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Old February 14, 2006, 10:10 PM   #1
SevenRoundMags
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Bullet to bore diameter ratio

Something I've been wondering recently; does the bullet diameter have to be bigger, smaller, or equal to the bore size? For instance, take a 9mm pistol. Is the bore diameter going to be .356, .354, or .355(9mm)? My logic tells me it has to be bigger, or the barrel would be destroyed when the bullet is fired. Also, if the bore has to be bigger (which once again I think it does) than the slug, how does the bullet not fall out the end of the barrel when you turn the gun upside-down?

thanks.
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Old February 15, 2006, 03:43 AM   #2
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Disclaimer: This is how I understand it to be. Anyone who sees me in error, please correct me.

The bullet diameter is typically equal to the distance between the grooves, or valleys, leaving it slightly larger than the diameter as measured by the lands. This is why you cannot stick a bullet down the muzzle (at least, you shouldn't be able to). So in the 9mm example, the bullet diameter is .355 inches. For argument's sake, we'll say the lands are each 0.001 inches high as measured from the grooves. So the bore diameter as measured from the grooves is .355 inches, equal to the bullet diameter, while the bore diameter as measured by the lands is .353 inches.

When the bullet is fired, the pressure forces the bullet down the barrel, and the lands dig into the bullet, slightly deforming it, and that grip is what forces the bullet to initiate it's spin. If you've ever seen, or get to see, a fired bullet, you should be able to see the indentations from the lands.

Hope this helps.
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Old February 15, 2006, 07:30 AM   #3
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For rifles , [similar with pistols] the groove and land diameters are about .008" different. For example for the 308Win and other 30 caliber bullets, the bore is .300" and the groove .308". Bullet diameter is the same as the groove diameter for jacketed bullets and about .001" bigger for lead bullets.
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Old February 15, 2006, 09:04 AM   #4
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Better get some "logic" training from a bright 4th grader!
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Old February 15, 2006, 12:36 PM   #5
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Twycross has it.

In a perfect world, all firearms bores would be made exactly to specifications, as would all bullets.

Bullets are sized to exactly fit the groove diameter of the firearm in question. The lands engrave the outer few thousandths of the bullet or jacket in order to impart spin upon firing.

Of course, this is not a perfect world. Both barrels and bullets are subject to 'tolerances'; that is a '30 caliber' barrel has a groove diameter of .3085" and a land diameter of .308" plus or minus a few ten-thousandths. Actually, modern machining methods produce barrels of superb uniformity; which is not to say the occasional clunker gets through the quality control. Modern bullets are very uniform as well. Most of the major bullet makers produce hunting ammunition superior in accuracy to match bullets of fifty years ago.

Military weapons of the past have varied greatly in specifications. 9x19 and .380 ACP pistols are both 'officially' .356 groove diameter; however, major manufacturers in various countries have produced such handguns with barrels ranging from .354" to .362". And they all shoot the same bullets. Wartime production of ammunition varied as well.

As you might expect, a small bullet in a large barrel produces only marginal accuracy. In addition, if the bullet is hard enough not to obdurate sufficiently, much power is lost in the propellent gases 'blowing by' the bullet. Larger diameter bullets simply 'smoosh down' (that's a technical term) to bore diameter.

Revolvers pose another problem. Not only must bullets fit the barrel, they must first pass through the throat of the respective cylinder. So, for maximum accuracy, cylinder throats should be reamed out to .0001" larger than the bore groove diameter. Sizing revolver bullets is a futile excercise; they get re-sized several times between chambering and the bullet leaving the barrel.

Now; one more cock-eyed thing about bore and bullet diameters. The popular name for a round may or may not have anything to do with the actual size of the bore.

.38 S&W, .38 Special, .357 Magnum, Super .38, .380 ACP, and most all the 9mm something or others are all in the range of .354" to .362". Except for extreme bullet weight differences, the projectiles are pretty much interchangable. I've loaded and shot 90 grain .380 ACP bullets in .357 Magnum cases. I've loaded and shot 158 grain lead round nose .38 Special bullets from a 9x19 autoloader. I've loaded and shot 125 grain JHP .357 Magnum bullets from a .380 ACP (they were too long to fit in the magazine, so I had to single load them).

.303 British is actually .323". .303 Savage is actually .308". 7.7mm Arisaka is the same as .303 British. 8mm Mauser is actually .323", or .312" in the earliest loading. 6.5x55 Swedish Mauser, .256 Newton and .264 Winchester Magnum are all the bore diameter; .256" is the land diameter, .264" is the groove diameter and 6.5mm is the metric equivilent of .256". Then, .257 Roberts and .25-06 are both .257 groove diameter.

