The Firing Line Forums

Go Back   The Firing Line Forums > Hogan's Alley > Handguns: General Handgun Forum

Thread Tools Search this Thread
Old May 8, 2001, 02:21 AM   #1
Junior Member
Join Date: January 9, 2001
Posts: 11
I tried using the search feature on the forum to figure this one out, but I got like 8000 results....

So here are my questions.

What is the difference between Caliber and MM?

How are the values calculated?

Why are there 2 different ways to measue a bullets size??

Kwyjibo is offline  
Old May 8, 2001, 03:10 AM   #2
Senior Member
Join Date: April 10, 2001
Location: Erie, PA
Posts: 236
I'm no expert, but...

There's really no important distinction between them. Both are the diameter of the bullet. More specifically, the diameter of the slug and the barrel it's fired from. The metric ones are easy enough to figure out. A 9mm is 9mm across, 10mm is 10, 7.62mm is 7.62mm in diameter. With the English measures, it's done in parts of an inch. .38 is 38 hundredths of an inch in diameter, while a .45 is 45/100 and .357 is 357/1000, making it about the same size as a .38.

In fact, a .357 and a .38 are either identical or very, very close, since a .357 can usually fire .38 rounds. Same thing with the .308 and the 7.62mm rounds. I think. I remember reading someplace that the English measurements are pretty inaccurate. The .44 is something like .425 in reality, and the .38 is closer to .357, when you put some calipers on it. I'm sure there's someone out there better equipped to answer this question, and maybe provide a link to some more thorough information.
PaladinVC is offline  
Old May 8, 2001, 03:20 AM   #3
Posts: n/a
Yes, caliber is the english system and mm is metric. As for some bullet diameters being slightly smaller than their names, I think (but don't quote me) that some of the onlder rounds were gauged by case width instead of projectile. I could be wrong, and if I am, please correct me.
Old May 8, 2001, 05:14 AM   #4
Apple a Day
Senior Member
Join Date: March 29, 2000
Location: Poquoson,Virginia
Posts: 1,493
You can convert back and forth by multiplying/dividing by 25.4

THose who use arms well cultivate the Way and keep the rules. Thus they can govern in such a way as to prevail over the corrupt.
- Sun Tzu, The Art of War
Apple a Day is offline  
Old May 8, 2001, 06:51 AM   #5
Bud Helms
Join Date: December 31, 1999
Location: Middle Georgia
Posts: 13,155
Caliber is the measure. The units are expressed in millimeters or inches. 9mm is a caliber. So is .452, as in .45 ACP.
Bud Helms is offline  
Old May 8, 2001, 08:36 AM   #6
Chad Young
Senior Member
Join Date: September 20, 1999
Location: Gainesville, FL
Posts: 601
More clearly:

"Caliber" or "Calibre" is the approximate diameter of the projectile or the bore of the barrel firing said projectile. This can be expressed in Metric units (like 9mm) or English units (like .44 Magnum).

With English units, the approximate bore diameter is usually expressed in inches with the assumption that the number starts with a period. A "357 Magnum" fires a bullet about .357 inches in diameter. A "300 Winchester Magnum" fires a bullet about .300 inches in diameter. Please note I said 'approximate' bullet diameter - some calibers are actually a little different from their name. A classic example is the .44 Magnum which actually fires a bullet .429 inches in diameter! Most "thirty caliber" cartridges are actually .308 inches in diameter.

Most cartridges with a metric designation reflect the actual bullet diameter fairly accurately. The 10mm really uses a 10mm bullet. The .40S&W also uses a 10mm bullet. To see the actual bore diameter of a given cartridge, I recommend the book "Cartridges of the World."

What gets really fun are cartridges with non-standard names or that have both English and Metric designations. Examples:

.30-30 - A ".30" caliber bullet (actually .308) over 30 grains of black powder in its original loading.

7.62NATO, 7.62x51, .308 Winchester - All are nearly exactly the same and are used interchangeably. Arguably, they are the same cartrifdge, just with different names.

.30-06 - .30 caliber service carridge, year 1906 (another that is actually .308)

.32ACP - in Europe usually known as 7.62 Browning.

Here are some common calibers and bore diameters:

.38 Special and .357 Magnum - .357 inches
9mm Parabellum and .380 ACP - .355 inches
9mm Makarov - .365 inches

As you can see, this leads to much confusion! The best thing to do with any gun/ammo combination that you are unsure of is to ask your gunsmith. There are some common rounds that can usually be shot interchangeably. I will list them here:

If your gun shoots .44 Magnum, you can also normally shoot:
.44 Special, .44 Russian.

If your gun shoots .357 Magnum, you can also normally shoot .38 Special.
Chad Young is offline  
Old May 8, 2001, 08:43 AM   #7
Senior Member
Join Date: June 15, 1999
Location: SW Florida
Posts: 212
A rule of thumb I've always used is multiply or divide by 4...

9mm X 4 = 36 caliber

40 cal divided by 4 = 10mm

This isn't exact, but pretty darn close
kingknives is offline  
Old May 8, 2001, 09:46 AM   #8
Senior Member
Join Date: January 11, 2001
Location: Dallas
Posts: 207

Want to add a few comments to the previous excellent posts.

It is important to remember that most cartridges have a unique historical context. In the non-metric world, there was little attempt at uniformity. The metric folks , at least, have defined cartridges by diameter of bullet and length of case.

A caliber is generally the diameter of the bullet, but has also been the diameter of the barrel at the rifling groves, or the diameter of the barrel at the lands.

In the nomenclature of czarist Russia, they split the inch into tenths called "lines", so a .30 cal was called a "three line" rifle.

