.284" Diameter Bullet does NOT equal 7mm

[DMP25-06]

I have some questions about the measured diameter of bullets .


Why is a .284" diameter bullet referred to as being "7 millimeter" , when 7 millimeters = .275590"

1 Meter = 39.37007874..." , divided by 1000 (each Meter = 1000 Millimeters) = .039370" (rounding back to 6 decimal places)

Therefore , converting Millimeters to inches :
7mm = .275590" ----- But , bullets labeled as 7mm are actually .284" diameter
6.5mm = .255905" ---- bullets labeled as 6.5mm are actually .264" diameter
6mm = .236220" ----- bullets labeled as 6mm are actually .243" diameter
8mm = .314960" ----- bullets labeled as 8mm are actually .323" diameter
10mm = .393700" ----- bullets labeled as 10mm are actually .400" diameter

When a barrel is drilled , the diameter of the drilling is always smaller than the final diameter of the bore after the rifling grooves have been cut , and that distance across the grooves is equal to the bullets's diameter , whether it is .284" , .264" , .243" , .323" , or .400" , and others .
The raised surfaces of the barrel lands is the diameter of the drilling before the rifling grooves are formed , whether by button-cut , single point-cut , or other methods .

My question is : How did we arrive at these Metric designations when the bullet's actual diameter , measured in inches , does NOT convert to those metric values ????

I am curious as to what the answer might be .


DMP25-06

 

[74honker]

DMP 25-06, this is a very interesting point and question. Surely someone here can explain this and I would love to hear the answer also.

 

[Muddyboots]

Why is .284 called a true 7mm?

Hey gang, curious as to why most people refer to the 2.84 bullet diameter as a true 7mm when .277 is actually closer mathematically? .277 being 7.036mm and 284 being 7.214 mm. I see everywhere people calling the .284 diameter the "true " 7 mil.

www.longrangehunting.com

See JE's comment which we will ALWAYS miss his input.

 

[74honker]

Thank you Muddyboots for that link. That helped alot and another reason JECustom was such a resource here! My next question to all that would be, in my cloudy mind lol, does this mean that theoretically that some calibers may have a "tighter" fit and potentially higher performance because of said "fit"? Or am I just crazy and not seeing this correctly.

 

[Muddyboots]

I think that us exactly what JE was saying. Some years ago I was "losing sleep" over this question and went to SAAMI for answers. JE was better source. His discussion on Palma bore diameters is good example.

 

[Liberator]

7 x 08 is easier to say than .275590 x 08 :-}

 

[MagnumManiac]

This is where the confusion starts.
Bore size is the diameter of the bore, ie, in a "308" as everyone calls it, the bore is .300" and the rifling grooves are .308".
In a .270", the bore is .270" and the grooves are .277"……see the confusion?!
This all started way back when, for instance, a 44 Special or Magnum is not .440", it is actually .429".
This must have started when the 44-40 was given it's name. Why this occurred I have no clue, but it is what it is.
My cast lead Bullets for my 44-40 Model '92 measure .432" as cast, then I swage them to either .430" or .431" depending on hardness.

The Pommy's were even worse describing their cartridges.

Cheers.

 

[kiwikid]

Most of the early metric cartridges were European Military based and they tended to use the bore diameter rather than the land diameter. For example the 6.5x55 has a bore diameter of 6.5mm or .2559". The same is true for the 7x57 Mauser.

Remington continued this practice with their 6mm Rem and 7mm Rem Mag.