Which is the bore measurement?

[Dannix]

This may seem really basic, probably because it is. But which is the bore measurement: the red line or the blue line? I looked high and low, but couldn't find this anywheres.

[trk]

GOOD illustration!
Red line is GROOVE diameter
Blue line is BORE diameter

[trk]

With that, a 'standard' .30 caliber bore will typically be .300 and .308 with the bullet being .308" diameter.

[NSP64]

Trk has it.
Think of boring a hole in the barrel, then cutting the grooves into it. The lands are what is left.

[Bret4207]

Correct. We size to a little over GROOVE diameter, not bore diameter. Common misuse of terms.

[Dannix]

Thanks trk. What .30 cal are .300 bore? I thought it was either .308, .311, or .312.

NSP64, now that I think about the way a barrel is made, that makes sense. Otherwise, if somehow lands were added rather than cutting groves, the red would be bore and the blue would be "land diameter."


Correct. We size to a little over GROOVE diameter, not bore diameter. Common misuse of terms.
That was my next question. The misuse of terms was starting to get me really cornfused.

[Rocky Raab]

Indeed a very good illustration, and yes, the bore diameter is the blue line.

Grooves vary in depth with the caliber (and caliber refers to the bore diameter, .30-caliber being .300", for example). Small calibers have shallower grooves, which is how a .22-caliber bore will have about a .224" groove diameter - each opposing groove is .002" deep. A .30-caliber, in contrast, will have a .308" groove diameter because the opposing grooves are each .004" deep. In between, the .25 and .27 calibers use grooves about .0035" inch deep - which makes their bores .257" and .277" respectively.

Fun stuff, huh?

[montana_charlie]

Dannix,
It's easy to keep the two dimensions straight in your head if you consider the traditional steps in making the rifled barrel.

First, a hole is bored through it. This bored hole is the same diameter as the name of the caliber. A .30-30 barrel will have a hole bored that is .300" in diameter.
Having used the 'bore' word several times, you probably get the idea that the hole is the "bore".

Next, they cut grooves into the inner wall of the bore to form the rifling. Those grooves will be a few thousandths of an inch deep, and will generate a diameter larger than the original "bore" dimension. If the grooves are .004" deep, that adds .008" to the .30 caliber "bore" diameter (mentioned above) and results in a .308" bullet diameter requirement.

CM

[Dannix]

I'm going to assume .303 and 7.7mm Japanese have a slight larger bore, and as a result use slightly larger bullets than .308, but what about 7.62x39? Does 7.62x39 simply have deeper grooves?

Edit: Also, why are some calibres labelled what they are, e.g. .44?

Neat stuff indeed. Thanks for getting me up to speed.

[onesonek]

Now with that knowledge Dannix,,,,,don't be starting any hunting camp arguements by asking some .270 owner what he thinks of shooting those 7mm bullets:wink:

[Dannix]

haha. More often then not, I have a hard time talking with guns with non-gungeeks. It's like I have a brain overload when a newbie asks me a question like "what gun should I buy" or "what AR calibre should I go for" or when a hunter says "blahblah cartridge isn't good for blahblah game".

I usually end up dipping my toe in with "it depends" and ask some basic questions, and don't talk freely unless the water seems fine.

[montana_charlie]

I'm going to assume .303 and 7.7mm Japanese have a slight larger bore, and as a result use slightly larger bullets than .308,
Both of those figure out to a nominal .303" bore. Add .008" for some grooves and you have .311"...the expected diameter of bullets used in .303 rifles.


but what about 7.62x39? Does 7.62x39 simply have deeper grooves?
What makes you think so? A 7.62mm barrel still has a .300" diameter bore.
The x39 or x51 or x54 numbers refer to the length of the cartridge case...not anything to do with the bore or groove diameters.

You may already know, but in case you don't...
7.62x51 would be an adequate way to designate the cartridge that is commonly known as 7.62 NATO. That same cartridge, in 'American talk', is the .308 Winchester...an everyday thirty caliber round with grooves .004" deep.
It was initially developed to create a shorter package...to go through autoloaders faster...which provides 30-06 performance.

CM

[Dannix]

With you on the lengths and NATO stuff. Correct me if I'm wrong though, but 7.62x39 does not shoot .308, and I think I'm correct in saying the same for 7.62x54R.


.to go through autoloaders faster
That bit I didn't realize. I thought they just wanted a shorter action for a lighter/handier rifle.

