Odd question about bullets...

[Diesle]

Questions about bullets...

When you take a single round of ammo and place it bullet first into the muzzle of your rifle, the round is stopped by the walls of the muzzle before reaching the rim of the case. In other words, the circumference of the bullet is greater than that of the barrel at the muzzle end.

Observation is that the bullet must be compressed by the barrel on its way out. It would have to be 'squeezed' out the muzzle end. Or, lenghtend and elongated....

I’ve read "the bullet is compressed fore and aft by the opposition of barrel friction restraining it, and combustion pressure pushing from behind. The bullet gets shorter. That volume and mass has to go somewhere, so it also gets fatter, literally stretching the bore diameter until the barrel’s compression force inward exactly balances the force of the bullet’s expansion outward."

So a bullet that is already a little bigger than the available space is further shortened and 'fattened' on its way out.

Is that correct?

I’m confused by this, please help.

Diesle

 

[Hkmp5sd]

It could be viewed that way. The gasses pushing the bullet cause it to deform and fill the area around the contours of the rifling. This continues until the bullet has completely entered the rifled portion of the barrel. The bullet also conforms to the curve of the rifling giving it the rotary movement which becomes spin.

If you inserted a larger diameter bullet, it would generate greater chamber pressure and possibly destroy the gun.

 

[Art Eatman]

The barrel is steel; the bullet is (usually) lead and commonly has a copper jacket. With or without a jacket, it's still a lot softer than the barrel.

The bullet diameter is within a thousandth of an inch or so of being the same as the bore. The lands, those little ridges which are spiralled and give spin to the bullet, are commonly about two thousandths of an inch in height.

A recovered bullet will show grooves impressed upon it by the lands.

For accuracy, you first need minimum deformation of the bullet. Then, whatever deformation occurs must be uniform from shot to shot.

The lands are a compromise: High enough to engage the bullet to give it spin, yet not so high as to notably deform the bullet or cut the jacket.

In a rifle, there is very little upset of the base of the bullet when it's fired. Any deformation is constrained by the barrel to be a very slight elongation. Very, very slight.

Hope this helps,

Art

 

[Oleg Volk]

And the need to reduce deformation is why some target rifles in the past had polygonal bore...down side was increased lead fouling.

 

[Meanoldfart]

Let Me try to clarify

This is really pretty simple, BORE size is the size of the hole drilled in the barrel. The size of the hole in a 7.62x39 ,7.62 x51, 30-06, 7.62x54R .300 Win Mag. are all exactly .300 inches.
Then using several different methods, the grooves are cut into the bore.
This is when we start to have some variation in the barrels, depending on how deep the grooves are and how many they are and whether they are equal in width to the lands.
About the minimum depth of the grooves will be.004 in., sometimes they will be as much as .006 in..
Bullet makers make the bullets to fit the GROOVE diameter, not the BORE diameter.
"Most" .30 cal. rifles use the industry standard bullet diameter of .308in., .300in. bore plus two grooves of .004 equals .308.
Some of the Milsurp rifles such as the 7.62x39 have a GROOVE diameter of .312. I just slugged the barrels on all thee of the SKSs that I have and the BORE diameter is .300in and the GROOVE diameter is .312.
But an important fact is that the grooves are twice as wide as the lands. I have never heard of a reason for this, but it may be that it is easier to clean a wide groove than it is to clean a narrow one.

 

[Diesle]

Meanoldfart


Ya, that’s what I was after. The barrel at the muzzle is .3 of an inch, making the bullet of a .300 Win Mag just over that. Say between, .308 and .312.

Your saying that the bullet is made to be the diameter of the barrel at its widest. Which would be the measurement inside the groove.

So the bullet is then compressed (like jello) into the groves on its way out.

When a round is shot into water or ballistic gel, can you see with the naked eye the rails that would be created as a result of this channeling...?

Sorry for the stupid questions, and thanks for the info! Bery helpful...


Diesle

 

[Archie]

Diesle, you got it pretty much right.

Yes. You can easily see the grooves cut by the lands. If you can, poke around the impact area of an outdoor range (if they let you.) About every other trip to the outdoor range I attend, I find some bullet on the deck between the firing line and the impact area.

Typically for 30 caliber guns (US) the bore (land) diameter is .308 and the groove diameter is .3085. (That's on the blueprint, anyway. Most are pretty dang close.)

If you don't have more than three loading manuals, get the latest Lyman book. It shows the groove diameters of the guns used in testing. The information is revealing. Read the intro, too. Lots of good information; some of it crucial, some of it just interesting.
Read it even if you don't reload.

 

[Meanoldfart]

Archie, No flame intended.
The BORE diameter is .300 inches.
The GROOVE diameter may be from .308 inches to .312 inches.

But .308 is standard.

I suppose it would be possible to have a groove diameter of .3085, but it would not be standard for the U.S.

 

[BHP9]

An intersting story about bullet compression is one that came from an experiment that the famous gunsmith P.O. Ackely performed many years ago.

He took a 6.5 (26 caliber) Janpanese Arisaka rilfe and bored the chamber out to 30-06 while leaving the barrel's bore still in 26 caliber. He then fired the gun by remote control (a string tied around the trigger while he hid behind a large tree. When he touched it off the 30 caliber bullet was instantly squeezed down from 30 caliber to 26 caliber. The gun's stock shattered and of course the bore was ruined but amazingly the action held together and did not even crack or suffer bolt lug setback. He rebuilt the action into a sporter and a friend bought it from him and I would not doubt if the gun is still in existance today and would be worth quite a bit to a collector if it could be found.

P.O. Ackley proved that the Arisaka was the strongest Bolt Action Military rifle of WWII because most of the other WWII guns he tested all blew up with overcharges of pistol powder that he loaded into the cartridge cases and none of their chambers were altered like the Ariska's was.