.243win unexpected velocity change

9

95GTSpeedDemon

New member
.243
H4350 40.8gr
loaded 5x of 2.9 coal (.035 from jam), 2.885, 2.845, 2.805
92*F
20" barrel
2.9 went an avg of 2881fps
2.885 2863fps
2.845 2845fps
2.805 2829fps

Gorndon's wont duplicate this COAL change to FPS change.
Gorndon's says the 2.9" is 65k, which is a lil spicy. book max is 41.1 @ 2.7"

Ideas/theories?
 
Gordons is a rough estimate. Used published data if you can get it. If not make sure you work up from start, slowly.

Generally the closer you get to the lands you increase pressure.

To me this would say that the longest seating depth is close enough to increase pressure. and the rest are all nominal.
 
GRT has bullet jump option. It is by default disabled, and zero bullet jump is assumed. Shorter coal would cause higher pressure and higher mv.

When bullet jump is enabled and a figure is entered, the simulation will work slightly differently. When coal is shortened, the bullet jump should need to be increased by the same amount. Pressure and MV increase much less, so much so that they may even appear unchanged. I think the assumption is that the primer ignition moves the bullet forward to contact the lands before powder ignition can reach peak pressure.

However I don't recall seeing decreased mv with shorter coal on GRT, as indicated by op's chrono data. Occasionally on chrono perhaps. I believe it is something along the line that little or no bullet jump could cause higher peak pressure. Whatever it is, GRT is unable to model.

-TL

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Im going to have to look at the other bullets i shot in .243 and the 6.5 ones and see if they all acted that way. I was a bit surprised and i dont understand it.
 
You have your average velocity listed for five rounds of each length. What are the SD's for each length? Are the velocities overlapping from group to group?
 
The other two were 95gr, a nosler BT and 95gr berger vld. Forgot to list the first bullet i asked about, the 108 berger elite hunter.

Velocity does overlap some. Sd is between 10.9 & 21.4. Sample size on the 10.9 was only 4 since the chrono didnt catch one.

I have the sd card data from the chrono, ill dig it out if necessary

I found jam on all of them then backed off it some.
 

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With velocities that close and the SD you have I would not form any theory. Every part of reloading operation could change the velocity, the first one being how do you measure the bullet length?
 
tangolima said:
GRT has bullet jump option. It is by default disabled, and zero bullet jump is assumed. Shorter coal would cause higher pressure and higher mv.

When bullet jump is enabled and a figure is entered, the simulation will work slightly differently. When coal is shortened, the bullet jump should need to be increased by the same amount. Pressure and MV increase much less, so much so that they may even appear unchanged. I think the assumption is that the primer ignition moves the bullet forward to contact the lands before powder ignition can reach peak pressure.

However I don't recall seeing decreased mv with shorter coal on GRT, as indicated by op's chrono data. Occasionally on chrono perhaps. I believe it is something along the line that little or no bullet jump could cause higher peak pressure. Whatever it is, GRT is unable to model.

-TL

Sent from my SM-N960U using Tapatalk
Click to expand...
where is the jump option, i cant find it.
 
95GTSpeedDemon,

What you see is normal. With a high-power rifle, if a bullet is in contact with the throat at ignition, peak pressure goes up about 20%. As you back the bullet out, it drops and keeps dropping until you hit the seating depth at which proximity to the throat ceases to be the dominant pressure term, and the elimination of powder space due to deeper seating starts to become the dominant term, after which pressure rises again. Here is a plot using data Dr. Lloyd Brownell developed with a round-nosed Remington bullet in 30-06.

attachment.php


As the case neck is pressurized, before the bullet jumps to the throat, there is a brief period during which gas escapes around the bullet and heads down the barrel ahead of it. It has to pass through the space between the ogive and freebore and throat opening. Dr. Brownell suggested¹ that the temporary stall in pressure rise that resulted would cause not only a lack of gas but let the bullet move forward enough to increase the powder space enough that burning was retarded a bit, and the bullet got further down the tube before the pressure peaked.

A spitzer ogive shape opens and closes the space around the ogive over a shorter distance than the very gradual taper along the sides of the round nose ogive do, so the amount of seating depth change needed to get this peak pressure change is greater for the round nose profile.

