Why is .284 called a true 7mm?

Oldschool280

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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.
 
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Edd

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I'll provide a little info that may or may not answer your question.

I have not heard anyone make that claim about bore diameter. I also don't know of any 7mm cartridge that is designed for a .284" bore. The generally accepted bore diameter for a new 7mm barrel is .277" and the groove diameter is .284". If you look at the SAAMI specifications for various 7mm cartridges, there will be some slight variations in those dimensions.

Perhaps you have heard someone refer to a 284 Winchester as being a true 7mm. The SAAMI spec for the bore diameter of that cartridge is .2755". That is real close to exactly 7mm. I suspect if you were to have someone build you a custom 284 Winchester, you would get a barrel with a .277/.284 bore/groove diameter.
 

TOM H

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I've heard it before and I think is easier to say than groove dia. I own 284 rifle and I would think most that own 7mm would understand bore/groove dia.
 

Oldschool280

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I've heard it before and I think is easier to say than groove dia. I own 284 rifle and I would think most that own 7mm would understand bore/groove dia.

I guess what i was trying to say was why does my 7 mm fire bullets that are actual .284 in Diameter or 7.2 mm and a 270 fires bullets that are .277 7 actual milimeters, the projectile sizes are what i was referring to .seems silly to call a bullet by the land diameter in a rifle than the actual size of the projectile
 

J E Custom

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Hey gang, curious as to why most people refer to the 2.84 bore 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.


There are many cases of cartridges being named something different than actual size. Most are very close so they can be in a known category But the cartridge name can be misleading.

As far as I know this practice all started when Winchester wanted to produce a new cartridge In 7mm but did not want to call it that because the Chinese already had a 7mm And they wanted a "new" cartridge so they called it a 270 Winchester. It was a true 7mm but it had its own name
And would be different so it should sell. (It did).

So now it has become a standard practice to give a new cartridge a catchy name to improve sales.

The bullet used, is determined by the grove diameter not the bore diameter because some cartridges have slightly different bore diameters based on rifling height.

The 270 win has a bore diameter of .270 but the grove diameter is .277 making the rifling .0035
in height and requiring a bullet diameter of .277 to fit the grove/actual bore of the barrel.

A 284 Winchester has a bore diameter of .2755 and a grove diameter of .283 but the rifling height
is .00375. It requires a bullet diameter of .284. The .0005 difference is used when the bullet is engraved by the rifling.

This all sounds tricky but it is simple and trouble free as long as the correct bullet diameter for the grove diameter is used regardless what the cartridge is named.

Look at how many cartridges use the same bullet but have different names. Example; 5.56 Nato, 223 rem, 222 rem, 224 Weatherby, 22/250 etc. they all use the .224 diameter bullet.

NOTE: Even the Europeans don't stick to the actual dimensions even though they are on the Metric system and most of there 7mm's are actually .284.

J E CUSTOM
 

just country

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question, why does the military call a .277 a 6.8?? the .308W a 7.62, .264 a 6.5??

mic. the highest point of the ogive of the bullet to get the true dia. of the bullet??lightbulbgun)
 

TOM H

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I guess what i was trying to say was why does my 7 mm fire bullets that are actual .284 in Diameter or 7.2 mm and a 270 fires bullets that are .277 7 actual milimeters, the projectile sizes are what i was referring to .seems silly to call a bullet by the land diameter in a rifle than the actual size of the projectile

When you order a barrel this is what you look at.

Bartlein Barrels, Inc. - Calibers
 

FEENIX

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Hey, looks to me like you guys are over-thinking this thing. Look at the box of bullets; it says 7mm/.284. That's all I need to know.
I lose enough sleep trying to figure out real-world problems. :D

Yep! This reminds me of ...


:):Dgun)
 
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J E Custom

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Hey, looks to me like you guys are over-thinking this thing. Look at the box of bullets; it says 7mm/.284. That's all I need to know.
I lose enough sleep trying to figure out real world problems. :D


We are all guilty of over-thinking some of these questions, but some things need some though and understanding. some of us take lots of things for granted because we have been dealing with them for over half a century but some of the new members have not dealt with them and need/want an explanation. so I for one don't mind answering a question if I can no matter how simple it seems to me.


Even some of us old timers can learn if we listen to others. taking care to understand what cartridge takes what size bullet is very important in my mind. a good example of this is the difference in barrel grove diameters in the same cartridge. Old 22 hornet = .222 and .223 the new 22 hornet has a .224 grove diameter. another example is the Palma barrels are sometimes .3065 or .307 or can be the standard .308 depending on what someone wanted, so it would be nice to know what you have. (All 3 sizes can be/are called a Palma barrel and most "Think" that it is a .308. so questions are good and knowing is better.

It is easier to get the wrong ammo/bullet than you think and it happens all the time. I personally don't want someone to "Not" ask a question because it may sound stupid to others. the only stupid question, is the one not asked.

No one has all the answers, but on this site someone will know the answer or know where to find it.
So keep'em coming.

Just my opinion

J E CUSTOM
 

FEENIX

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We are all guilty of over-thinking some of these questions, but some things need some though and understanding. some of us take lots of things for granted because we have been dealing with them for over half a century but some of the new members have not dealt with them and need/want an explanation. so I for one don't mind answering a question if I can no matter how simple it seems to me.


Even some of us old timers can learn if we listen to others. taking care to understand what cartridge takes what size bullet is very important in my mind. a good example of this is the difference in barrel grove diameters in the same cartridge. Old 22 hornet = .222 and .223 the new 22 hornet has a .224 grove diameter. another example is the Palma barrels are sometimes .3065 or .307 or can be the standard .308 depending on what someone wanted, so it would be nice to know what you have. (All 3 sizes can be/are called a Palma barrel and most "Think" that it is a .308. so questions are good and knowing is better.

It is easier to get the wrong ammo/bullet than you think and it happens all the time. I personally don't want someone to "Not" ask a question because it may sound stupid to others. the only stupid question, is the one not asked.

No one has all the answers, but on this site someone will know the answer or know where to find it.
So keep'em coming.

Just my opinion

J E CUSTOM

Jerry,

I'm pretty sure I am not alone in looking forward on you posts esp. taking your time to explain it in layman's terms and well thought off response(s) as you always do.

Cheers!

V/R

Ed
 
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