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Can anyone point me to trajectory charts for the 5.56 x 45 rounds for the 55 gr M193 bullet and the M855, 62 gr bullet? Or better yet, can you supply the Ballistic Coefficient for the M855 round so I can use the calculators?
 

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.304 for the M855.
 

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.243 and .304 are correct for UNFIRED bullets, however when rifling is engraved, it lowers the BC by quite a bit. I have a software program called "Ballistic Explorer" where I can chronograph a test batch of ammo at 4 yards and again at 100 yards then plug those average velocities into Ballistic Explorer to determine actual BC. I did the BC test for M-193 bullets (factory L C headstamp, 55gr FMJ) and fired them in my AR556 with a 1:8 rifling twist. The result was a BC of .201 …. notably lower than that of a virgin bullet. I did not test M-855 bullets but I'm sure they would be lower than the published BC.

I have tested many other rifle cartridges with a host of different bullets and all of them had actual BCs that were lower than the BCs stated in reloading manuals by the bullet manufacturer. Once I established the actual BC, my bullet drop charts match nearly perfect with actual fired cartridges out to 500 yards …. probably further but that was the max distance I could shoot at my son's range. If I used the published BCs, my targets at all distances would all indicate notably low POI.

If you stop and think about it …. there's no way a bullet manufacturer can determine what gun you are going to shoot …. it's twist rate or depth of rifling so they publish factual data for virgin bullets. I used to think that was a marketing issue …. making bullets look better than they really are but I don't know how they could do anything else other than to inform reloaders that actual BCs are lower in fired gun than with virgin bullets.
 

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Discussion Starter #6
.243 and .304 are correct for UNFIRED bullets, however when rifling is engraved, it lowers the BC by quite a bit. I have a software program called "Ballistic Explorer" where I can chronograph a test batch of ammo at 4 yards and again at 100 yards then plug those average velocities into Ballistic Explorer to determine actual BC. I did the BC test for M-193 bullets (factory L C headstamp, 55gr FMJ) and fired them in my AR556 with a 1:8 rifling twist. The result was a BC of .201 …. notably lower than that of a virgin bullet. I did not test M-855 bullets but I'm sure they would be lower than the published BC.

I have tested many other rifle cartridges with a host of different bullets and all of them had actual BCs that were lower than the BCs stated in reloading manuals by the bullet manufacturer. Once I established the actual BC, my bullet drop charts match nearly perfect with actual fired cartridges out to 500 yards …. probably further but that was the max distance I could shoot at my son's range. If I used the published BCs, my targets at all distances would all indicate notably low POI.

If you stop and think about it …. there's no way a bullet manufacturer can determine what gun you are going to shoot …. it's twist rate or depth of rifling so they publish factual data for virgin bullets. I used to think that was a marketing issue …. making bullets look better than they really are but I don't know how they could do anything else other than to inform reloaders that actual BCs are lower in fired gun than with virgin bullets.
This makes a lot of sense. the fact that your measured ballistics data matched the calculated theoretical ballistic curve when the lowered value of BC was used is a strong argument in your favor. Incidentally, I found values of the M855 round BC of 0.370 down to 0.270 in various articles and forums on the internet. It could be that someone did the same thing you did and hence the lower BC.s. I don't know how they could have gotten a value of 0.370 however.
After thinking about it some, I imagine BC is also effected by the rifling twist not only because of the different spin but the different a angle of the rifling grooves on the projectile.

Thanks for your reply.
 

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You learn something new everyday. And it is one of those your you slap your head and go it’s obvious also.

Thank you learned something new today.
 

