Is the barrel hole of a gun the same size as the bullet?

Yes, no...sometimes....maybe......

Cartridge nomenclature tends to be pretty random and often doesn't have anything to do with bore dia.

the .357 and .38Special both use .357 dia bullets (tho they can also use 358s, depending on bullet design material...and even 9mm/.355/.356 in a real pinch).
There are also .38s that use .360 bullets...the Colt and S&W (not special)........

it gets confustrating......


And even if you DO have a bullet that matches the Cartridge name, your bore dia is still going to be smaller....and right now I cant remember the normalities for measuring from lands or grooves.....

No. The "barrel hole" is divided between lands and grooves. The grooves are deeper than the lands.

There is very little rhyme or reason to cartridge nomenclature. Revolvers such as the Remington New Model Army and Colt Dragoon that were 44 cal in percussion nomenclature referring to diameter at the lands, became 45 caliber as the Colt Single actii , although the barrel diameter remained @ .448 in the lands to .452 in the grooves. Take the 38 Special, a nominal .357 diameter. It the 38-40 Winchester, derived from the 44-40. The latter, parent cartridge had a nominal .427" bore, while the 38-40 has a nominal . 401" bore. The 32 acp and 32 S&W long are nominally .3125" and . 312".

Things become no clearer with rifles. The 300 Winchester Magnum, 30-06, 308 Win, and 30-30 are all .308" while the . 303 British is .311" nominal.

Jacketed bullets are, generally made to fit nominal groove diameter. Cast bullets will generally be .002" over groove dimater.

Glad I could clear that up

The SAAMI (Sporting Arms and Ammunition Manufactures Institution) Standards provides a wealth of information on this subject. It does take a little effort to find the information. When looking at the drawings pay attention to the tolerances given for the dimensions. This seems to confuse many people.

Link to SAAMI cartridge and chamber drawings.
https://saami.org/technical-information/cartridge-chamber-drawings/

DocRock said:

Jacketed bullets are, generally made to fit nominal groove diameter. Cast bullets will generally be .002" over groove dimater.

Glad I could clear that up

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I think that's what the initial question actually was, not the usual questions about naming.
I've been curious about that, too, and haven't had a gun I could properly get calipers to.
So to clarify: the bullet diameter is the same as the groove diameter, or meant to swage down to it, not splitting the difference for the lands to deform it out to the groove diameter.

After one fires the bullet through the barrel, yes, they will be the closest to the “same size” they will ever be.

A bullet’s diameter is about the same size as the grooves, a thousandth or two bigger for a good gas seal for plain lead.
The lands are slightly smaller to engrave into the side of the bullet and spin it for stability.

Proper fit is essential for accuracy and safety.
Too big and the pressure will jump, possibly causing the barrel to burst.
Too small and the bullet will “rattle” down the bore and have no accuracy or stability.

I will add this to the Thread simply because some folks forget and some do not yet know.



Both my .30'06 and my .308Win rifles sport the same diameter bullets, but they are both .30 caliber rifles, referring to the Bore diameter.

Grooves for these are usually about .004" deep, with that on either side, .008" more than the .300" bore, and so .308".

Clear as mud.

As others have indicated, for reloading .38/.357 (both normally have a .357 groove diameter) you would usually use .357 jacketed bullets and, .358"-.359" lead (to assure a good seal).

Enjoy!

....and then there are the shotgun bores.

Thanks for the picture, GBExpat. I cannot count the times I read of a barrel with a "bore" of .308".

What I call the American Plan is for jacketed bullets to be of groove diameter and cast bullets to be perhaps .002" larger than groove.
For a revolver, bullets should fill the chamber throat which is best at or slightly larger than groove diameter.

Then manufacturing tolerances rear their ugly heads. I think the worst example is 9mm P, guns and ammunition made all over the world to whatever dimensions the boss thinks best. The nominal groove and bullet diameter is .355" but there are barrels out there all the way from that to .359". And they shoot well enough with jacketed bullets. Fitting a cast bullet to a Slobbovian Surplus Service Pistol can be a challenge.