Okay, that's more than you asked, but I was on a roll.

Rivers, I do not follow your thought at all?
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Old February 15, 2006, 03:48 PM   #6
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Close enough

OK, so you got a lot of info from the guys on bore diameter and groove diameter, etc. The thing that keeps a round from simply falling out of the barrel when you tip it down (that was one of the things you asked) is the grip the case has on the bullet (called tension or pull). The case itself is held in place by being larger than the bore and fitting into a chamber, restrained by headspacing on either the shoulder, the rim, or the case mouth. When you pull the trigger, the striker or firing pin ignites the powder, increasing the pressure and launching the bullet into the rifling. It seals (obturates) the bore, and is forced out of the end of the barrel by the expanding gases. You hear the report (BANG) when the escaping gases collide with the atmosphere. The crack you may hear when someone shoots a round that travels past you is the shock wave caused by the bullet exceeding the speed of sound.
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Old September 27, 2012, 09:29 PM   #7
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resurrection

i am hoping to resurrect this post if but for a second or two...
archie, what you said is right where i am studying on right now. i took a .357 handi rifle that had fired all types and styles of bullets for years, and i rechambered it for .357 max. now i was getting ready to load some hardcast BIG bullets. 200 gr, 235 gr etc. these things are long and heavy and on a whim i decided to slug the bore. i slugged it with a hard cast(not easy at all) and the measurements i got were .347 and .353. kinda tight. but i have shot hundreds of mixed .38 .357 thru it with no problem. now i am only planning on putting gaschecked lead thru this as a .357 max so far... but those measurements have me a bit apprehensive. any thoughts? i dont know if i should resize to .354 or just try it as is. it was plenty accurate before...
tim
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Old September 27, 2012, 09:47 PM   #8
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SevenRoundMags..If you really want to get confused..try and get your mind around the reasoning of the various .38's using .357" dia. bullets! This came about when the cap and ball revolvers were converted to fire metallic ctg.
Nominal I.D. of cyl. chamber was .380..cases were made to fit, & bullet had a short "heel" of reduced dia. that fit inside case..like our .22 rimfire.
Bullet bearing dia. was same as outside case dia. These were true .38's.
Then around 1890..fashon changed..folks wanted cleaner handling ammo..so...bullet was now inserted inside case..only problem..was it was now nominaly .357 dia & WOULD drop right thru bore! It was given a hollow base to expand when fired. Later, revolver mfg.s changed to smaller bore dia.
Hows that for a Rube-Goldburg solution!
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Old September 27, 2012, 10:42 PM   #9
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AcmeTim

The 'normal' sized bullet should work without major problems, HOWEVER...

Since the .357 Maximum is - according to my late night memory - loaded to higher maximum pressures than either .38 Special or .357 Magnum, you would do well to begin at the lower end of the load spectrum and cautiously work up. Squeezing a bullet down a couple thousanths of an inch is not generally a major problem with pressure, but it does have some effect increasing the pressure.

You may also find the longer bullets do not stabilize properly. Longer bullets require a faster twist to stabilize. On the other hand, you may find no problem in that regard at all. If you find bullets keyholing on target, it's not due to the tight bore, but due to under stabilization from under spin.

If you feel better about sizing the bullets to fit the bore exactly, it surely will not hurt anything. Just don't start with the hardest bullets you can find and maximum loads.
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Old September 28, 2012, 07:21 AM   #10
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archie,
thanks. i did a retest last night on the bore. this time i just tapped the bullet in the muzzle end and knocked it back out. lots of lead shavings where it entered. it measured the same. out of curiosity i pulled out the old trusty dial caliper. now i could watch the hand travel as the bullet was twisted. i got .352 and .357. cleaned the ends of the digital really well and got a little closer to the dials readings. bottom line... i dont think its as tight as i originally thought, and i dont think any two people would ever get the same measurements when trying to do this. i will go slow and build up a load.
i think my twist rate , from what i can find is 1:15.5. maybe that is not fast enough to stabilize that 235 or 200 grain bullet. we will see. it seems like a waste to go up to .357 max and still shoot the 170-180 gr bullets i shot as a .357 mag.
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Old September 28, 2012, 12:46 PM   #11
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Quote:
Also, if the bore has to be bigger (which once again I think it does) than the slug, how does the bullet not fall out the end of the barrel when you turn the gun upside-down?
Even if the bore was .002" larger than the bullet, the bullet wouldn't just fall out of the barrel.
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Old September 29, 2012, 01:36 AM   #12
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Before this thread is killed by the grace of God.

