
Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics 

06292006, 05:37 PM


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Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics
Well, I've been "hard at it" trying to come up with a calculation of some sort of Figure of Merit that incorporates Optimal Barrel Timing and the output of QuickLoad. Here's the latest version.
The "Score" you see in the 11th column is derived from evaluating velocity, chamber pressure, barrel timing and efficiency. The equation looks like this:
ABS ((ABS (MAX ($D$12:$D$37)D12))/50+1/(ABS($G$3E12))*1000(ABS('OBT Tool'!$O$36G12)*1000)ABS(MAX($J$12:$J$37)J12)/10)*10
Yikes! Lemme explain. (ABS means "take the absolute value"  in other words, ignore whether the value is positive or negative).
The first term  ABS(MAX($D12$:$D$37)D12)/50  calculates the difference between the velocity produced by the powder in question (located at cell D12) and the highest velocity produced by any of the powders in question (determined by MAX($D$12:$D$37)), then divides that difference by 50.
The second term  1/(ABS($G$3E12))*1000  subtracts the value of the chamber pressure of the powder in question (located at cell E12), from the maximum SAAMI chamber pressure (located at $G$3), takes the inverse of that number, and multiplies it by 1000.
The third term  (ABS('OBT TOOL'!$O$36G12)*1000  calculates the difference between the actual timing of the powder in question (located at G12) from the "ideal" barrel timing for a barrel of the specified length (more on that later), and multiplies the difference by 1000.
The final term  ABS(MAX($J$12:$J$37)J12)/10  calculates the difference between the "efficiency" of the powder in question (located at J12) from the highest efficiency produced by any of the powders in question (determined by MAX($J$12:$J$37)) and divides the result by 10. (Efficiency is the muzzle energy in ftlbs, divided by the charge. The units are ftlbs/grain.)
The initial "ABS", and the final "*10" simply get the "Score" to a positive value, and scale it so that we're not looking at differences of 0.01 in "Score".
I'm sure it'll help to walk through an example. Let's use the numbers from the first table below.
The powder with the highest Score is Alliant Reloder19. The charge is 46.72 grains; the % of the case used by that charge is 100; the velocity is 2804 f/s; the max chamber pressure is 56091 PSI; the amount of powder burned before the bullet exits the bbl is 98.3%; it takes 1.2290 ms for the bullet to exit the muzzle; the pressure at the muzzle is 12705 PSI; and the resulting Score is 0.2. Here's how we get that score:
First, the max velocity in this group is 2874.6 f/s. Therefore, 2874.6 minus 2804.4 equals 70.2 f/s. 70.2 divided by 50 equals 1.404. Therefore the velocity "score" is 1.404.
Second, the maximum SAAMI chamber pressure for this cartridge is 61641 PSI. (Pounds per square inch.) The chamber pressure for the Rel19 load is 56091 PSI. The difference is 5550 PSI. The inverse of that is 0.00018 SI/P (Square inches per pound); multiplied by 1000, we get 0.18.
Third, the "ideal" timing for node number 5 for a 24" bbl is 1.22852 ms. (More on "timing" later.) The timing for the Rel19 load is 1.22897. The difference between the two is 0.000448 ms; times 1000 equals 0.448.
Finally, the maximum Efficiency among this group is 63.51 ftlbs/gr. Rel19's Efficiency is 52.32 ftlbs/gr. The difference is 11.19 ftlbs/gr; divided by 10 is 1.119.
So, we have... 1.404 + 0.18  0.448  1.12 = 0.018 * 10 = 0.18
Rounded to a single decimal place, that’s 0.2.
Let’s look at a powder with a lower Score – say I3031.
Velocity is 2732 – velocity score is 2.85
Chamber pressure is 59311 – chamber pressure score is 0.429
Timing value is 1.2290 – timing score is 0.505
Efficiency value is 61.10 – efficiency score is 0.241
For a Score of (2.85 + 0.429 .505 .241)*10 = 25.34
In closing, let me explain the multipliers/dividers a bit. The variables; velocity, chamber pressure, timing, and efficiency all have vastly difference scales. In order to insure that one did not have undue influence simply due its scale, each needed to be appropriately resized. Differences in velocity aren’t too big a deal to me. Therefore, it takes a difference of 50 f/s to equal one whole point in the final Score. Chamber pressure is very important to me, but chamber pressure is measured in tens of thousands of units. Hence the inversion and subsequent 1000 multiplier. Ditto WRT the Timing variable – the differences are measured in microseconds, hence the 1000 multiplier. Efficiency wasn’t that important to me, so it takes a difference of 10 ftlbs/gr to make one unit of difference in the Score.
This is just the first installment. Next come what this means to building a rifle – namely barrel lengths. There are some very interesting results. If you stayed this long, stay tuned. The best is yet to come.
Paul
Last edited by gitano; 09022019 at 08:12 PM..


Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics 

06292006, 10:08 PM


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Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics
Here's the data for the same bullet in the same cartridge with the barrel 2" longer.
Paul


Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics 

06292006, 10:10 PM


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Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics
Here it is for a 28" bbl, using the "fastest" timing node.
Paul


Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics 

06292006, 10:13 PM


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Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics
And here is the data for the same 28" bbl, but using the "slow" node. This illustrates the significance of barrel timing. After yu've had a chance to digest this a bit, We'll discuss it.
Paul


Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics 

06302006, 05:51 PM


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Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics
Hello Paul. I don't own a 6.58mm, however, I just don't understand why you are going through this drill.......I know, you love math and plugging things into spread sheets, but quite frankly, this post has put me out to sea. You have so many scales and variables it seem impossible without applying your own weighted averages based on your experience.
Then, how do you intend to confirm the data?
You might come up with the load/barrel length matematically but what happens when the real world kicks in? What about barrel perfection?
It looked to me like my IMR4064 was right up there, but what happened to IMR4831? Retumbo is the most spastic powder I have ever used and it scored well. If you come up with an optimum barrel length of say....29 inches, it would not be feasible in a hunting rifle. Far be it from me to rain on anyone's parade, however, I guess I'm not smart enough to figure out what your end state/goal is. Regards, Rick.
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Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics 

07012006, 03:21 AM


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Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics
No sweat Rick. Your question/statement is perfectly reasonable. Lemme 'splain a little of this.
As I fiddle (paperwhip) this stuff, the smoke is really clearing on some longstanding "issues" I've had. Most notably is the ofttouted "inherent accuracy" of a particular cartridge, or the converse, a cartridge that "just won't shoot". Both of those "things" bug me, because they don't make sense from a physical perspective. If you'll stay with me through the tedium of the tables, I think I can illustrate what I'm getting at.
I'll explain in words, then follow with the charts. By the way, the reason the charts are so 'busy' is that I don't want to "offend" anyone by leaving out their favorite powder, adn there are some very interesting numbers off to the right. We can discuss those later after I clear up, hopefully, the bigger issues.
First, I chose the 6.508 because very few people have one. That way, I don't get caught up in "Well mine does better than that" stuff. However, I just completed an analysis of the .308 Win, and the .338 MAI... both of which I have and have pressure sensors attached to. What's showing up with this analysis is that "accuracy" is tied directly to bbl length, and longer bbls don't necessarily shoot faster IF you keep in mind accuracy. Furthermore, the reasons behind the oftobserved situations where either 1) a rifle shoots most accurately at a relatively "slow" muzzle velocity, or 2) it shoots amost accurately right at the point where signs of pressure start showing, (with a corresponding high MV), are becoming clear.
In the .308 Win, I analyzed four bullet weights  110, 130, 150, and 180  and five barrel lengths  20, 22, 24, 26, and 28 inches. Here's another matrix of the MVs for each length bbl and each of the bullets. Note that as bbl length goes up, MV does not necessarily go up. (I chose the MV of the powder with the best score.)
. . . . 110 . . . . 130 . . . . 150 . . . . 180
20" . 3188 . . . 3008 . . . . 2701 . . . 2628/2518
22" . 3188 . 3150/2976 . . 2925 . . . 2619
24" . 3161 . . . 3181 . . . . 2875 . . . 2645
26" . 3115 . . . 3137 . . . . 2842 . . . 2570
28" . 3515 . . . 3092 . . . . 3055 . . . 2837
Let's take 'em one at a time.
The 110 MVs go steadily down from 20" to 26", and finally go up  dramatically  at 28". The reason is, the timing nodes for the longer bbls. The max chamber pressure is fixed, regardless of bullet or barrel length. If you make the barrrel longer, the timing changes. So even though you can make the bullet leave the muzzle faster with a longer bbl, if you're trying to make it leave at a specific time (a node), then you won't get any increased velocity until you get the barrel long enough to "jump" up to the faster node while keeping the pressure within specs.
There are two good examples at the 20" 180 and the 22" 130. For both of those, you'll see that there are two MVs. That's because there's just enough pressure headroom, to 'catch' the "faster" lower timing node. However, in these cases, of the hundreds of powders to choose from, there is only a few (sometimes only ONE) powder(s) that actually stay(s) within the pressure limits AND keeps the proper timing.
In the 130, the MVs actually go down from 24 to 28 inch bbls.
In the 150, they go up, then down then back up, as the MV, chamber pressure and timing nodes 'align'.
The same is basically true in the 180.
OK, so what's the bottom line? You're right about barrel vagaries and other variables. However, I'm not looking for the "exact", "right" bbl length from this exercise. What I'm looking for, is a choice of powders, with an attendent MV, for a specific bullet, and barrel length. What this analysis provides for the .308 Win is a short list of pwders (all those with scores lower than "10"). From that list, I can pick the one I "like" to use, and that has the most pressure "headroom". That pressure headroom allows me to fiddle with seating depth and charge in order to fine tune the load to account for those variables you mention. This analysis tells me that the 130 grain bullet is the most difficult (least choices of powders, and least pressure headroom) of the four bullet weights I chose, to get to shoot acccurately from a 24" barrel.
Finally, if I were putting a rifle together, I'd select a bullet or two that I thought I would shoot most frequently, and I'd work on estimating that barrel length that would give me the widest "sweetspot" with respect to powder choices and pressure. After it was built, I'd fine tune the loads by using seating depth and charge.
A "view through the haze" is what I'm after here, not a view through a microscope.
Hope this has made a bit more sense.
Paul


Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics 

07012006, 02:57 PM


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Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics
Pretty interesting once you study it awhile. I guess I'll keep getting along with RL22, 4064, 4831 and 4350. I really can't afford to stockpile lots of powder. I've never even tried Vitavouri....heck, I can't even spell it! I probably will stick with the 2224 inch barrel....longer ones just seem to get in the way. Keep up the good work, Paul. Regards, Rick
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Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics 

07022006, 10:09 AM


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Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics
Actually Rick, NOT having to "stockpile lots of powders" is one of the primary reasons I'm going through this exercise. The results I'm getting hin the final analysis are very interesting  especially with respect to reducing the list of powders one "needs". The next post I make in this thread should "close that loop". Hopefully later today. I appreciate you hanging in there.
Paul


Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics 

07022006, 01:05 PM

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Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics
Hi Guys,
Hmmm all that information makes my head hurt but if your ideas come to bear fruit and I can slim down the powders I have by a few it would be most helpful indeed Please keep up the good work Paul Just because I don't understand it all does not mean that I am not interested just confused .
I wonder how using your formulas H335 comes out in the 3030 at pressure equal to the .308 using 125130 grain Spire point bullets. This is the combination I use in that Medwell & Perritt rifle but if a different powder show promise then I would be daft not to look into it .
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Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics 

07022006, 04:35 PM


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Re: Of Optimal Barrel Timing, Choosing Powders & Paper Ballistics
When trying to include the plethora of powders available, the data tends to get a bit "dense". I appreciate you guys hangin' in there. I think this final table will clear the smoke... I hope so anyway.
In this table, you have all of the powders that "make the cut" when looking at 4 bullets and 5 bbl lengths for the .308 Win cartridge. "Making the cut" means that you could actually fit enough powder in the case to make the bullet "go" (between 75% and 115% of case capacity), and the chamber pressure wasn't above the SAAMI max.
The number you see in the cell across from a powder and down from a bbl length is the "score" it got relative to all the other powders that "made the cut". This "score" is strictly based on the "best"  meaning the highest velocity, the lowest pressure, the closest to the timing node, and the most "efficient". None of those scorings required any "scaling" or manipulation by me. They are simply the basic math of ballisitcs  namely velocity, pressure, timing, energy, and charge.
Look at the first row  I3031  for the 20" bbl with the 110gr bullet it was 3rd out of all the powders. For the 22" bbl it was 1st. For the 24" bbl it was first. In fact of the 20 possible bbl lengths, I3031 provided the fastest MV, with the lowest chamber pressure, the best timing and the best efficiency, for 7 of them. It was in the top 4 for 16 of them.
The righthand three columns are the Average Score, the Sum of all the Scores, and the Sample size (N). Some powders (4 of them), only "made the cut" for a single bbl length and a single bullet weight.
Here's the point.
1) QuickLoad, Load From A Disk, loading manuals and gunwriters (ptooey), list hundreds of powder from which to choose. I really wanted some way to sort the wheat from the chaff and narrow my choices to something reasonable  say 5 or 10 "good" powders from which to choose when working up loads for a specific cartridge.
2) I wanted to figure out how to get the Optimal Barrel Timing theory "into the mix" as I tried to select what would be close to the most accurate load for a specific bullet and barrel length.
3) If I was gonna build a rifle, ala the .338 MAI that Ol' John conceived  I wanted a way to focus in on what bullet, powder and bbl length combinations might prove to be the the most satisfying to me.
Here is my perspective on the results.
I'm kinda flabbergasted at I3031's "performance". Even acknowledging that this is only "paperwhipping", I3031 is head and shoulders above most of the other powders. Only I4895 and H335 even get close. I was never a big I3031 fan. However, this analysis and its results explain why I3031 may not have "worked" for me in the past. I was always striving for the highest velocity I could get while still trying to get the best accuracy. Clearly, when OBT is considered, max velocity and best accuracy, are often mutually exclusive. I will now strive to match bbl timing to bbl length, charge, and bullet weight.
As for me, I'm gonna give I3031 a seriously hard look for my .308 Win load workups. I'm not interested in "convincing" anyone else to quit using their favorite powders, nor am I interested in "convincing" anyone to limit their powder uses. This analysis is for those that might be wanting to "experiment" a little, or are considering taking on a riflebuilding project.
Oh yeah, I should add that the 185 and 200 grain bullets in the .338 MAI with a 26" bbl look "very good", and Ol' John came to that conclusion without the 'benefit' of any of the above analysis. However, even though I have a pretty good load worked up for the 200 grain CTBST I'm still working on the "best" choice of powders. "Things" actually might get even better.
Quote:
I wonder how using your formulas H335 comes out in the 3030 at pressure equal to the .308 using 125130 grain Spire point bullets.

I'll have a look at it, BH.
Paul
PS, I'll try to make the table a bit more "readable"  we'll see how it comes out.
Last edited by gitano; 07032006 at 11:16 AM..

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