7.62x54(.308), 7.62x25(.308), 7.62x39(.310)

1.) If you do the actual conversion from metric, 7.62mm
is 0.300", which happens to be about the bore diametre of
your average, garden-variety 0.308" barrel. I *think*
the original rifles chambered for 7.62x39 had somewhat
deeper grooves, being military and all, but the bore is
often nominally *close* to 0.300", thus it's also 7.62mm.
The metric designations *usually* reference nominal bore
diametre.

2.) The x54, x25, whatever, is case length in mm. There
may also be an "R" for rimmed cases, as in the 7.62x54,
which is, more specifically 7.62x54Rmm

breakopen308

I own a 7.62 X 54 and 3 - 7.62 X 39's. I am far from an expert on these cartridges but have always considered the "by" 54 pretty much equivalent to a .308 and the "by" 39 equivalent to a 30-30. I have seen the casings for a "by" 25 and it looks a tad larger than .22....... I'm sure someone will have the answer. I'm going to guess the 2nd number measures the diameter of the bullet in millimeters. The 1st number stumps me unless it might possibly be just a term to classify it as a military round?

Interesting topic and I'm looking forward to the answers we will hear.

still didn't answer either question.
1)Why would a 7.62x39 which is in between 7.62x54(.308) and 7.62x25(.308) be considered a .310

2)I already know that the second number is some sort of measurement but what exact part is the x54, x25, or x39 SPECIFICALLY referring too.

The last one refers to white_bass.
Hey thanks for the info so far Mossymo

white_bass

The "by" 25 case is significantly smaller than the other two rounds which it should be. But in relating numbers what I am saying is like 20 to 16 to 4.
Looking at a "by" 25 it seems to short of brass to make the 2nd numbers a measurement of the length of the cartridge case.
But as usual, I always reserve the right to be wrong !!!

Mossy,

25 millimeters is ~ 1". If you take a set of calipers and measure the
length of a 7.62x25 case, you'll get ~0.97-0.99". 7.62x54 case is
~2.1" long. Ain't metric fun? :-)

Todd

yeah, who's the idiots, the guys who created metric or the ones who decided not to use metric?!?!?

I found out from doing more research that the secondary # possibly refers to the case length like, white_bass said. No confirmation on that though yet.

Still not sure what makes the 7.62x39 a .310 instead of a .308 ???

ping

I had a customer come in yesterday and tell me that the x25 shoots a pistol round and the other 2 are rifle rounds. He reloads his own ammo. I got it confirmed that first is the diameter, and the second is the length. But he said that the x54 is considered more of a 30-06 than a 308, even though my ordering book for ammo shows the x54 at a 308. Still puzzling.

Ok guys... here it goes...

7.62x54mmR is a Rimmed (hence the R) Russian cartridge used in Mosin Nagant Rifles like the Mosin M91/30, Mosin M39, and Mosin  M54...  Being Russian has nothing to do with the "R".  The cartridge contains a .308 caliber bullet in the range of 175-225 grains, and is roughly equivalent in performance to a 30-06.  The case is 54mm long, not including the projectile, and has a rim instead of a detent at the end of the cartridge so the extracting lip is wider than the rest of the case.

7.62x39mm is also a Russian cartridge, but not rimmed, which is fired by rifles like the SKS and AK-47.  The cartridge contains a .310 caliber bullet in the range of 122-154 grains, and is roughly equivalent in ballistic performance to a 30-30 Winchester.  This bullet is .310 because the Russians in their infinite wisdom, when creating this cartridge, didn't have the precision tooling that we had here in the United States.  The larger diameter bullet was used so the bullet would better grip the rifling in the not so perfect barrels in the Soviet barrels.  The case is 39mm long and has a standard detent in the cartridge, so the extracting lip is flush with the rest of the case.

