Another way to look at it: 6mm = .236, 6.5 = .256, 7 = .276, 8 = .315.

Is it just some tradition we are stuck with, like pipe size? ie, 1/2" pipe = .840 od. 3/4" pipe = 1.050, etc.

 

[Nomad47]

Try to the grooves for the "inch" calibers and to the lands for metric. I have thought about it also and I think that is the difference.

 

[M-61]

Try to the grooves for the "inch" calibers and to the lands for metric. I have thought about it also and I think that is the difference.

I think you are correct with this. Isn't this the way "LAZZERONI" refers to their caliber designations?

 

[Steve Blair]

The designations are all over the map, regardless of who developed the cartridge. A .30-'06 has a Ø.30" bore but .308" groove dimension. A .243 Winchester has ~.243" groove dimension. A 7X57 cartridge usually has a Ø7.036 bore. Just accept that the designation may or may not mean anything and, if it does, it may not be consistent to anything else.

 

[Old Navy]

7.62X.03936=.2999= .30 Caliber Later! Frank

 

[JRS]

Pipe is measured ID. Tubing is OD.

 

[jonbearman]

I think all they did was round up or down for a whole number to not further people's understanding of calibers.I wouldnt worry as they develop more cartridge's they will do the same.

 

[ScottMcc]

Pipe is measured ID. Tubing is OD.

No, all pipe of a given size has the same OD, 1/2" = .840 od, 3/4 = 1.050, 1" = 1.315. Wall thickness and ID varies with the schedule.

 

[ar10ar15man]

it has little to do with bore or groove, more to do with "name" / marketing..its just a name...don't get wrapped around the math and numbers..its a name......9mm makarov is .363....all 38's/357's are 35 cal EXCEPT 38 smith and wesson which is 36 cal....none are 38 cal....
its a name game plain and simple