In specialty loading, my .40-65 Browning BPCR has a Badger barrel with .408" groove diameter, but its throat is larger and the rifle shoots best with .411" cast bullets that are aligned immediately they start out of the case neck.

Demi-human said:

After one fires the bullet through the barrel, yes, they will be the closest to the “same size” they will ever be.

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Often before it gets to where it’s going.

If the OD of the bullet was the same diameter as the minor ID of the bore, the lands couldn’t impart the rotation of the bullet that stabilizes the bullet in flight. If the major ID of the bore (grooves) were exactly the same size, the bullets ability to not let gasses pass it would be reduced, diminishing their ability to accelerate it on the way out of the barrel. Bullet construction is considered in how much larger the projectile is though.

If it were the same size a fired bullet (right) would look more like an unfired one (left).

P.O. Ackley said of oversize bullets, "by the time it has traveled its own length, the bullet will FIT the barrel.

I called bullet diameter = groove diameter the American Plan. There is a tradition in Europe to make bullets slightly under groove diameter so the bullet metal displaced by the lands will have someplace to go.

GBExpat said:

I will add this to the Thread simply because some folks forget and some do not yet know.

View attachment 984808

Both my .30'06 and my .308Win rifles sport the same diameter bullets, but they are both .30 caliber rifles, referring to the Bore diameter.

Grooves for these are usually about .004" deep, with that on either side, .008" more than the .300" bore, and so .308".

Clear as mud.

As others have indicated, for reloading .38/.357 (both normally have a .357 groove diameter) you would usually use .357 jacketed bullets and, .358"-.359" lead (to assure a good seal).

Enjoy!

Click to expand...

jmorris said:

Often before it gets to where it’s going.

If the OD of the bullet was the same diameter as the minor ID of the bore, the lands couldn’t impart the rotation of the bullet that stabilizes the bullet in flight. If the major ID of the bore (grooves) were exactly the same size, the bullets ability to not let gasses pass it would be reduced, diminishing their ability to accelerate it on the way out of the barrel. Bullet construction is considered in how much larger the projectile is though.

If it were the same size a fired bullet (right) would look more like an unfired one (left).

View attachment 984809

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Nce pics with the comments.

Jim Watson said:

P.O. Ackley said of oversize bullets, "by the time it has traveled its own length, the bullet will FIT the barrel.

I called bullet diameter = groove diameter the American Plan. There is a tradition in Europe to make bullets slightly under groove diameter so the bullet metal displaced by the lands will have someplace to go.

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I’ve read that is also the reason Barnes and other mono-metal bullet makers cut rings or bands around their projectiles. It allows displaced metal somewhere to go and helps with lowering pressure a bit.



I’m not a ballistic engineer, but this concept does make sense. .

Stay safe.

I use a 237 bore on my 6mm (243) barrel

Jim Watson said:

... Fitting a cast bullet to a Slobbovian Surplus Service Pistol can be a challenge. ...

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<chuckle> That brings to (my) mind all "7.62mm" (=.30") Russian/Soviet/ComBloc bullet requirements ... for both pistols and rifles.

O'course, max groove diameter on my vintage .32-20 Colts is .313" while on my vintage .32-20 S&Ws is .312" and my vintage Marlin 27S Lever Pump is .311" so the Russian arms industry was not alone in experiencing some uniformity issues. .

And then to further confuse things . . .

You have sabots:



and squeeze bores:

The “bore”.

LookAtYou said:

is the bullet the same size as the barrel?

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As noted above, it wants to be "close" so as to freely enter the barrel, then 'upset' to engage the rifling.

Ignition Override said:

The “bore”.

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And, a machinist would tell you the "bore" is the diameter of the drill bit or cutting tool used to make it.