[MtGun44]

Normal US "30 cal" barrels ARE .300 bores. BUT they have .308 +/- groove diameter and
the groove diameter (RED LINE) is what you need to use as a basis for sizing your boolits.

Typically groove diam plus .002 or .003 for .30 cal boolits works well.

As said previously, think of boring and reaming the hole to .300 and then cutting .004" deep
grooves in it. You wind up with .308 groove to groove - 'groove diameter'.

Bill

[462]

As a sign at the Smith & Wesson factory is reputed to say: All .38 calibers are .357" except the .38-40 which is .400".

And, .44 caliber is nominally .429".

[geargnasher]

7.62mm is .300000000".

The "51" or "39" part is length of case in mm. It's just a metric designation, that's all. Like tire sizes.

A .308 Winchester is known as 7.62 NATO, and also as 7.62x51mm, 'cuz the case is 51.18mm long.

A 7.62X63mm is better know as .30-'06 Springfield in this country.

A 7.62X54R is a 30-caliber Russian rimmed case.

The 7.62X39mm is also known as Russian, since it was developed for the SKS by Kalashnakov (sp), or some argue by Germany.

Oh, and here's a good one: Wanna guess what a 7.62X51Rmm is? It's a .30WCF or .30-30 Winchester!

Don't ask me what a metric .307 Winchester (rimmed .308 Winchester case) is, I don't think they know what it is in europe.

Of course there are variations in how each manufacturer cuts the rifling, and some of the metric calibers can be a little weird in translation, that's why there are so many boolit moulds in the .312"-.316" range, to get a good fit in some of the overbored calibers.

Hope this helps.

Gear

[montana_charlie]

With you on the lengths and NATO stuff. Correct me if I'm wrong though, but 7.62x39 does not shoot .308, and I think I'm correct in saying the same for 7.62x54R.
I don't load for either cartridge, so I'm not up to date on any of the pertinent dimensions.
I just know that 7.62 millimeters equals .300 inches.

CM

[Dannix]

As a sign at the Smith & Wesson factory is reputed to say: All .38 calibers are .357" except the .38-40 which is .400".

And, .44 caliber is nominally .429".
I guess there's more madness than method in the naming of some calibres then? Why call did the originators call it .44 calibre when the bore is (I'm guessing based on .429 jword size) somewhere between .42 to.426?


Typically groove diam plus .002 or .003 for .30 cal boolits works well.
Thanks bill. It seems pretty remarkable when I stop and think about it that the lead will swage down just a bit without jacking up the pressures. Am I correct in assuming the actual desired boolit oversizeness (e.g. plus .002 vs .003) depends on the hardness of the alloy? At some point the material is too hard to swag down quite so much, decreasing the required plus size to prevent blowby and lead sublimation, and eventually, depending on the cast/lathed projectile alloy, no plus size is required, correct?


Of course there are variations in how each manufacturer cuts the rifling, and some of the metric calibres can be a little weird in translation, that's why there are so many boolit moulds in the .312"-.316" range, to get a good fit in some of the overbored calibres.

Apparently the soviet ammo, at least x39 and x54R, simply uses a larger projectile i.e. .311-.312 jwords instead of .308. Same sort of situation with 9x18mm Markov and 9x19mm, with again the soviet round shooting a larger projectile. I was just curious if the soviet designs simply use deeper grooves, and the bore is indeed .3000 cal or 9.000mm respectively, Or if the bore simply does not match what the name would seem to imply, and if so what's behind it deceptive naming. Anyone know?


I don't load for either cartridge, so I'm not up to date on any of the pertinent dimensions.
I just know that 7.62 millimetres equals .300 inches.

CM
No problem. Guys loading 7.62x39 with jwords have a pretty limited selection compared to .30cals on this side of the pond. (Again, I don't know why. Either bigger bores, or deeper grooves). The ability to utilize the plethora of .308 jwords is one attractive aspect of the 7.62x39 "equivalents" that seem to keep popping up, albeit often with their own twist, e.g. Marty's 30HRT, .30Apache/7.62x39USA,7.62x40, .300Blk. Of course for a caster, jword product offerings are irrelevant, but I know not if the projectile diameter differences are an issue of bore or of groove depth.

