Range Report Lex Talus PSW/FFS programs and Dk

[James in Wyoming]

Most of us are familiar with the difficulties of using G1 ballistic coefficients to get really accurate drop data at distance and thanks to Bryan Litz's work, we have alternatives using the G7 coefficients. Most programs other than the one included with his book do not, however, allow use of the G7, and the two programs referenced above allow modification of the curve through use of the Dk, or deceleration constant. I happen to like a lot of the features of the Lex Talus programs and have found that with a properly calculated Dk one can get very accurate drop data out to 1000 yards and beyond. My question is for those of you, like Lindy, who have used these programs a lot...
Once you have an accurate Dk for a particular bullet, isn't this figure "transferable" to other cartridges/velocity combinations using the same bullet? Could a "reference library" of known Dks be constructed and incorporated in the program? Certainly there would be some differences shooter to shooter and cartridge to cartridge, but couldn't I, for instance, use your computed Dk for a particular bullet and be much closer to real world results than simply using the default value built into the program? If the figures are to a certain degree transferable, would a reference list of known Dks be a worthwhile addition to that program? Is this something Lex Talus might incorporate into future versions of the program or could the ability to use the G7 be added to the next iteration? Comments appreciated.
Best,
James

 

[Lindy]

Re: Lex Talus PSW/FFS programs and Dk

James:

As one of the beta testors for the FFS program, I probably have as much time on it as anyone except the developer.

I've never compared DKs with anyone else, so I can't say how transferable they are.

But I suspect that the DKs are relatively load-specific, shooter-specific, and rifle-specific, and, as such, are probably not very transferable. My suspicion is reinforced by watching different shooters using the same rifle - by which I mean <span style="font-style: italic">exactly</span> the same rifle, not similar rifles - with the same load under identical conditions at long ranges and getting different dope.

I have Bryan's program, which yields the same results as JBM does when using a G7 BC.

I find it interesting, though, that I have pretty good long-distance (1000 yards) data on the load I use most often. And I used that data to develop the DK I use in FFS for that load.

In order to get the other ballistics program I run - Bryan's program, JBM, Exbal, Perry Systems new program GModels (which uses G7 and other BCs) and Atrag - to match that data, I have to modify the BCs in all of those programs.

There are a lot of possible reasons for that, which include chronograph inaccuracy. It's pretty hard to eliminate all of the possible systemic sources of error. I have eliminated inaccuracies in the scope adjustments by careful calibration of my scope.

Optically Checking Rifle Scopes

However, the usefulness of a model is a function of whether it produces good predictions. And all of those programs, suitably modified to match my long distance shooting data, do so.

As such, in recognition of the fact that elimination of all of the possible sources of error which result in program predictions which differ from my shooting results is impossible, I've decided to be satisfied with getting good predictions.

I also think it's interesting that after I have modified the BCs to match my long distance shooting data, all of the programs produce predictions out to 1000 yards which match within 0.1 milliradians. Since that's about the difference in elevation at 1000 yards produced by the standard deviation of the muzzle velocity of the load, I consider those predictions to be functionally identical.

From that, I conclude that all have satisfactory atmospheric models, and, given that, I pick my primary ballistic program on the basis of the feature set of the program.

I'm not an extra-long-range shooter, and it may be that one or more of those programs is better at ELR predictions than another, which is an area of experimentation for someone else.

 

[James in Wyoming]

Re: Lex Talus PSW/FFS programs and Dk

Thanks Lindy. I've had similar experiences with same rifle, same load, different shooter and different dope, especially beyond 600, so I know that the values are not perfectly "transferable" between shooters. As you say, the value of the model is whether it produces good -- not perfect -- predictions, and it is most interesting that all the commonly used software, when tweaked by modifying the bc, yields good results. That begs the question of whether the same "modified bc" works equally well in the different programs or whether each program has to have a specifically modified bc to work well, but in the last analysis it doesn't really matter for me as the FFS works quite well in the field on a handheld when adjusted with Dk. I have not really played with Bryan's program and the G7s yet but am curious to try it out at some length this fall and see if it produces "better" approximations with less tinkering, as I would rather shoot than mess with the programs.
Best,
James

 

[Lindy]

Re: Lex Talus PSW/FFS programs and Dk

Well, I would rather shoot than mess with the programs, too - but I have a lot more time to mess with the programs. As someone who wrote software for a living for many years, I'm curious about the implementations.

