Reading load data

[Throwin Lead]

I have a question about reading the load data charts. This is for 40SW.

For example - 180gr bullet OAL 1.135 Starting load 4.0 gr = 780FPS Max load 5.0 gr=900 FPS

My goal is to make a minor load. Using the starting data the PF should be about a 140 PF

If I divide the 780FPS by the 4 gr that value equals 195. Am I correct to assume that each tenth would be worth 19.5 FPS? I could work the load from the max value too. 900/5 = 180FPS per grain - the difference here is that at the lower charge weight the velocity is 15FPS greater. Would you average the 2 values getting a result of 187.5 fps per grain?

Using the low charge calculation from 4.0 gr:

A charge of 3.9 gr times 195 should equal a velocity of 760FPS making a 136.9 PF.

A charge of 3.7 gr times 195 should equal a velocity of 721.5FPS making a 129.87 PF.

I understand this load or any load would be subject to a chrono test and an accuracy test to verify expected results.

I'm just trying to understand the relationship of charge weight to velocity for working up loads.

Another question regarding bullet type. If you find a load for your bullet weight but it is not the exact same type of bullet how does that factor in regarding the velocity? For example if the load data states 180gr JHP and you are using a FMJ RN. What is the variance of using one bullet type over another? Is a JHP faster/slower than a FMJ RN? Is there a chart published somewhere that explains the velocity variances of one type of bullet to another?

How does OAL affect the load? My guess would be longer = slower. If correct how much slower?

Is there a significant difference in FPS between loading at 1.125 vs 1.130or 1.135?

[HoMiE]

I have a question about reading the load data charts. This is for 40SW.

For example - 180gr bullet OAL 1.135 Starting load 4.0 gr = 780FPS Max load 5.0 gr=900 FPS

My goal is to make a minor load. Using the starting data the PF should be about a 140 PF

If I divide the 780FPS by the 4 gr that value equals 195. Am I correct to assume that each tenth would be worth 19.5 FPS? I could work the load from the max value too. 900/5 = 180FPS per grain - the difference here is that at the lower charge weight the velocity is 15FPS greater. Would you average the 2 values getting a result of 187.5 fps per grain?

Using the low charge calculation from 4.0 gr:

A charge of 3.9 gr times 195 should equal a velocity of 760FPS making a 136.9 PF.

A charge of 3.7 gr times 195 should equal a velocity of 721.5FPS making a 129.87 PF.

I understand this load or any load would be subject to a chrono test and an accuracy test to verify expected results.

I'm just trying to understand the relationship of charge weight to velocity for working up loads.

Another question regarding bullet type. If you find a load for your bullet weight but it is not the exact same type of bullet how does that factor in regarding the velocity? For example if the load data states 180gr JHP and you are using a FMJ RN. What is the variance of using one bullet type over another? Is a JHP faster/slower than a FMJ RN? Is there a chart published somewhere that explains the velocity variances of one type of bullet to another?

How does OAL affect the load? My guess would be longer = slower. If correct how much slower?

Is there a significant difference in FPS between loading at 1.125 vs 1.130or 1.135?

Too many variables to give definitive answer, but for the most part if you stay within min-max range charge/velocity is "nearly" linear relationship. But, this varies with different bullet/powder/case/primer/OAL/etc. combinations.

To figure out the slope (FPS) of the linear equation you take the change in Y (Velocity) divided by the change in X (Charge Weight gr.). Change in FPS = (900-780)/(5.0 - 4.0) = 120 FPS or 12 FPS per 0.1 gr. But like I said, it's almost a linear. Your "real world" testing will probably be higher than simple interpolation. Start low and work it up.

Regarding different bullet types, that depends also. Some bullets vary by diameter size and jacket hardness/composition just by different manufacturing processes. Some bullets copper jackets deform more or less affecting pressure/velocity. Barrels vary also, you might not get the same velocity as your buddy with the same pistol/rifle.

OAL is another variable that affects your case capacity and pressure curve. Some powders burn "faster" with more unused area in the case others don't. As you start to make cartridge longer you increase case capacity usually making pressure lower for same powder charge, up to a point. As the bullet starts to get closer to the leade/lands then pressure curve will change, quickly sometimes leading to overpressure, so you have to be careful when trying any different changes while reloading.

[Kingman]

simple answer.

It has to do with volume curves, pressure curves etc.

So no it is not linear.

[HSMITH]

The ONLY way to know is to chrono map the loads. Some powders in certain combinations are very linear and others are not even close.

[George]

DO NOT expect linearity in charge weight versus velocity. Even if you find linearity by careful experimentation, do not assume it will translate to any degree.

[revchuck]

Good advice above. One thing nobody else touched on is FMJ vs. JHP OAL and its influence on pressure.

When you think about it, JHPs are longer than FMJs (assuming same bullet weight). That's because they have this hole taking up space. If you use JHP OAL and seat an FMJ, you'll get a lower velocity and pressure because the base of the bullet isn't as deep as it would be with the JHP. Conversely, if you use FMJ OAL and seat a JHP, you'll get higher velocity and pressure - not a problem when loading down as you are, but could cause excessive pressures if you're at a max load.

Another thing - some powders work great at high pressure and lousy at low pressure. If cleaning your gun involves a shovel, you might want to reconsider your powder choice.

[Stoats]

In principle, the following equation applies:

Kinetic energy = (constant * potential energy of powder[ proportional to mass of powder]) - (constant losses)

0.5Mv^2=(KMp)-©

expressed as velocity, and given a given bullet, case, seating depth, crimp etc, :

v = k * sqrt ((Mp) - C)

or

v^2 = (k^2 * Mp) - C

Where v= muzzle velocity, Mp= mass of powder, k and C are constants.

Given two data points, you can calculate k and C. it is possible that C will be so small as to be effectively zero

Given three data points, you can check how close your "model" is to reality if you calculate the two constants based on the outliers.

Do not, however, attempt to extrapolate too far outside the range for which you have data.

Edited for wonky maths

Edited July 10, 2007 by Stoats

[Brazos SC Shooter]

Reading some of these posts made my head hurt.

The only thing I can, which I read on this site, is do not use OAL as a means for changing PF. Focus on matching bullet weight and powder charges.

Here is a prime example of a linear relationship not existing. I worked up 5 different 10 round loads starting at 4.7 gr of TiteGroup with 185 pr Precision moly bullets at an OAL 1.225. On this particular day 4.7, 4.8, 4.9 and 5.0 grains all chrono'd at the exact same PF of 183-185.

Short story is work up several recipies and try them in different conditions, hot, cold, mild temperatures so you know what your load will do in each.

[HoMiE]

Too many variables that can mess with the linear regression techniques. The smaller your extreme spread, the more closely it will follow a linear relationship.

I know this article is about shotshells, but there is a linear relationship between Peak Pressure and Velocity. Pressure/Time curve is not linear.

http://www.claytargettesting.com/study2/Study2.3.pdf

[38superman]

Yes.

No.

Maybe.

Sometimes.

Sort of.

Pick any of the above.

All are correct answers to your question.

As you can see from the above posts, you have asked a question that one could write a thesis trying to answer.

Pressure curves and the physics associated with them along with dozens of variables make it a very complex issue.

The loading tables are to be used only as a general guideline.

They are by no means absolute.

Trying to predict how much of which powder with what bullet will give a specific power factor is an exercise in futility.

There are just too many variables.

There is no substitute for load development with your gun and a good chronograph.

I wish there was. I could have saved a lot of time, money and effort over the years.

Tony

Edited July 10, 2007 by 38superman