Here is an image of what changing seating depth from contact with the lands to 0.030" off the lands does for a pointed bullet shape in 6 PPC:

attachment.php


Again, this is just changing the amount of gas bypassing the bullet before it reaches the throat of the bore.

The bottom line is that your results are expected and surprisingly average for a sample size as small as 5.



¹http://deepblue.lib.umich.edu/bitstream/handle/2027.42/3866/bac6873.0001.001.pdf?sequence=5, pp 44-51 (55-61 on the pdf file page counter).
 
Ideas/theories?
Click to expand...

Lots, but only one that really matters, which is that your rifle, ammo, shooting conditions, and results are unique to you and the day you shot them.

There are a huge number of factors involved in the velocities your chronograph displays, which can even include the angle and strength of the light and how you hold the gun when firing.

No computer program can account for all the variables, no matter how many data points it allows you to enter, the programs must rely on certain constants that may, or may not be precisely what your factors are.

You seem to be expecting precision from a system designed to give general guidelines, and that nearly always leads to some level of disappointment or frustration.

What you have is one example of an observed trend. You're shooting loads in excess of industry standard max loaded length. You did it once, and noted slightly different speeds for different seating depths.

Do it again, as close to exactly duplicating what you did before and see if you get similar results. Then do it a few more times. IF your results show a consistent, repeatable pattern, then you have data. But, it is data specific to your gun, your loads, your shooting, and your conditions at the time.

And when variations are only a handful of fps its tought to say what factor, or combinations of factory might be the likely cause.

Just for example, did you consider the differences in seating depth might change the amount of tension holding the bullet in the case, and that different might accout for 20,30, 50, 60 +/- fps ??

OR perhape the slightly different amount of space in the case due to different bullet seating depths changing the amount of pressure? OR both factors together, and possibly several others also, all at the same time??

I think you're concerned with how many angels are dancing on the head of your pin, and wondering why a computer program doesn't tell you its 19 or 17 when you can see there is a difference...

Good luck with your quest, but other than "when you make a change, you get a change", I've got no specific help, only possibly things that neither of us can know, or prove with certainty.
 
This has been one of the most puzzling and esoteric threads I've seen.

Thanks for showing me how little I know about reloading.

And good luck.
 
No, just curious.
I may play with jamming it, so ill just drop to min charge and try it.
Knowing that pressure increases as case volume decreases, pressure should rise. If its jammed, pressure also rises. I didnt expect the results so i asked for answers. Its nice to see software emulate/calc real world results, but data in doesnt always compute.
Just trying to learn.
 
In general, in high-power rifle cartridges, a 10% reduction in charge weight will lower pressure by around 20%, so it's about right to keep the pressure behind a jammed bullet load a match to the one set back 0.030" or so. Some pistol powders can change pressure more than that for a 10% reduction, so this rule of thumb is only for high-power rifles.
 
OP.

I will check and let you know where the bullet jump option is located when I get home. I used that option quite regularly.

Unclenick.

Thanks for info. You just confirmed what I thought. Just didn't realize the inflexion point is at that much of bullet jump. I shoot quite a bit of milsurp with eroded throut, so mostly to right of the inflexion. Recently I have been doing newer AR rifles. Now I recall seeing lower MV when I tune the coal shorter. Does quickload or GRT model such phenomenon? I think not.

-TL

Sent from my SM-N960U using Tapatalk
 
Unclenick said:
In general, in high-power rifle cartridges, a 10% reduction in charge weight will lower pressure by around 20%, so it's about right to keep the pressure behind a jammed bullet load a match to the one set back 0.030" or so. Some pistol powders can change pressure more than that for a 10% reduction, so this rule of thumb is only for high-power rifles.
Click to expand...
I looked into this issues myself, by analyzing pressure data I could get hold off. 10% increase in powder charge brings about 30% increase in peak pressure. That's what I have been using. I have been reluctant to disclose it on the forum, lest I would be censured for encouraging unsafe practices.

-TL

Sent from my SM-N960U using Tapatalk
 
Bullet jump on GRT is the last row in the cartridge definition block on the left.

-TL
8b3caa5215ad8cad88604ec07640afe7.jpg


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The row above it is for specifying gas port and cylinder gap etc. Pretty handy.

-TL

Sent from my SM-N960U using Tapatalk
 
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