Muzzle energy?

[andym79]

Where can I find details of muzzle energy in ft/lbs or equivalent for different cartridges and bullet grain? So that I can easily compare the ‘power’ of different combinations of casing and bullet.

I know for example that a .454 Casull 260 gr has 1871 ft. lbs at the muzzle (7.5" barrel), whereas a .480 Ruger 325 gr has 1315 ft. lbs (a lot less, given a greater weight), but I want someting else to compare it to!

Am I right in thinking that the longer a barrel the higher muzzle velocity tends to be?
Does this get to a point where it starts to decrease, would say a 36" barrel slow the bullet or not?

 

[grendelbane]

I am not a mathematician, nor do I play one on TV.

However, if you take a pocket calculator, and multiply the velocity by itself, (the sqare of the velocity).

Then divide that number by 450240. Then multiply that number by the weight of the bullet in grains.

The number you get will be foot-pounds of energy. Not a very useful number, but that is it.

Example; .45 ACP with 230 grain bullet traveling at 840 FPS.

840 x 840 = 705600

705600 / 450240 = 1.5671641

1.5671641 x 230 = 360.44774

Have fun

 

[MachIVshooter]

Am I right in thinking that the longer a barrel the higher muzzle velocity tends to be?

Correct.

Does this get to a point where it starts to decrease, would say a 36" barrel slow the bullet or not?

No. Within reasonable parameters (no 150" barrels, etc.), a longer barrel will always produce a higher velocity. However, each cartridge (more specifically, each load) has it's optimum barrel length. Especially with rifle cartridges, there comes a point when the velocity increase may not be enough to justify the additional weight and length. For example, a .308 win completes the majority of the powder burn in about 22", thus adding 6 or 8" does not increase velocity enough to be feasible. Also, super long tubes are less rigid and tend to "whip" more than moderate length barrels. This hinders accuracy. Most rifle cartridges develop the majority of their velocity in 20-26" barrels. Magnum handgun rounds are usually optimal in 7-10" tubes, standard handgun rounds are best suited in 5-6" barrels. Obviously, there are many variables that can contradict these generalizations, but this is a good starting point.

 

[pauli]

22lr starts losing velocity around the 18-20" mark, as i understand it. there's no ballistic reason for a longer barrel in this case, though there are other possible reasons (higher capacity tube mag, longer sight radius, looks, sound).

 

[MTMilitiaman]

Actually, the majority of any handgun or rifle's velocity is achieved within the first several inches. Velocity gain is proportionally less the further down the barrel the projectile gets until eventually, the forces of friction are greater than the pressure behind the projectile. At this point, the bullet will actually lose velocity. This point is usually a fairly long ways down the barrel but it means you probably won't gain anything by putting a 26 inch barrel on a 9mm carbine, for example. Also, heavier bullets will tend to be less affected by an increase or decrease in barrel length.
For example, if you have a 24 inch barreled .30-06, all projectiles, regardless of weight, will gain less velocity in inches 22-24 than they will have in inches 20-22, and so on. If you wish to cut the barrel down to 22 inches, the generally accepted rule to is discount 40 to 50 fps per inch for most centerfire rifles. Heavier bullets will less affected than lighter bullets--for example, a 180 gr bullet may lose only 60 or 70 fps from two inches of shorter barrel whereas a 150 gr projectile could probably suffer 100 to 120 fps.

Energy can be important and is the most common way to compare the relative power of a moving projectile, but you should keep in mind that the equation heavily favors velocity because it is squared, but there are plenty of instances where velocity is no substitute for bullet mass and frontal diameter, regardless of energy. Hunting dangerous game, for example. Likewise, bullet performance is at least as important as energy and bullet placement is obviously much more so.

 

[Vern Humphrey]

The powder BURN is completed within the first inch or so of travel down the bore -- at the point where peak pressure occurs. The rest of the trip down the bore is driven by the expansion of gas. The longer the bore, the more the gas expands and the lower its pressure drops.

At some point, the pressure is so low that it cannot overcome the coefficient of friction of the bullet, and acceleration stops. This would be a long way, of course. But somewhere between 16" and 36" (depending on the cartridge) the pressure is so low that increases in velocity by adding barrel length just don't make sense.

 

[miko]

Actually, the majority of any handgun or rifle's velocity is achieved within the first several inches.

