Bullets: "tightness" in barrel and pounds of pressure?

[DFW1911]

I'm not sure if I'm using the right terms, but hopefully you'll get the idea!

How "tight" does a bullet have to be in a barrel to make it function properly and take advantage of the rifling? If you've ever had to use a wooden dowel to dislodge a bullet you'll know they're in the barrel pretty "tight."

Based on this, the force to move the bullet down the barrel and to distance needs to be significant.

So: we hear about cartridges' pressures, but what does that translate to in pounds of force? Using a .45 230 grain as an example, how many more pounds of force does the average +P generate over its non +P counterpart?

I know these are pretty broad / generalized questions, but I don't know what I don't know

 

[Vern Humphrey]

Let's take them in order.

How "tight" does a bullet have to be in a barrel to make it function properly and take advantage of the rifling?

In general, the bullet should be groove diameter. For example, a .30 caliber barrel is .300 inches in bore diameter, with grooves .004" deep. The "groove diameter" measured from the bottom of one groove to the bottom of the opposite groove is .308". And standard .30 caliber bullets are .308" in diameter.

Early in the Smokeless Powder era, it was believed the rather unpredictable powders available then needed "windage" -- space so excess pressure could blow past the bullet. The German 8X57 Mauser is an example of a cartridge and bullet designed on that theory. The groove diameter of the barrels were about .323" but the bullets were only .318". This arrangement was hard on barrels, resulting in gas-cutting, and the S-type bullet was developed with a full .323 diameter. The rifles were re-throated to make them safe with the new, larger bullet.

Another example would be the early Savage rifles. Arthur Savage was fascinated by high velocity and found that oversize bullets gave higher velocity (because they result in higher pressures.) Some early Savage cartridges, such as the .22 Imp, were designed for oversize bullets.

we hear about cartridges' pressures, but what does that translate to in pounds of force?

Cartridge pressure traditonally was measured with a crusher guage -- a cylinder was screwed into a hole in the breech, a piston inserted in the cylinder, followed by a pellet of lead or copper, and the cylinder head screwed on. When the gun was fired, the piston crushed the pellet, which was carefully measured and the results translated into Lead Units of Pressure (LUP) or Copper Units of Pressure (CUP). These are reasonably close to pounds per square inch. Modern electronic pressure systems give pounds per square inch directly.

To use the .45 ACP as an example, pressure runs from around 15,000 CUP to about 17,000 CUP, depending on the powder used, and that will drive a 230 grain bulllet to somewhere between 835 feet per second, and 900 feet per second. Using the formula for kinetic energy, e=1/2*mass*v^2, that gives us from 347 foot-pounds to 413 foot pounds

Using a .45 230 grain as an example, how many more pounds of force does the average +P generate over its non +P counterpart?

For the .45 ACP, +P loads run from about 19,000 to about 20,000 CUP. Depending on the powder used, these loads will drive a 230 grain bullet to between 970 to 980 fps. This gives us about 485 foot-pounds.

 

[Lee Roder]

Foot-pounds is energy, not force.

From my Lee manual, the chamber pressure for 230gr bullet loads looks to be about 20K psi typical. A 45 caliber bullet's cross sectional area is about 0.16 square inches.

Therefore, this chamber pressure applies a force of
(20K pounds per square inch) * (0.16 square inches)
or
3200 pounds to the back of the bullet on its way down the barrel.

Roughly.

 

[hammerklavier]

Interesting... I'm glad the OP asked this question.

 

[SiRed91]

good info, I have wondered this myself

 

[DFW1911]

Thanks, Vern and Lee, good info. Definitely helps answer my question and gives me a bit of perspective.

I appreciate it much.

DFW1911

 

[MichaelK]

Early in the Smokeless Powder era, it was believed the rather unpredictable powders available then needed "windage" -- space so excess pressure could blow past the bullet. The German 8X57 Mauser is an example of a cartridge and bullet designed on that theory. The groove diameter of the barrels were about .323" but the bullets were only .318". This arrangement was hard on barrels, resulting in gas-cutting, and the S-type bullet was developed with a full .323 diameter. The rifles were re-throated to make them safe with the new, larger bullet.

I'm sorry, but this statement is utter nonsense! Where did you get this information? Pick up any reloading manual and you can read in the 8X57 section that the 8X57 was originally designed in 1888 with a groove diameter of .318" and was supposed to shoot heavy round-nose .318" diameter bullets.