FFS is my primary program, because of the feature set. I suspect most people would do well to learn one program well, rather than spend a lot of time trying to figure out which is the "best" one.

 

[IsleofGough]

Re: Lex Talus PSW/FFS programs and Dk

I thought G7 was only for VLD bullets. G5 or G6 is supposed to be better with 308 boattail match. Am I mistaken?

 

[ScottyS]

Re: Lex Talus PSW/FFS programs and Dk

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">James:

As one of the beta testors for the FFS program, I probably have as much time on it as anyone except the developer.

I've never compared DKs with anyone else, so I can't say how transferable they are.

But I suspect that the DKs are relatively load-specific, shooter-specific, and rifle-specific, and, as such, are probably not very transferable. My suspicion is reinforced by watching different shooters using the same rifle - by which I mean <span style="font-style: italic">exactly</span> the same rifle, not similar rifles - with the same load under identical conditions at long ranges and getting different dope.

I have Bryan's program, which yields the same results as JBM does when using a G7 BC.

I find it interesting, though, that I have pretty good long-distance (1000 yards) data on the load I use most often. And I used that data to develop the DK I use in FFS for that load.

In order to get the other ballistics program I run - Bryan's program, JBM, Exbal, Perry Systems new program GModels (which uses G7 and other BCs) and Atrag - to match that data, I have to modify the BCs in all of those programs.

There are a lot of possible reasons for that, which include chronograph inaccuracy. It's pretty hard to eliminate all of the possible systemic sources of error. I have eliminated inaccuracies in the scope adjustments by careful calibration of my scope.

Optically Checking Rifle Scopes

However, the usefulness of a model is a function of whether it produces good predictions. And all of those programs, suitably modified to match my long distance shooting data, do so.

As such, in recognition of the fact that elimination of all of the possible sources of error which result in program predictions which differ from my shooting results is impossible, I've decided to be satisfied with getting good predictions.

I also think it's interesting that after I have modified the BCs to match my long distance shooting data, all of the programs produce predictions out to 1000 yards which match within 0.1 milliradians. Since that's about the difference in elevation at 1000 yards produced by the standard deviation of the muzzle velocity of the load, I consider those predictions to be functionally identical.

From that, I conclude that all have satisfactory atmospheric models, and, given that, I pick my primary ballistic program on the basis of the feature set of the program.

I'm not an extra-long-range shooter, and it may be that one or more of those programs is better at ELR predictions than another, which is an area of experimentation for someone else.

</div></div>

Hell Lindy, that needs to be a sticky.

 

[IsleofGough]

Re: Lex Talus PSW/FFS programs and Dk

just to show how little these programs differ:
Ballistics of 175 gr 308 M118LR at 58 degrees, 78% humidity, 0 altitude, Velocity 2600, G1 .495, zero 100 yards, drop in mil. distance in yards:
yards____JBM____FFS___Patagonia__Exbal___RSI
100________0______0_______0_______0_______0
200______0.6____0.7_____0.6_____0.6_____0.6
300______1.5____1.5_____1.5_____1.5_____1.5
400______2.4____2.5_____2.5_____2.4_____2.5
500______3.5____3.7_____3.6_____3.5_____3.6
600______4.8____5_______4.9_____4.8_____4.9
700______6.2____6.4_____6.3_____6.1_____6.3
800______7.8____8.1_____8_______7.7_____8
900______9.6___10_______9.8_____9.5_____9.8
1000____11.6___12.2____12______11.5____11.9

However, slight miscalculation in velocity, BC, atmospheric conditions, distance, or wind will create fairly large differences esp at larger range. (Much more than differences between programs). So.... what Lindy said.

 

[Lindy]

Re: Lex Talus PSW/FFS programs and Dk

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">I thought G7 was only for VLD bullets. G5 or G6 is supposed to be better with 308 boattail match. Am I mistaken?</div></div>

From Bryan Litz's book Applied Ballistics for Long Range Shooting :

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">The G7 standard projectile has a shape very similar to that of modern long range bullets. Unlike the short nosed, flat based G1 standard, the G7 standard has a long pointed nose and a boat tail.</div></div>

The choice of a standard is going to be based somewhat on the availability of BC data. Bryan has done extensive testing to provide G7 data for a lot of bullets.