7.62x25mm is a Russian/Czech cartridge that is used in handguns and submachine guns, like the Romanian Tokarev T33 pistol, Czech CZ-52 pistol, and Russian PPSh-41 submachine gun.  The cartridge contains a .308 caliber bullet ranging from 85-110 grains (85 is stock load) and has a muzzle velocity in a 5" pistol barrel of over 1500FPS, making it the fastest pistol ammunition in the world... until the 5.7mm round was developed, which is basically a .30 caliber rifle casing with a tiny bullet.  the 7.62 Tokarev and 5.7mm rounds are the only pistol rounds capable of piercing personal body armor and Kevlar helmets reliably.  The case is 25mm long, not including the projectile, and like most ammo, has a detent instead of a rim.

Caliber is a measure of diameter of a bullet.  .308 caliber means ".308 inches in diameter", .310 means ".310 inches in diameter" and so on.  ".30 caliber" is a general term used for bullets ranging from .300" in diameter to .315" or so... anything larger is considered .32 caliber.

Other rimmed cartridges include the .45-70, which is a .45 caliber created in 1870 by the US Military, hence the designation of "-70".  The 7.62x54mmR is the only cartridge I know of with the "R" in the designation.  30-06 is a .308 caliber that was created for the US Military in 1906, and is not a rimmed cartridge, but I wanted to use it as an example of naming conventions for cartridges.  The 30-30 Winchester is a .308 caliber that was created in 1930, again for Military service.

Other conversions are as follows...

5.56mm NATO is .223 caliber.  You can use a .223 caliber cartridge in a 5.56mm barrel, but you can't use a 5.56mm cartridge in a .223 caliber barrel.  Goofy, I know, but it's true.

5.7mm is .224 caliber.  It uses a huge rifle casing to fire a tiny - 45grain - bullet at velocities over 2000FPS from a long barreled pistol, 1700+FPS from a 5" barreled pistol, and 2200+FPS from a submachine gun or rifle.

.50 caliber is 12.1mm.  There are so many applications of .50cal, that it wouldn't even pay for me to go through them all... the most common are .50BMG which is a rifle cartridge, and .50AE which is a handgun cartridge.

8mm is about .35cal

9mm is roughly .40cal

.45cal is about 11mm

Hope this lengthy post helped answer your questions.

And now, I'm going to get banned for dredging up a 4 year old post. =P

Dragon is correct about the second number being the number of mm of the length of the case.  Not quite on the .45 - 70.
The 45 - 70 was named for the .45 cal bullet and the case would  hold 70 grains of black powder.  It was adopted in 1873, with the .50 - 70 being the more-or-less standardized military armament till then.  Again, a .50 cal 450 grain bullet with the case holding 70 grains black powder. 
The 30 - 30 again having a .30 cal bullet and holding 30 grains black powder, and the Quigley .45 - 120 holding 120 grains black powder with the same 405 grain bullet as the .45 - 70 and 90 variations.  The 25 - 35 a .25  cal bullet in front of 35 grains black.  .25 - 20 only 20 grains of black powder.  And so on.  
A bit of trivial history - The .45-70 and .50 - 70 would hold 70 grains black powder in those days but as cases nowadays have different webs and case thickness and construction they will only hold about 55 - 60 grains at most.  Those old stamped out copper cases were part of the story behind Custer's downfall!  First of they were expensive and the military was give few rounds to practise with.  A lot of the raw recruits had very little firearm training and actual shooting training consisted of only a handful of shells during their training.  Second, as they were pure copper, they would expand when fired and with the weak extraction of the .45 - 70 Springfield trapdoor they were using that was the Allin conversion of the original Springfield Civil War muzzleloader, they often jammed solidly in the breech, leaving the trooper up the Native American creek without a weapon!  They'd have to ram it out with the attatched ramrod before they could reload.  Try that when you are outnumbered 10 to 1!  
A lot of the old European rifles, especially the classic doubles, were rimmed and you will often see the R behind the case designation.  Like the 7 X 57R (7mm bullet, 57 mmlong case which is rimmed) 
Dragon was right about dates for some cartridges.  The .30 - 03 was the 30-06 case designed in 03, and modified with a different bullet in 1906 it became the famous 30 -06! 
Lots of metric and non metric confusing names.  The .303 British was actually .313 bullet and the .22 hornet originally had a .223 cal bullet, later went to .224.