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Here's what I know about twist rate versus ballistic coefficient ….. Rifling engraving on a bullet acts much like an impeller in a jet engine but instead of creating thrust, it does just the opposite and creates additional air friction. The twist rate and depth of the rifling engraving will determine the amount of added air friction based on a text book bullet with no rifling engraving. Here's where the math gets fuzzy …. faster twist rates (or deeper engraving) will reduce the bullet's BC but at the same time, a faster spin rate will keep the bullet stable in flight for a longer distance. So the net result for long distance shooting with a fast twist rate barrel is …. velocity will slow down from the extra air friction on the rifling engraving, which takes the bullet longer to reach the target and in turn, causes the bullet to drop more. However a faster twist rate will maintain stability farther downrange than a slower twist rate. For long range shooting, it's all about bullet stability. When a bullet starts to yaw and tries to swap ends (because the base is heaver than the nose) it becomes unstable and accuracy goes down the tubes.
 

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Are the riflings a complete negative? The aircraft tech in me has to ask, if they have aero affect they are spinning in the direction they need to act as a screw, if not a propeller.
A lot to think about other than a lump shoved through the air isn't it?
 

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Sr40ken, What happens when the engine in a small aircraft dies and the prop can't be feathered? The propeller tries to keep the engine turning and because it resists air friction, the aircraft slows down rapidly …. no different than a bullet.

Many people think rifling engraving on a bullet keeps it spinning and that would be true if the bullet's velocity was higher than when it left the muzzle. However, bullets start slowing down the moment they clear the muzzle so the rifling engraving becomes a drag, not a bullet spinner. The formula for determining a bullet's spin rate when it leaves the muzzle is: RPM = velocity times 12 divided by twist rate, times 60. As an example, let's say a rifle barrel had a twist rate of 1:9 and the bullet exited the muzzle at 3000 fps. 12/9=1.33, 1.33X3000=4000, 4000X60= 240,000 RPM.

In this example, when the velocity of the bullet drops below 3000 fps, rifling engraving will actually slow the bullet down. How much the bullet's velocity decays will depend directly on the bullet's ballistic coefficient and the depth of the engraving. If somehow a bullet could be rocket propelled where velocity increased, the bullet's spin rate would also increase.
 

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More bullet spin stuff:

Rifling in a rifle barrel was invented centuries ago when muzzle loaders were used with round lead balls. When round lead balls were forced into a bore, they reshaped a little …. somewhat resembling a football. If the bullet was not spun, the football shaped bullet would travel end over end through the air and not stay on a direct path and not maintain very good accuracy. When bullets were spun by rifling, they developed a gyro effect, much like a toy top. As long as the bullet was spinning fast enough, the gyro effect would keep the bullet from toppling end over end, thus accuracy was vastly improved. A very slow 1:40 twist rate was enough to keep round lead bullet stable in flight.

When cartridges loaded with more modern bullets started being used, rifling became even more important because the noses of nearly all bullets are lighter weight than the rear end. That means without the gyro effect from rifling, it's much like throwing a dart tail first where it wants to reverse ends and go heavy end first.

Air friction not only slows the bullet's velocity, it also causes the bullet's spin rate to decay at a proportional rate. At some point downrange when the bullet's spin rate drops below the point of gyro stability, the bullet will start to yaw and soon after it will begin to tumble, adversely affecting accuracy. Bullets with higher ballistic coefficients will travel farther downrange before they become unstable. Likewise, bullets that are spun faster by the barrel's rifling will also maintain enough spin to stay in a stable path. In high quality "very low drag" bullets that are spun very fast, it is not unusual for them to maintain stability for a couple thousand yards whereas lighter and less aerodynamic rifle bullets may run out of stability after just a couple hundred yards.

Handguns are not intended for long range shooting so they typically have a very slow twist rate to prevent inaccuracy from barrel torque. As such, handgun bullets may begin to lose stability and keyhole as soon as 50 yards. A couple exceptions ….. hollow base wad cutters are nose heavy so they don't want to swap ends and tend to be very accurate provided they are not spun too fast. Dual ended wad cutters (DEWC) are extremely stable in flight and will maintain stability until they finally hit the ground. They are not spin rate sensitive nor velocity sensitive but they are wind drift sensitive because of their very poor ballistic coefficient. A low BC makes them a poor choice for rifles at longer shooting distances.
 
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