[Rocky Raab]

The problem with all these names is that they aren't absolutes. Just because something is called a 7.62mm something doesn't necessarily mean it uses bullets of that exact diameter. The old Soviet bloc habitually used bullets a bit larger than the nominal named number - for reasons that probably made sense to them.

A parallel situation exists in US ammo. We have cartridges with numerical names from 219 through 225 - and all eight of them use .224" bullets.

Converting from metric to US also causes confusion because if you actually measure things, you often learn that a given item is neither its metric OR US measurement, but somewhere in between. Tolerances come into play, but the root of the matter is that we name things for convenience regardless of their actual dimension.

[MtGun44]

Hardness has little relation to diameter. Not much to swage down the extremely tiny amount
with a soft alloy like "hard" lead alloy.

Ah - standards of boolit diameters. A common question, and there are a few answers.

If you look at history, the first cartridge was the .22 Short rimfire. All the immediate followers
were also rimfires, with a heeled boolit (pull the boolit from a .22 LR and look at it) with
the boolit OD the same as the case OD. Think about that for a minute. Now, let's suppose
that you are going to go to "Inside lubricated bullets" (note that .22 LR is outside lubricated,
wax and little grooves rolled into the full diameter part to hold it). You then put the bullet
wholly inside the case. It has to be smaller than the case, so a .38 boolit had to drop down
to .357. No particular reason that I know that they chose to keep the case diameter and not
the boolit diameter. Maybe resharpening the old reamers which makes them smaller, meant
they could salvage some expensive tools and maybe the chamber reamers were still in good
supply in the tool room. History hinges on such things, perfectly rational at the time, lost to
history very quickly.

The .44 had to drop down to .427 (.44-40). In each example, the final boolit diam
is about 0.013 smaller than the nominal - for about a .0065 brass thickness. Note that the
.44-40 brass has very fragile and thin case mouths - probably left over from the rimfire thickness.
Not sure why .44 Russian wound up at .429 groove diam, but it is the parent of the .44 Spl and
the .44 Mag. S&W did that, whereas Winchester did the .44 Henry (in Henry rifle, first repeater,
and a rimfire) followed by the .44-40 - once again keeping their chamber diameter (at the front) the same
perhaps for tooling and experience reasons. Remember, when you are first - anything is OK as a standard,
since you are making history. The .22 rimfires are the only survivors of an ancient and bypassed
ammunition technology.

Things that seem particularly stupid today are often perfectly logical from the standpoint of the
practical engineers and designers that were solving the gun problems and literally making it up
as they went along. So we have weird measurements in our various calibers, esp the ones that harken
back to the rimfire days.

Bill

[Multigunner]

S&W straight cased proprietary cartridges , other than the .357 magnum, are designated by the nominal chamber diameter.
Colt heel base bullet cartridges are also designated by chamber diameter, but bullets may or may not be of that same diameter, though in general larger than the comparable S&W bullet or designed to bump up to a larger diameter. The .38 Colt Navy cartridges for example, being designed to allow use of the original cylinder bored through to allow breech loading, this did not allow for a reduced diameter throat to the chamber.

The Remington .46 revolver cartridge intended for use in converted Remington front loaders, was actually a .44 bullet of around .446 diameter, not sure of case diameter.

Some caliber designations do not coincide with actual bullet or case diameters, these are marketing ploys to prevent the cartridge from being confused with a similar cartridge.

Anyway I like the British method of using the terms "Minor diameter" and "Major Diameter", the minor being the nominal bore size, the major being the projected circle which would encompass the bottom of the grooves.
Unfortunately this method came as a result of wide variations in groove depth of British military barrels. This is my major complaint with British milsurp rifles which had an acceptable variation of up to .006 in major diameters.
To be fair the British were experimenting with various groove depths and even reverse tapered bores to try to obtain the best ballistics from the shortened rifle barrels as they transitioned from the 19th century long infantry rifles.

US manufacturers were able to keep tolerances of bores much tighter, but even then 19th century small bores (circa .30) varied by as much as .002. By the WW1 era US barrels were far more consistent, seldom varying by more than .0005.
The WW2 rush to mass produce barrels by the tens of millions resulted in greater variations than had been noted for WW1 era barrels.

[Dannix]

Thanks for the history lesson and some of the method to the madness. It's remarkable how old a lot of this is.

The heeled boolit cartridges reminds me of what has to be one of the most interesting projects I've come across, perhaps simply because it is so well documented: the 480 Achilles.

[MtGun44]

Ok ------- :-)