As you may know, shotguns are in "gauge", which it the number of lead balls of that diameter that can be cast from a pound of lead. This was not that crazy, because in the old days many folks had bullet molds and cast their own bullets. So a guy with a twelve gauge bore could get twelve balls from a pound of lead.

There has often been a degree of salesmanship in naming new cartridges, rather than strict effort at accuracy. Research any cartridge that you want to use carefully.
RazorsEdge is offline  
Old May 8, 2001, 09:51 AM   #9
Senior Member
Join Date: February 12, 2000
Location: Birmingham, AL
Posts: 1,124
An obscure point

It about NEVER comes up in dealing with sporting arms or reloading, but the term caliber is also used in describing artillery pieces, and the length of the barrel. It's the ratio of barrel length to bore diameter. For example, the USS Iowa mounts a main battery of 9 of the 16 inch 50 caliber rifles. Thus the barrels are approximately 67 feet in length (16 inches times 50), while the 5 inch 38 caliber dual purpose guns have barrels of about 16 feet in length. This may not help, but I love these trivia.
Hutch is offline  
Old May 8, 2001, 10:12 AM   #10
Senior Member
Join Date: February 5, 2000
Location: Vienna, Austria
Posts: 279
Please bear in mind that the metric calibers describe land diameters (the inner diameter), and imperial calibers the rifling diameters (from groove to groove).

Also, the 8 x 57 JS (infantry special) uses a slightly larger bullet than the 8 x 57J (infantry) cartridge. The former may not be fired from a rifle chambered for the latter, as it would cause excessive(!) pressure, and is therefore marked with a colored primer and a bullet with a grooved ring.
Gunter is offline  
Old May 8, 2001, 10:54 AM   #11
Join Date: February 25, 2001
Location: Gillette, WY
Posts: 60
Some of the "inaccuracy" of caliber is due to changes in bullet design. In the late 1800's most bullets were the same diameter as the case and had a small "heel" that stuck inside the case. Just like a .22LR.

When it was decided that this was undesirable, often the bullet was made smaller to stick into the case but the caliber name did not change. This is why a modern .38 is really a 36 (.357). The old .38 with the outside lubed heeled bullet was really close to 38/100". When they switched to an inside lubed (that is the lead bullets grease grooves are inside the case now) they kept the same case and made a smaller bullet, and because they could still shoot in the older guns , albeit with an undersized bullet, they did not change the cartridge name and it became a tradition.

In fact this all started with Smith and Wesson. The Russian army wanted to buy S&W No. 3 Revolvers but did not like the outside lubed bullet of the .44 Smith cartridge. So they shrunk the bullet (now .429) and called it the .44 Russian. Which was later made with a longer case to become the .44 Special. Which was then stretched again to become the .44 Magnum.
Marauder is offline  
Old May 8, 2001, 11:35 PM   #12
Registration in progress
Join Date: March 2, 2001
Location: CA :(
Posts: 77
OK, now what is the difference between 9mm, 9x23, and 38 super?
Are the 38 super and the 9x23 interchangeable?
I believe the 9x19 is the same as 9mm, right?

For the help!
Those who stand for nothing fall for anything."
-Alexander Hamilton
HET is offline  
Old May 9, 2001, 12:27 AM   #13
Senior Member
Join Date: October 28, 1999
Location: AL
Posts: 472
9x19 - 9mm Para, or Luger (9mm bullet / 19mm long case)
9x21 - slightly longer case used in countries where military
cartridges are illegal. About the same ballistics.
9x23 - 9mm bullet with longer case and alot more power.
.38 super - some call it the first 9x23. It has a slightly
different shaped case and operates at slightly
less pressure than the 9x23. It was originally
made for the Colt 1911 to improve on the 9x19
9x25 - actually a 10mm case necked down to 9mm. Very hot.
.357sig - 9mm bullet is modified necked down .40 case.
Actually it's not a .40 case, but close enough.
Brasso is offline  
Old May 9, 2001, 04:58 AM   #14
Join Date: July 13, 2000
Location: E. Wa
Posts: 36
If you are familiar with "Pulp Fiction" the answer is "Royale with Cheese". If you are not familiar the mm is metric and Cal is basically inches. .45 cal is roughly .45 inch. 9mm is roughly .355 inch. -Coug
Coug is offline  
Old May 9, 2001, 10:53 AM   #15
Chad Young
Senior Member
Join Date: September 20, 1999
Location: Gainesville, FL
Posts: 601

Actually, there are TWO 9x23 rounds. One is the 9mm Largo or 9mm Bergmann-Bayard. this is a early 20th century round of limited power. The 9x23 Winchester is an early 90s IPSC round designed to make Major in IPSC. In theory, one could fire the 9mm Largo in 9x23 guns, but going the other way would almost certainly destroy your gun!

For even more confusion, there are three 9x18 rounds - 9mm Makarov, 9x18 Police (or Ultra), and new Russian 9x18 "super" which is kind of a +P Makarov round.

All are different and not readily interchangeable!

Chad Young is offline  
Old May 9, 2001, 09:11 PM   #16
Junior Member
Join Date: October 19, 1999
Location: Jackson, MS
Posts: 2
Thanks for that bit of information. When I noticed this subject, I was going to ask that just to stir the pot. I've always wondered how a .38 and a 5" could both be 38 caliber!


GustusJ is offline  

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

All times are GMT -5. The time now is 07:08 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
This site and contents, including all posts, Copyright © 1998-2018 S.W.A.T. Magazine
Copyright Complaints: Please direct DMCA Takedown Notices to the registered agent:
Contact Us
Page generated in 0.08095 seconds with 7 queries