Theory vs. load data, what gives?

[huntsman]

In keeping with the safety rules of reloading, I am sticking to the published data and starting low and working up my loads, just so we are all clear before I begin this thread.

However, in studying the load data for the 6.5x55 SE with 129 grain Hornady bullets, I couldn't help but notice that with certain powders the velocity DECREASES with the 129 gr compared to the 140 gr with the same charge. How can this be? I just doesn't seem logical that an equal force (the exploding charge) could push less mass with less friction area(shorter 129 gr bullet) at a slower velocity than it pushes a greater mass with more friction area (longer 140 gr bullet).

Can anyone out there explain this confusing scenario?

[skb2706]

One theory could be that there is an error in the data. Another would be that there is more pressure built up behind the heavier bullet with the exact same charge weight of powder. Higher pressure = higher velocity.....in theory.

[Robert]

The reason is that the lighter bullet is a lot easier to get moving down your barrel.  It gets out of the muzzle a lot faster.  The heavier bullet will actually burn the powder more efficiently.  If you just increase the powder with the lighter bullet, it will blow un-burned powder out the muzzle, especially with shorter barrels.  However...you can use faster burning powders with the lighter bullets and possibly get the same velocity.  I dont know what data or powders you are looking at, but Accurate lists the 6.5 with considerable increase with the lighter bullets with most powders.
  It could also be possible that the accuracy was bad with hotter loads, so they listed only the best loads for that bullet.

[huntsman]

skb,

I'm looking really hard at your first theory. Nothing else seems to make sense given the laws of physics.

On your second theory, I'm going to turn the tables on you.

Another would be that there is more pressure built up behind the heavier bullet with the exact same charge weight of powder. Higher pressure = higher velocity.....in theory.

Higher pressure (with the same explosive force=powder charge) is only possible if there is less volume/time in the chamber and barrel (P=f/a), which in turn can only occur if the bullet moves more slowly down the barrel. A slower bullet would have less velocity, not more.

Robert,

I am looking at Accurate's data, along with Lee's and Loadbooks USA for the 6.5x55. All of them show a drop in velocity from 140 grain down to 129 grain with the same charge. No other bullet weights show a similar relationship (160 gr, heavier, has decreased velocity, and 120 gr and down show a significant increase in velocity).

You said regarding why the 129 gr has less velocity than the 140 gr:

The reason is that the lighter bullet is a lot easier to get moving down your barrel. It gets out of the muzzle a lot faster.

A bullet moving faster out of the end of the barrel is the definition of higher velocity. If this scenario were true, then the 129 grain would indeed have a higher muzzle velocity than the 140 grain. My contention is that it must have a higher velocity for this very reason, and the data must be wrong.

For the 129 gr bullet to first go faster down the barrel and then end up exiting the barrel slower than the 140 gr, the 129 gr bullet must decelerate (slow down) before leaving the barrel. Pressure would have to drop off much faster with the 129 gr than with the 140 gr. This does not seem logical given the fact that both bullets are being pushed by the exact same force. Thus the conundrum.

Thanks to both of you for the input. I'm still scratching my head...

[Black Jaque Janaviac]

I think what was trying to be said is that some powder depend on pressure to burn properly.  This can particularly be noticed with magnum pistol powders.  

Smokeless is known as a "progressive" powder.  In other words, if you just touch it off as it lays in a pile on the ground the powder will just burn like a flare.  But if you confine it and ignite it the pressure builds, then the powder burns faster, which builds more pressure, then the powder burns faster, which builds more pressure, then the powder burns faster, which builds more pressure, then the powder burns faster, which builds more pressure, then the powder burns faster, which builds more pressure, then the powder burns faster, which builds more pressure, then the powder burns faster, which builds more pressure, then the powder burns faster, which builds more pressure, then the powder burns faster, which builds more pressure---KABOOMM!!!

For some powders there is a magic minimum pressure.  Below, say 25000 cup, a powder might blow the bullet out the muzzle, but the velocities will be erratic, fouling will be great, and accuracy miserable.  Once 25000 cup is reached the powder suddenly burns more consistently and the shooter is happy.

I don't know if that's the case with your situation.  I'd say you're best contacting the publisher of the data for the explanation.   The rest of us can only speculate.

[bpjon]

"For the 129 gr bullet to first go faster down the barrel and then end up exiting the barrel slower than the 140 gr, the 129 gr bullet must decelerate (slow down) before leaving the barrel. Pressure would have to drop off much faster with the 129 gr than with the 140 gr. This does not seem logical given the fact that both bullets are being pushed by the exact same force. Thus the conundrum."


Maybe, maybe not.  The lighter bullet may be easier to get going, and starts more quickly down the barrel, causing the pressure to drop more quickly, thus slowing the burning rate of the powder.  The heavier slug starts more slowly, allowing more pressure to build, which increases the burning rate of the powder, which causes a pressure increase, etc., until the bullet exits the muzzle.  In effect, the heavier bullet causes the powder to burn more completely, thus extracting the available energy more efficiently.

This powder behavior is not uncommon in handgun cartridges loaded with very slow (for handguns) powder.  For example, a 325 grain bullet may outrun a 250 grain bullet over identical charges of 296/H110.

[huntsman]

Now we have a missing piece of the puzzle! And the former statements by skp and Robert make a whole lot more sense to me now. The key here seems to be that the powder behaves differently in each case because the two bullets exert different resistance forces on the expanding gases. In essence, then, we are NOT dealing with the same FORCE pushing the two bullets (as I had assumed) even though we have the same powder charge.

By this theory, we have the 140 grain bullet exiting the barrel at about the peak of the pressure buildup, whereas the 129 grain bullet is exiting the barrel somewhere before that peak in pressure. Thus the exit pressure pushing the 140 gr bullet is higher than the exit pressure pushing the 129 gr bullet. The 129 grain bullet has been pushed to the end of the barrel with less pressure (and force) because it has less resistance than the 140 grain bullet. This would explain how the velocity of the heavier bullet could actually be higher.

But there is still the puzzle of why the 120 grain bullet does not behave in the same manner as this theory would predict. The above theory would have the 120 grain bullet exiting by even less force (pressure) and at a lower velocity even than the 129 grain, but the opposite is true according to the data. Still have that durned itch on my scalp....

Thanks BJJ and bpjon for your insightful analysis.

[The Shrink]

Huntsman

You've got two variables not yet accounted.  One mentioned above is the speed of burn of the powder.  This will affect the inital impulse that starts the bullet moving and will affect the continuous pressure once the bullet is out of the case and throat.  The second is the presence or absence of a crimp on the case that holds the bullet in place to increase that initial impulse.

Stepping back to black powder, the USArmy found that the lighter 405 gr bullet in the 45-70 required a heavy crimp while the heavier 500 gr bullet did not.  

Wayne the Shrink

[unspellable]

Any powder will perform best in a certain pressure range.  In general,  shotgun and pistol powders will burn properly at the lowest pressures.  In any reasonable rifle load the peak pressure will be reached within the first six inches of bullet travel up the barrel.

For the two loads in question here, if the data is correct, I would assume they happen to land around some quirky point in the powder's burn characteristics.  If the light bullet starts moving too soon the pressure and burn rate never reach the point they do with the heavier bullet.