In 1905 the 8X57 was redesigned, with the bore diameter increased to .323" but instead shooting a 150 grain spitzer bullet. It shoots just fine, but ammunition makers are concerned that modern .323" ammo will be shot in an old 19th century gun with a .318 bore.

By the way, my information comes from the Speer Reloading Manual, 14th edition, page 576.

 

[wishin]

How "tight" does a bullet have to be in a barrel to make it function properly and take advantage of the rifling? If you've ever had to use a wooden dowel to dislodge a bullet you'll know they're in the barrel pretty "tight."

I'm convinced that the reason Savage rifles have a very good reputation for accuracy out-of-the-box is the exacting tolerances in their barrels.

 

[unspellable]

Bullet vs groove diameter

It's quite true that the 8x57 Mauser's bore was originally enlarged from .318 to .323 for pressure reasons. With the original .318 diameter a number of blow ups were experienced. The bore was enlarged simply to reduce pressure. It was not fully realized at the time that the real culprit was erratic powder performance. Later, with more consistent powder, the bullet was enlarged to match the enlarged bore.

This concept of a using a bullet smaller than grove diameter turns up in other firearms of the period. The original Luger cartridge, the 7.65 mm Parabellum is an example. The bullet is .309 but the groove diameter is .310 or .311.

The change in the 8x57 bullet diameter had unintended consequences here in the USA. Winchester found themselves with a large batch of the smaller sized bullets and no real market for them. They loaded them in the 32 Special which has a groove diameter of .320. If an undersized bullet is fired in a badly worn bore it will produce poor accuracy. This resulted in the 32 Special's undeserved reputation for poor accuracy with a worn bore even though the problem does not arise with a properly sized bullet.

 

[griz]

I can answer one part of the question. A 45 has an area of just under .16 square inches. So if you have a pressure of 20,000 PSI, that's 3180 pounds pushing it out the barrel at that pressure. Of course that would only be for a fraction of the short time that it spends in the barrel, but it gives you a ball park idea of the max force.

 

[DFW1911]

This is interesting stuff.

Thanks for the info. It really helps give perspective on just how strong gun barrels and frames are, not to mention the material the bullet is made out of.

It also sheds a good deal of light on how some "ka-booms" happen.

As I say, the "I don't know what I don't know" quotient of my knowledge base is diminishing a bit.

Thanks again,
DFW1911

 

[Vern Humphrey]

I'm sorry, but this statement is utter nonsense! Where did you get this information? Pick up any reloading manual and you can read in the 8X57 section that the 8X57 was originally designed in 1888 with a groove diameter of .318" and was supposed to shoot heavy round-nose .318" diameter bullets.

In 1905 the 8X57 was redesigned, with the bore diameter increased to .323" but instead shooting a 150 grain spitzer bullet. It shoots just fine, but ammunition makers are concerned that modern .323" ammo will be shot in an old 19th century gun with a .318 bore.

Nope. The cartridge was redesigned, necking it for the .323" bullet. By opening the throat of original 8X57 Mausers, .323" bullets can be safely shot. The danger in shooting older Mausers is that the neck is held too tightly in the chamber (you have to forcibly shove the bolt home) and will not freely release the bullet without raising pressures to dangerous levels.

 

[Lee Roder]

[this] helps give perspective on just how strong gun barrels and frames are ...

yep what happens when that pin initiates everything that follows is truly nothing to fool around with. just check out saxonpig's recent post about the savage. lots of "banana peeled" barrels on youtube too.

 

[unspellable]

The Mauser

This is getting a bit murky.

To begin with, both the service rifle groove diameter and the bullet diameter were .318.

Following a few blow ups, the groove diameter was increased to .323.

After improvements in powder consistency, the bullet diameter was increased to .323. This probably would require throating out the existing service rifles with .323 groove diameter to allow for the increased neck diameter. From that point on new service rifles were manufactured fully chambered for the .323 cartridge.

Off to one side, the military did not have control of sporting rifles, so it was hit or miss as to what bore they ended up with, thus the old .318 diameter cartridge stayed in production for sporting rifles.

 

[jpwilly]

I'm convinced that the reason Savage rifles have a very good reputation for accuracy out-of-the-box is the exacting tolerances in their barrels.

I own a Savage and can wholeheartedly disagree. The bore in my rifle has machine chatter marks the whole length. No doubt the bore size is right and the button rifling method works but there's much more to a Savage's accuracy than their barrels...there's got to be! JMO