A bullet moving at 1500 fps has more than twice the energy of a bullet moving at 1000 fps. The first 1000 fps may have been achieved in the first several inches but it's the last 500 fps that determine most of the bullet's energy.

miko

 

[GunGoBoom]

a .22 starts to lose velocity between 12 and 16 inches. So yes, CERTAIN lower powered cartridges more powerful than .22 may start to lose velocity at or around 36 inches, when the powder is burned, and friction overtakes the expanding gases. So, the answer is, it depends on the cartridge. And yap, the formula is weight of bullet times velocity squared = energy. Another useful formula is momentum (sometimes called 'knockout' or 'knockdown' factor) - it is simply weight of bullet times velocity.

 

[Cosmoline]

Check the ballistic charts in Gun Digest. They're handy, and keep expanding every year. I learned an enormous amount just by comparing the cartridges on the chart when I was first learning about shooting. Also, Chuck Hawk's on-line cartridge information has a lot of comparative ft. lbs.

For calculations on handloads, I cheat using this on-line ft. lbs. calculator

http://members.aol.com/vintairgun/fpcal.htm

http://www.archeryexchange.com/information/info_pages/kinetic/kinteic-calculator.shtml

They're on an airgun and bow site, but the physics are the same. Just enter the grain weight and velocity and you get the Ft. lbs.

The weight of the bullet is important as was noted, but arguably its sectional density is even more important for a hunting round as this determines how easily it will penetrate. The higher the SD, the less force is required to get a set amount of penetration. This is why heavy 6.5mm and 7mm bullets will still drop big game with such inconsequential ft. lb. readings.

There is also the velocity at which bullets begin to have an exposive effect on flesh. IIRC this is about 2,200 fps. This is one factor that gives smaller, faster bullets some advantages over larger, slower bullets--at least up to a point. You can accomplish the same basic result with very different ballistic profiles. For example, a 200 grain .30 caliber SP bullet at 2,400 fps. is considered adequate against brown bear, and so is a 400 grain .45 caliber hardcast slug at 1,900 fps.

 

[Vern Humphrey]

A bullet moving at 1500 fps has more than twice the energy of a bullet moving at 1000 fps. The first 1000 fps may have been achieved in the first several inches but it's the last 500 fps that determine most of the bullet's energy.

Kenetic energy increases with the square of the velocity. And yes, a 50% increase in velocity is worth quite a bit.

But in most cases, we're talking not about a 50% increase, but more like a 1% to 5% once you reach optimum barrel length. For example, a .30-06 might drive a 150 grain bullet to 3,000 fps with a 24" barrel, and to 3075 fps with a 26" barrel. That's not enough to worry about.

 

[miko]

3000 to 3075 - that's 5% increase of energy for 8% increase in barrel length. As much energy as the same 150 grain bullet has travelling at 675 fps.

Considering that the barrel may be tapered and is only a part of the whole rifle, one may be getting 5% increase of energy for 4% increase of length and 3% increase of weight of the rifle and 2% increase of recoil.

The choice depends on particular situation/preference but the values are not trivial.

miko

 

[MachIVshooter]

For example, a .30-06 might drive a 150 grain bullet to 3,000 fps with a 24" barrel, and to 3075 fps with a 26" barrel. That's not enough to worry about

3000 to 3075 - that's 5% increase of energy for 8% increase in barrel length

Actually, the velocity gain for a 150 gr. bullet going from 24 to 26 inch barrels is a mere 43 FPS, or a gain of 86 FPE. To gain 75 FPS you would have to go to a 28" tube.

The point Vern Humphrey is trying to make is that it simply isn't worth an extra 4" of barrel for ~150 ft/lbs at the muzzle. If you have ever hunted with a 26" barrel rifle, that is bad enough. I am 5'11", and my .375 Ultra's 26" tube is always getting snagged. I find it much easier to carry the 24" .30-06 or 22" .280. Of course, the .375 is burning between 95 and 103 grains of powder, so it needs all 26". But for most centerfire rifles burning 45-65 grains, 22" or 24" is optimal. And only a few cartridges need more than 26", such as the .50 BMG.

Would you be willing sacrifice the handling of your sports car to gain 5 or 10 MPH top speed? Same concept.

 

[miko]

I am 5'11", and my .375 Ultra's 26" tube is always getting snagged.

That's what you get for using 14-th century design. You need a bullpup.

miko

 

[Vern Humphrey]

If you have ever hunted with a 26" barrel rifle, that is bad enough

There is a reason why old-timers cut the 30" barrels of their surplus Krags to 24" or less.