I say aspect ratio may play a roll here. Width vs length with velocities play a roll in the whole aspect of the query. A 22lr is only .22, but to length might be .33? That is what you call an aspect ratio of 33%.

Don't quote me on those numbers, but you get the point.

The 223 is "only" .22". But the length is about .75" in length. Well, at 3000 fps, when that .75" starts to tumble at 3000, well, sorry, your guts are hamburger.
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Old September 29, 2012, 05:41 AM   #13
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Quote:
I say aspect ratio may play a roll here. Width vs length with velocities play a roll in the whole aspect of the query. A 22lr is only .22, but to length might be .33? That is what you call an aspect ratio of 33%.
non sequitur

Aspect ratio is defined for the specific application. For vehicle tires it is height-to-width. It could as easily have been defined as width-to height.

For a given "caliber" length equates to bullet weight for the simple reason you can't significantly expand the bullet diameter (caliber) to gain weight. It must be done by extending the length of the bullet. Hence 150 gr spire point bullets are only significantly different from 180 gr spires in length.

Bore diameter is usually somewhere between the groove-to-groove diameter (widest) and the land-to-land diameter (narrowest). It is the designer's or marketer's choice what to name it.

There seems to be little science involved, as through the years, technical variations have appeared which took a variation of the name of the parent cartridge with it.

A bullet accelerating down a barrel approaching muzzle velocity does not need to have an airtight seal. It only needs a pretty good seal for a fraction of a second. One of the reason cast bullets are so accurate (assuming care in their fabrication) is that they match the bore diameter better than jacketed bullets. We hear that explained as, "They form a better gas seal", but that is actually a side effect, not the goal. One of the reasons rifling in a barrel is usually so shallow is to have only enough interference to engrave and impart the spin we know is needed for accuracy and to improve the bullet-to-bore fit, also for accuracy. The best seal between a bullet and its bore approaches the firing cycle of a bomb. The slug in a shotgun is nothing more than a pressure relief plug flung out with some degree of accuracy. A bullet in a rifled bore is similar, though more sophisticated.

Let's move this to Handloading, Reloading, and Bullet Casting. There are a couple of guys there that can explain this better ... if there is still interest.
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Old September 29, 2012, 06:22 AM   #14
Jim Watson
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Quote:
Bore diameter is usually somewhere between the groove-to-groove diameter (widest) and the land-to-land diameter (narrowest). It is the designer's or marketer's choice what to name it.
I always thought the bore diameter was the diameter of the hole bored (and reamed) through the bar of steel in which rifling grooves were to be cut, and therefore the land-to-land diameter of the finished barrel. There seems to be a lot of confusion on the matter these days (including some really strange numbers from six years ago.)

The American practice has been to make jacketed bullets equal to groove diameter, within barrel and bullet manufacturing tolerances. Lead bullets shot with smokeless powder are typically a couple of thousandths over groove diameter. An undersize lead bullet is a source of metal fouling.

As P.O. Ackley said of oversize bullets, by the time it has traveled its own length out of the case it will FIT the barrel. The key in preventing excess pressure is letting the bullet leave the case readily, avoiding what Clark calls "pinch."

As far as heavy bullets in .357 Maximum, it depends on the rifling twist and velocity whether a given weight/length of bullet will be stable and accurate. I would expect a 200 grain bullet to do fine, they work in .38-200 British (.38 S&W) after all. A 235 will just have to be tested.
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Old September 29, 2012, 10:15 AM   #15
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My bullets measure size on size with the grooves for fmj and .001-.002 larger for lead. Bullets smaller than bore (grooves) are not good at all as they allow hot gasses to bypass the bullet and blowtorch your barrel. Cant say as much as to the former for rifles though, i mostly shoot pistols. The later is true for both.
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Old September 29, 2012, 04:25 PM   #16
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Bud Helms

Quote:
non sequitur

Aspect ratio is defined for the specific application. For vehicle tires it is height-to-width. It could as easily have been defined as width-to height.

For a given "caliber" length equates to bullet weight for the simple reason you can't significantly expand the bullet diameter (caliber) to gain weight. It must be done by extending the length of the bullet. Hence 150 gr spire point bullets are only significantly different from 180 gr spires in length.

Bore diameter is usually somewhere between the groove-to-groove diameter (widest) and the land-to-land diameter (narrowest). It is the designer's or marketer's choice what to name it.

