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Metric to caliber conversion

Discussion in 'General Gun Discussions' started by pwillie, Jan 30, 2011.

  1. pwillie

    pwillie Well-Known Member

    What is a 25 caliber in metric terms?:banghead:
  2. Ian

    Ian Well-Known Member

    6.35mm. The conversion is 25.4mm per inch.
  3. Ruger GP100 fan

    Ruger GP100 fan Well-Known Member

    Multiply by 25.4.
  4. Ruger GP100 fan

    Ruger GP100 fan Well-Known Member

    I gotta learn to type faster.
  5. bigfatdave

    bigfatdave Well-Known Member

    The .25 ACP (Automatic Colt Pistol) (6.35x16mmSR) centerfire pistol cartridge is a semi-rimmed, straight-walled pistol cartridge introduced by John Browning in 1905 alongside the Fabrique Nationale model 1905 pistol.

    From Wikipedia:

    This stuff isn't hard to look up in the information age, particularly cartridge info.

    But if you just want a conversion, plug it into google as {number} {units you have} in {units you want} ... example: "0.25 inches in millimeters"
    or "1200 feet per second in meters per second"

    I have a few handy reference charts for manual conversion, and I have a few basic conversions in my own head, but I'm spoiled by google ever since they started letting you do calculations in the regular search bar.
  6. pwillie

    pwillie Well-Known Member

    Thanks all,....so my 25 caliber is 6.35 mm?
  7. vaupet

    vaupet Well-Known Member

    Caution: an inch is always 25.4 mm
    a 7.62 mm is always a 7.62 mm
    but a XX caliber is not always 0.XX inch (for instance: 38 special and 357 magnum = same diameter)


  8. PapaG

    PapaG Well-Known Member

    And a 44 is really a 43 (.429 or .430) and a 44 cap and ball is a 45 (451 normally) and a 30-30, a 308, a 30-06, a 7.62, a 300, are all really .308 dia.

    Do we dare get into what the second number "might" mean.....45-70, 30-06, 30-30, 38-40, 7.62 X 51 and so on....
  9. rambler

    rambler member

    Dare dare dare!


    Also, why is 380 called 380?
  10. M2 Carbine

    M2 Carbine Well-Known Member

    Because it's .355 diameter, like the 9mm, and someone didn't want to call it 9mm.

    and a .223 bullet is actually.224 diameter.

    and a 9x18 Makarov bullet can be .363, .364 or.365 diameter.

    Confusing ain't it?:)
  11. Jim Watson

    Jim Watson Well-Known Member

    Your .25 caliber WHAT?

    6.35mm = .250"
    A .25 ACP bullet is typically .251", pretty close.
    But a .25-06 rifle bullet is .257" which is 6.53mm. But it will be cataloged as a 6.35mm in Europe because the BORE diameter is about .250".
  12. pwillie

    pwillie Well-Known Member

    So,a 25 06 and a 257 is the same? Is this how Weatherby came up with a 257 Weatherby?:scrutiny:
  13. Gromky

    Gromky Well-Known Member

    rambler the second number can indicate several things. For the .30-06 the 06 is when it was brought into use by the army. For the .30-30 the 30 is the original powder load. And when it's something like 7.62x51mm the 51mm is case length. Nice and standardized, right?

    I think a lot of it is differentiation on the market. When you come up with your new round you want it to stand out on the market, one easy way to do it is to go with a number that's different from your competitors. Otherwise you have to name it after your company, add an improved, or make sure there's magnum in the title.
  14. Smokey Joe

    Smokey Joe Well-Known Member

    The one basic rule...

    P Wille, is that THERE IS NO BASIC RULE!!

    When you design a new cartridge, you can call it whatever you want. Usually, marketing possibilities are a factor.

    In strictly measurement terms, 1.00 inches = 2.54 cm or 25.4 mm. That can be used as a conversion factor, but it may or may not tell you anything true about the diameter of the bullet for a particular cartridge.

    My favorite marketing example is the .250-3000, which was marketed as such back when 3000 fps in a standard commercial rifle cartridge was a big deal. The same cartridge is now sold as the .250 Savage, since 3000 fps is no longer a big deal.

    You can call your new cartridge something easy-to-remember, like .38 Special (which started out as the .38 Smith & Wesson Special!) or name it after the actual bore diameter, like .357 Magnum, which sounds ominous but is actually the same bore as the .38 Spl.

    The military in the US has its own cartridge designations, which get transmogrified into commercial designations for the same cartridge when that cartridge goes commercial , as in .30-'06, which was a design change from the earlier .30-'03.

    And so on. Any cartridge collector, or firearms historian (neither of which designations fits me) can probably supply a dozen more examples without straining.

    For us ordinary blokes, the thing to do is to know for sure, the current and past name(s) of the cartridge we want to use in our firearm, and when handloading, know exactly what diameter bullet is proper to use.

    But as for rhyme or reason in the names of cartridges, well, there ain't much.
  15. 9mmepiphany

    9mmepiphany Moderator

    45-70 - the 70 refers to the original blackpowder charge that it was loaded with...it is the same with the 30-30

    30-06 (30'06) - as already mentioned, the second number refers to the year it was adopted for military service. However, 25-06, 7mm-06, 8mm-06, refer to the parent cartridge which was necked down/up to produce a newer cartridge (most started as wildcat cartridges)

    7.62x51 (actually 7.62x51mm)- is a metric designation and usually more accurate and informative as it gives both the diameter of the projectile and the length of the casing. That's how you know the M14 round (7.62x51mm) is longer than the AK-47 round (7.62x39mm) or that a 9mm (9x19mm) is longer than a .380 (9x17mm)

    One of my favorite trivia cartridges is the 38-40. It shoots a .401 diameter bullet. ah, you say...that's the second number. No, that 40 still refers to the original blackpowder charge

    at least the .44 spl/mag being .429" make sense. The .44 American, parent to the .44 Russian and .44 Spl was originally loaded with a heeled bullet , like a .22lr, and was the same diameter as the case. When they loaded non-heeled bullets for the calibre, they had to be reduced in diameter to fit into the case
  16. hirundo82

    hirundo82 Well-Known Member

    Except for when it is .312 or .295.
  17. hirundo82

    hirundo82 Well-Known Member

    IIRC neither .44 Special nor its parent round .44 Russian ever used a heeled bullet. However, their predecessor the .44 S&W American did, and the designation stuck for its descendents, later including the .44 mag.
  18. oneounceload

    oneounceload member

    The diameter bullet both cartridges use is the same - the cartridges themselves, however are not.

    IMO, metric is MUCH easier as the numbers applied are true to the diameter and case in the majority of instances
  19. exeagle

    exeagle Member

    Some good info in this thread....caliber designations have always confused the heck out of me. :confused:
  20. Cosmik de Bris

    Cosmik de Bris Well-Known Member

    ...and it depends whether you measure the bullet, the distance between the rifling grooves, or the distance between the rifling lands.

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