Rifling is a subtractive operation to remove material to a known greater diameter of the bored blank.

Our British friends gave us a reversed reference (groves not lands) to better represent the size of lead ball wanted, rather than drill bit diameter used. American usage might reflect adding rifling to a given musket bore--or not, language is goofy.

Riomouse911 said:

I’ve read that is also the reason Barnes and other mono-metal bullet makers cut rings or bands around their projectiles. It allows displaced metal somewhere to go and helps with lowering pressure a bit.

View attachment 984820

I’m not a ballistic engineer, but this concept does make sense. .

Stay safe.

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I think the reason is that with a monolithic bullet, there’s no lead core to swage down so the rings are cut to diminish the surface area engaging the lands, thereby diminishing surface tension and barrel pressure. Similar thing is done with artillery shells.

DocRock said:

I think the reason is that with a monolithic bullet, there’s no lead core to swage down so the rings are cut to diminish the surface area engaging the lands, thereby diminishing surface tension and barrel pressure. Similar thing is done with artillery shells.

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Larger steel shells, really only touch the barrel in two places, the bourrelet and the rotating band:



The rotating band is what engages the rifling and imparts the spin on the projectile and provides the gas seal, it was usually copper, although recently they can be seen made of soft iron or steel, or more often plastic. The bourrelet is machined to the diameter of the lands (or "bore"), and just slides along.

There have been shell designs with multiple rotating bands.

DocRock said:

I think the reason is that with a monolithic bullet, there’s no lead core to swage down so the rings are cut to diminish the surface area engaging the lands, thereby diminishing surface tension and barrel pressure. Similar thing is done with artillery shells.

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Yes the original X-bullet from Barnes without the grooves has a pressure spike problem cause by the extra force required to drive the bullet into the riflings. The triple shock and most all copper bullets since have had groove to reduce the amount of copper that needs to be displaced during the swaging into the rifling. This also reduces barrel friction by reducing the contact area.

Anything much bigger than 50 caliber and the body of the projectile is made to land diameter. Frequently most of the projectile body is actually smaller than land diameter except for a band near the base of the Ogive that is a running fit with land diameter and called a bourrelet. The only part of the round that is groove diameter is a driving/rotating band near the back of the projectile, usually made of soft iron or a gilding metal like bronze. The driving band forms the gas-seal and allows the riflings to spin the projectile.

ETA: @lysanderxiii beat me to the punch.

LookAtYou said:

So using a FMJ out of the S&W 686+, would the wound in a person be roughly the same size as the barrel hole at the front?

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Wound channels are a different matter. The temporary channel would be .357 or larger, the permanent less so. Complicated topic full of people with shocking opinions....

LookAtYou said:

would the wound in a person be roughly the same size

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@DocRock has the right of it.
Skin is elastic, and will actually shrink down around a puncture wound. The ER docs tend to segregate handgun GSW by <3/10 inch and >3/10 inch (in very, very broad terms). Rifle injuries tend to be an order of magnitude greater (to match the energy differences).
Exit wounds will tend to be larger, on average, but, that's only on average.

Note, too, that, once in the ER, the docs really can't tell the difference between a .327 magnum and a .357magnum, or a .32acp from a .38super--it much more about if the round expands, and how much and a whole bunch of variables along with all that.

The size of the hole in the barrel is far, far less important than where it, and the shooter, put the projectile.
Put a hole in a human aorta--5mm, for instance, about the lumen of the carotid artery--and that person is in serious trouble. Hit the vagus nerve bundle in there and you are likely to hit the pulmonary vein as well as the aorta--there are few good prognoses from such an injury, barring being shot in an operating room.

But, that's a target maybe 2" square, and the effects diminish rapidly an inch or two to either side. Which why 5 out of 6 people recover from gunshot wounds, even multiple ones.
Shot placement remains far more important than shot diameter.

On a shotgun it is much larger than the size of the shot but smaller than the size of a slug. LOL