Change in FPS per 1/10th Grain of Powder

[Bart B.]

Used Sierra Bullets data for two cartridges to calculate how much average muzzle velocity would change for a 1/10th grain change in powder charge weights. A mid-range velocity's powder is listed in order of high to low for fps changes; those either side will be close, but there are some interesting differences.

'Tis interesting to me how some powders have about twice the velocity change per tenth grain than others. And if the observed average muzzle velocities have a 40 fps spread, the odds of seeing a change with a small change in powder weight are not too good.

 

[LeonCarr]

Wow, thanks for posting that.

It is amazing how you get perfectly linear results with several powders increasing the exact same fps per 1/10 grain of powder all the way up the velocity scale.

Just my .02,
LeonCarr

 

[Walkalong]

Very interesting.

 

[spitballer]

Facts speak for themselves even if they're not always convenient, I've found that out. Frankly I'm surprised that the IMR 4064 didn't have more velocity with that combination...

 

[Bart B.]

I think those numbers for the .308 Win can be confusing at times. While the fps change for ball powders H380, BL-C2 and AA2520 are smaller per tenth of a grain, Extruded ones IMRs 4064 and 4895 produced better accuracy with 168 to 180 grain match bullets with a 3/10ths grain spread than the ball powders with a 1/10th grain spread.

There's obviously more to accuracy than charge weight spread.

 

[ArchAngelCD]

Yes, very interesting. Thank you for the posts.

 

[Hanshi]

Thanks for the fascinating information. I've never tested 1/10 grain changes before in smokeless powder rifles but have tested 5 grain changes in black powder muzzleloaders. The results there are seldom linear. Example:

Flintlock .40 caliber firing a patched round ball, barrel 38" 89 grain .390" ball, Goex's 3F Jacks Battle Powder.

30 grains of 3F JBP (Goex) ..............................1427 fps

35 grains of 3F Jacks Battle Powder (Goex) .......1475 fps (48 fps/5 grains) 9.6 fps per grain

40 grains of 3F JBP ........................................1701 fps (226 fps/5 grains) 45.2 fps per grain

60 grains of 3F JBP.........................................1923 fps (222 fps/20 grains) 11.1 fps per grain

 

[berettaprofessor]

Now there's some data we can use. Great work!

Gotta dial down my a-retentiveness on 0.1 gr charge variation as a result.

 

[MCMXI]

It is amazing how you get perfectly linear results with several powders increasing the exact same fps per 1/10 grain of powder all the way up the velocity scale.

Hmmm ... makes me think that Sierra might have used linear interpolation to propagate their load data tables.

 

[Bart B.]

Sierra's ballistic tech told me back in the 1960's to work up loads in half grain increments for charge weights under 50 grains; whole grain increments for heavier ones. Fairing in the plots would work and save time and components. Their "Best accuracy" load listed in their data is one of those in their table and it's checked in several rifles to verify its authenticity; 39.5 grains of Viht N540 for their 175 in the .308 case.

Wouldn't be surprised to learn they're still doing that. As well as testing for best loads with shoulder fired rifles from a benchtop. Which explains why their load that's the same as others is several fps slower.

 

[WelshShooter]

Hmmm ... makes me think that Sierra might have used linear interpolation to propagate their load data tables.

I was thinking the exact same thing, I don't get a warm fuzzy feeling from this data. I'd rather see either the completed table showing actual velocity versus charge weight or better yet, have this as a chart. As you've sussed it out, plotting a fitted linear regression misses out some important information. When I was performing load development for my .308 and 6.5x47 I recorded the velocity for each powder charge. I found that there was a certain point where I could increase the powder charge by 0.3gr and the average velocity did not significantly rise - if you plotted a straight line plot this powder charge didn't follow the line. This showed me stability in velocity at this powder charge which tells you that you don't have to get your weight to be exactly the same everytime to get good results. As it is, with my 6.5x47 my powder loads can vary by +/-0.1gr and I see no significant effect on velocity (or rather extreme spread).

 

[Bart B.]

WelshShooter,

If your velocity spread is 40 fps and change per 1/10th grain is 5 fps per Sierra's data, you'll need to shoot at least 20 shots (30 is much better) with each to have statistical confidence above 90% to show a 2/10ths grain change in charge weights changes average velocity 10 fps.

How many shots per charge weight did you shoot? 5 shots have about a 30% confidence level for anything shooting groups or measuring muzzle velocity.