There seems to be little science involved, as through the years, technical variations have appeared which took a variation of the name of the parent cartridge with it.

A bullet accelerating down a barrel approaching muzzle velocity does not need to have an airtight seal. It only needs a pretty good seal for a fraction of a second. One of the reason cast bullets are so accurate (assuming care in their fabrication) is that they match the bore diameter better than jacketed bullets. We hear that explained as, "They form a better gas seal", but that is actually a side effect, not the goal. One of the reasons rifling in a barrel is usually so shallow is to have enough interference to engrave and impart the spin we know is needed for accuracy and to improve the bullet-to-bore fit, also for accuracy. The best seal between a bullet and its bore approaches the firing cycle of a bomb. The slug in a shotgun is nothing more than a pressure relief plug flung out with some degree of accuracy. A bullet in a rifled bore is similar, though more sophisticated.

Let's move this to Handloading, Reloading, and Bullet Casting. There are a couple of guys there that can explain this better ... if there is still interest.
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Read, and still reading.

Sanction density does play a roll here, also flexibility as does rigidity.

As to length vs width, they all play a vital roll and the time frame against certain obscurities that might be the barrier of impact causing a counter productive reaction all conduit to velocities in a variable way, but that depends.

LOL, how'd I do?
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Old September 29, 2012, 04:35 PM   #17
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Purty good!

I got it!

[Edited to add: All rifling is not cut. Here is an article on Button Rifling. Some rifling is forged. Then there is polygonal rifling. I do believe Jim Watson accurately repeats P.O. Ackley's assertion on that topic.]
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Old September 29, 2012, 06:39 PM   #18
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I believe Jim is right on the origin of the term bore. Some well stated responses here.
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Old September 29, 2012, 06:56 PM   #19
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Yes, barrels are rifled by cutting, broaching, buttoning, forging, and ECM.
Springfield National Match barrels were rifled by scrape cutters which only took out a very small amount of metal per pass. I think they were the ones who lightly scraped the tops of the lands so all the tool marks would run longitudinally instead of leaving crosswise reamer marks. It has been done to ball burnish the bore to smooth out reamer marks.
There is one barrel maker who cuts or broaches the rifling but then runs a button through the barrel to iron out the tool marks

A hammer forged barrel can hardly be said to have a bore in the sense of machine tool operation, the blank had an oversize hole that was whacked down against the mandrel forming lands and grooves. I still refer to the land to land measurement as "bore diameter" out of tradition.
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Old October 1, 2012, 06:54 AM   #20
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Folks producing best accuracy typically use bullets larger than the groove diameter. But even bullets smaller than groove diameter will still leave a copper wash in the grooves all the way to the muzzle evidence that they expand up from pressure to the groove diameter.

My experience is with 30 caliber bullets. I and others have got excellent accuracy with any good bullet that's larger than the barrel's groove diameter. Best example is with barrels with groove diameters of .3077" to .3079" would shoot .3082" Sierra match bullets, .3087" and .3088" Western match bullets and .3092" Lapua match bullets with near equal accuracy.

Winchester knew what they were doing in the late 1950's when some of their Model 70 match rifles were used by the US Olympic team for 300 meter free rifle matches. Their broach cut barrels' groove diameters were in the .3083 - .3084" range and Western Cartridge Company was told to make their 197 OPE (Open Point Expanding hunting bullet; their most accurate one at the time) and 200 grain FMJBT match bullets at least .3086"; hence the .3087" and .3088" diameters mentioned earlier. Their Palma (and other M70 match) rifles in the 1970's with hammer forged barrels also had groove diameters in that range so LC M118 match ammo with their .3086" diameter bullets shot very well. But Sierra's .3082" match bullets never fared well in those Winchester factory barrels.

The Brits have to use arsenal ammo in their long range fullbore match rifles. Those 7.62 NATO bullets are about .3075" in diameter (about the same as many commercial 30 caliber hunting bullets in the USA and US made 7.62 NATO service bullets). They use barrels with bore diameters s everal tenths under .3000" and groove diameters of .3065" to .3070" for best accuracy. Those with .3065" groove diameter barrels were impressed with the accuracy of Sierra's .3084" diameter Palma bullet when they first shot it in 1991; that's almost 2 thousandths bigger bullet than groove diameter.

I've known a few folks who used .311" diameter hunting bullets made for the .303 and 7.7mm cartridges in their .30-06 Win. 70 factory barrels with somewhat oversize groove diameters. They shot more accurate than any 30 caliber hunting bullet made in the USA.

Last edited by Bart B.; October 1, 2012 at 10:21 AM.
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