Dumb Question on Grain and Round Performance

[JWH]

I am having a hard time trying to grasp the concept of bullet grain amount, and its performance. It is most dumbfounding whenever I run across the following situation....

-Two bullets of same caliber
-One bullet has less grain
-Bullet with less grain, has higher fps?

A simple explanation would be greatly appreciated.

[Reynolds]

Caliber would be roughly defined as the diameter of a circular cross section of the bullet or "size" of the bullet. It can be measured in either metric or SAE units or both (AK bullets are 7.62mm or .311 inch). Grain measures the weight of the bullet. 7000 grains equals one pound. Other countries also use the metric gram as a unit of measure for bullet weight (453.6 grams are equivalent to one pound). Bullets may be the same caliber and have different weights, usually by being either longer or shorter than another bullet. It takes less force to overcome the inertia of a lighter bullet and put it in motion, therefore the same amount of force will impart a greater speed to a lighter projectile if all other factors are equal. That is why lighter bullets move faster.
However, a lighter bullet also has less kinetic energy to transfer to the target if it is traveling at the same speed as a heavier bullet (inertia, again). So, lighter bullets have disadvantages too and aren't the solution to all applications.

[JWH]

Thanks for the info!

[CR]

The difference in bullet weights is more obvious in pistols. Let's compare 110gr and 158gr .38spl rounds.

Generally, lighter bullets need slightly longer barrels to get to their higher velocity. A 2" snubbie would fire a light bullet almost as slowly as a heavy one.

Lighter load usually has higher velocity at the expense of brighter muzzle flash and noise. Heavier bullets produce more felt recoil.

On target, performance depends on bullet design as well as on the weight. Heavier bullets generally keep their momentum (and velocity) out to longer ranges.

Corrections to my take on the issue are welcome.

------------------
Cornered Rat

http://dd-b.net/RKBA
"Juden Haben Waffen!"

[flashguy]

Originally posted by CR:
The difference in bullet weights is more obvious in pistols. Let's compare 110gr and 158gr .38spl rounds.

Generally, lighter bullets need slightly longer barrels to get to their higher velocity. A 2" snubbie would fire a light bullet almost as slowly as a heavy one.

Lighter load usually has higher velocity at the expense of brighter muzzle flash and noise. Heavier bullets produce more felt recoil.

On target, performance depends on bullet design as well as on the weight. Heavier bullets generally keep their momentum (and velocity) out to longer ranges.

Corrections to my take on the issue are welcome.


Heavier bullets fired at the same velocity will produce more felt recoil, which is roughly proportional to the momentum (mass X velocity); if the bullets are fired at velocities which produce the same momentum, there should be little difference in felt recoil. As a general rule, though, heavier bullets can be safely driven to velocities which do produce more momentum and felt recoil than light bullets. (Many factors contribute to this.)

Another consideration might be whether the handgun being used has a "compensator" (a device attached or built in to the muzzle of the barrel which redirects high-velocity gasses upward to counter muzzle flip). Light-weight bullets driven faster usually also have a larger volume of gasses and make the compensator work better. Compensators are usually only found on speed match specialty pistols (action shooting).

[Kukov]

Originally posted by CR:

Generally, lighter bullets need slightly longer barrels to get to their higher velocity. A 2" snubbie would fire a light bullet almost as slowly as a heavy one.

Lighter load usually has higher velocity at the expense of brighter muzzle flash and noise. Heavier bullets produce more felt recoil.

Corrections to my take on the issue are welcome.


Internal ballistics (what goes on inside the barrel) are not as cut and dried as that suggested above.

Burn charateristics and pressure curves in modern powder are reliant upon bullet resistence generating pressure and that pressure in turn generating ignition. Double base handgun powders contain nitrogylcerine which ignites from pressure.

All things being equal, a lighter bullet in a short barrel revolver will not produce the back pressure necessary for complete ignition. The result MAY be less terminal pressure and less terminal velocity in the lighter bullet.

A heavier bullet MAY produce more initial pressure in the short barrel and, consequently, generate a steeper pressure spike as it exits the barrel. The result MAY be a higher working pressure, and a higher terminal velocity with the heavier bullet.

BUT, a lighter bullet MAY accelerate faster and achieve a higher velocity at a lower total muzzle energy in a shorter barrel. Inertia plays a role here.

There are numerous variables at work in internal ballistics. Primer ignition, case crimp, true bore diameter, rifling characteristics (depth/width/rate of twist), bore wear, bullet material, external temperature, humidity . . .

That's why it's useful to test your loads in a specific gun, and why reloaders work up loads for specific powders, bullets, and guns. It IS almost rocket science.



[This message has been edited by Kukov (edited 10-07-2000).]