 

[spitballer]

Whew! I'm trying to digest this stuff and what I think I'm hearing in essence is that when charges increase to the point that they're getting a lot of resistance they no longer increase in velocity in a linear fashion, irrespective of pressure.

Dumb question here: so when I'm reloading to get larger groups without significant fouling, may I infer that I should stay just below this point of diminishing returns?

 

[WelshShooter]

Sorry OP, this post is a little off-topic but I wanted to ensure good statistics are mentioned here.

WelshShooter,

If your velocity spread is 40 fps and change per 1/10th grain is 5 fps per Sierra's data, you'll need to shoot at least 20 shots (30 is much better) with each to have statistical confidence above 90% to show a 2/10ths grain change in charge weights changes average velocity 10 fps.

How many shots per charge weight did you shoot? 5 shots have about a 30% confidence level for anything shooting groups or measuring muzzle velocity.

Hi Bart, what program or table are you using to generate your sample size and confidence level? I'm using Minitab v16 to determine sample size.

For my testing I shot 5-shots per measurement group but upped my charge around 0.3gr per time. Minitab can perform a calculation for me which requires two of the three following inputs, along with the standard deviation:
1. Sample size
2. Difference
3. Power value (i.e. confidence interval)

It will then calculate the one field which is left empty. Since I had a sample size of 5, and I want a confidence interval of 95% (so a power level of 0.95) then I can detect an average velocity difference of 28.9fps. The standard deviation of the velocity was 13.1fps.





I completely agree with your thought on increasing sample size. If I use the above rationale:
1. Sample Size = 30
2. Difference = (we want to find out)
3. Confidence of 95%

We can then use this information to detect an average velocity change of 8.92fps with 95% confidence (based on the same standard deviation of 13.1fps).

Minitab (and myself) don't care about how much you want to change the charge weight. The 2-sample t-test is a statistical test which compares the mean values between two groups (e.g. two different charge weights, two different bullet brands with the same charge weight and so on). The 2-sample t-test produces results with a 95% confidence band, therefore the reasoning behind the 95% confidence for sample size as above.

 

[dh1633pm]

Funny how the chart matches my own testing with IMR4895 in 308. I get 2600 FPS on the Chrony with 41.2 grains using LC Brass. A very consistent load. Thanks for sharing.

 

[Metal God]

Bart , were you able to conclude in that testing if there is an OCW . The theory is at some point in a complete charge spectrum you will see less of a spread in a .4 to .8gr spread giving you a zone in which tenths of grains have less of a change in pressure and velocity for a given firearm and load combo .

Your test would seem to indicate that theory is suspect but then again it does not look like you tested every charge through out a complete minimum to max charge spectrum . So there may still be an area where those numbers level off and become less noticeable .

After running this test do you have an opinion if there is a charge zone like the OCW theory ?

EDIT

Hi Bart, what program or table are you using to generate your sample size and confidence level? I'm using Minitab v16 to determine sample size

Just realized this could just be all a computer simulation . I'd think Bart actually shot each one of those charges in a machine rest . If not then disregard my questions . One theory can't prove or disprove another theory because they are both just theory .

 

[Bart B.]

Metal God,

From my first post:

Used Sierra Bullets data for two cartridges to calculate how much average muzzle velocity would change for a 1/10th grain change in powder charge weights.

My data was derived from Sierra Bullets' Infinity Software in the Reloading Manual section. Same type of data is in their reloading manuals printed on paper.

As for OCW, I think it's one of the biggest fallacies in load development. It's based on the theory that a bullet must exit when the shock wave from firing has returned back from the muzzle and is at the breech end and the muzzle diameter will enlarge .0002" or more if the shock wave's there. Too many flaws in its developer's reasoning. One's the fact that all four of the 7.62 NATO barrels I wore out in Garands had their muzzle diameter enlarged .0020" from steel cleaning rods rubbing on it (no copper wash at all for the last 3/4" of the bore), but they still tested 1 MOA or so at 600 yards when worn out having started out an near 1/2 MOA when new. I'll explain later' I'm working on a spreadsheet showing positions of the shock wave and bullet in the barrel to prove OCW's a joke.

WelshShooter,

I didn't use any specific math to get my data. It's a generalization based on the average that several got. Also with guidelines sent to me from Lake City Army Ammo Plant after inquiring a bad lot of M118 7.62 NATO match ammo issued to me back in the late 1960's. Basically the same way maximum/minimum group sizes are estimated based on the size of the first group of so many shots.

 

[WelshShooter]

Hi Bart, I wasn't questioning your data at all, I was only questioning how you arrived at the below statement. I work with statistics and modelling software as part of my job so always need to quote how I derive my confidence intervals. I see a lot of mis-used statistics so I just had to question it.

With regards to complaints on OCW, I'd be really interested to see your data to disprove it. I'm also of a similar opinion on OCW, but I don't have any data to disprove the idea. How can Blackhills ammo shoot well in 99% of rifles, yet somehow handloads must be "unique" and developed for each rifle? I'm always open to new ideas.

[...]you'll need to shoot at least 20 shots (30 is much better) with each to have statistical confidence above 90% to show a 2/10ths grain change in charge weights changes average velocity 10 fps.

How many shots per charge weight did you shoot? 5 shots have about a 30% confidence level for anything shooting groups or measuring muzzle velocity.

 

[Bart B.]

WelshShooter,

Here's two links with statistics used to estimate two things about shooting tests:

http://s854.photobucket.com/user/unclenickmod/media/GroupSize95probability_zps010ad21e.gif.html

http://s854.photobucket.com/user/unclenickmod/media/ChangeinGroupSize_zpsec927581.gif.html

I think they are close to what could be determined about muzzle velocity spreads.

About, OCW, your Black Hills ammo reference parallels OCW'c claim that Federal Gold Metal match ammo's ability to shoot sub MOA in most rifles. We both know any lot of either will not have the same muzzle velocity across all barrels of the same length and all lots will also differ in their velocity from a given barrel. Way too many variables for all to have the same barrel time for each bullet and exit when the shock wave's at the breech end. I don't think OCW believers and its originator considered that.

 

[au_prospector]

I am no mathematician, however I would think trying to predict to the tenth of a foot per second velocity based on a charge weight variance of a tenth of a grain is a bit of an optimistic stretch. There are way too many variables in shooting and shortcomings in measuring equipment to have confidence in a tenth of a foot per second. Just me I suppose.

 

[Orcon]

Bart,

I use the OCW load development method and routinely find a load in under 20 rounds fired. Whether it is a function of pressure/velocity, case fill or OBT is almost irrelevant to me if it works . I have noticed that almost all of my loads are compressed or nearly so. What method do you use for load dev?

 

[Bart B.]

What method I use for load development?

None. I don't work up loads for every barrel or component lot I get.

I use the same loads the match winners and record setters use and get accuracy in proper tests equal to what they get. Two exceptions for bullet and cartridge combos not used. One was for Sierra's new 155-gr. Palma bullet in .308's; several of us came up with 45.3 grains and it shot 1/2 MOA at 600 yards in a couple dozen Palma rifles with all sorts of chamber, bore and groove dimensions. The other was for 139-gr. bullets in a .264 Win Mag; Sierra said to try 72 grains of H870 and that shot 1 MOA at 1000. All used 20-shot groups.

Sierra Bullets uses the same load for each new test barrel for a given cartridge as well as when changing component lot numbers. They don't prep cases and throw charges direct from measures. Nobody shoots their match bullets any more accurate than they do in their test barrels.

 

[Captcurt]

I am no mathematician, however I would think trying to predict to the tenth of a foot per second velocity based on a charge weight variance of a tenth of a grain is a bit of an optimistic stretch. There are way too many variables in shooting and shortcomings in measuring equipment to have confidence in a tenth of a foot per second. Just me I suppose.

I gotta go with this. A tight chamber vs a large chamber or differences in neck tension could even affect velocity change of more than the datas 1/10 gr change. It is something to study but don't bet your favorite rifle on it.

 

[Bart B.]

'Tis my opinion the biggest contributor to muzzle velocity spread is how hard/soft the human holds the rifle against their shoulder.

People comparing the same rifle and ammo shot in free recoil to hand held against their shoulder as it rests atop something on a bench see 1/3rd to 1/4th the spread. But average velocity is lower for free recoiling rifles compared to hand held. And the rifle's barreled action in a fixed mount (no recoil at all) will have the highest average muzzle velocity and the lowest average spread.

 

[Orcon]

I use the same loads the match winners and record setters use and get accuracy in proper tests equal to what they get.

I guess that would work if you have access to that info and only shoot OTM bullets. Care to share your sources?