Trued MV not even close to Chrono?

[dk17hmr]

Is it common to have a very large difference in trued MV and Chronoed velocity when you really push velocity?

I have been shooting 122gr Cayuga's from my 6.5 SS at 3390fps average from 20 shots measured with my labradar. I shot data verification out to 750 and to get my curve to match my kestrel needs to be at 3140fps.

I don't really care as long as I'm getting hits in different conditions and DAs. So far it seems to line up.

My slower stuff matches pretty decently, just my high velocity loads and cartridges seem to have large velocity gaps.

 

[Huskydriver]

1.... Why are you truing at 750?

2 have you checked dope father than 750?

 

[hafejd30]

On my StrelokPro I often adjust the BC of the bullet in the program to get it to match down range

 

[dk17hmr]

It's my hunting rifle, my self imposed killing distance on big game is 600. I trued MV in my kestrel at 600 but have tested out to 750. I treat it as a killing right not as a range rifle.

My 25-06AI at 3550fps kinda same deal.

 

[Huskydriver]

It's my hunting rifle, my self imposed killing distance on big game is 600. I trued MV in my kestrel at 600 but have tested out to 750. I treat it as a killing right not as a range rifle.

My 25-06AI at 3550fps kinda same deal.

Well if you follow kestrels ins... You are supposed to true at the distance near trans speed

 

[dk17hmr]

Fair enough. 1300 yards wasn't available the day I was able to test.

Would it make that much difference if my data is matching inside my kill range? I'm willing to test it when I have time and conditions.

I guess I'm wondering if there is a correlation between measured and trued velocities. It seems like 2850 and less line up pretty decent in other calibers for me but when I get 3300+ I'm noticing substantial differences.

 

[Huskydriver]

Fair enough. 1300 yards wasn't available the day I was able to test.

Would it make that much difference if my data is matching inside my kill range? I'm willing to test it when I have time and conditions.

I guess I'm wondering if there is a correlation between measured and trued velocities. It seems like 2850 and less line up pretty decent in other calibers for me but when I get 3300+ I'm noticing substantial differences.

Yes there is a massive difference when I have played with it

 

[tthomp302]

You also don't true the velocity. You know your velocity. You true the BC.

 

[Huskydriver]

You also don't true the velocity. You know your velocity. You true the BC.

False....if u are using an ab curve on the kestrel

If he's using a g1 or g7 it's a little of both usually

 

[Matches Malone]

I can’t remember if kestrel recommended it or not, but true kestrel (AB) MV around mach 1.2. Here’s you’ll notice ranging errors, velocity errors, scope errors, etc. often enough, when you’re trying truing largely away from measured MV, you very well could be truing out those errors.

If you have the elite, you can use DSF for tuning subsonic. It won’t change the BC.

For BC, you might consider stepping the BC, or maybe using a CDM if you have a kestrel Elite.

 

[JustSendit]

False....if u are using an ab curve on the kestrel

If he's using a g1 or g7 it's a little of both usually

This is true and I think too often it’s misunderstood.

Personally I will CalMV for G1/7 too. If the data lines out, run it. But I don’t use G1, pretty much ever anymore and typically G7/CDM are very close to the furthest distances.

I think a lot of people build in errors like, not a perfect zero, wrong DOF, not accounting for wind for AJ, misranged targets, incorrect HOB, not updated atmospherics, wrong BC inputted, shitty scope, bad hold, too close distance to true…

If all that’s correct and my MV doesn’t match my chrono after I CalMV, then it’s probably a BC variation and weather I correct that with adjusting the BC or the MV, it’s the same outcome. From my experience.

 

[Huskydriver]

This is true and I think too often it’s misunderstood.

Personally I will CalMV for G1/7 too. If the data lines out, run it. But I don’t use G1, pretty much ever anymore and typically G7/CDM are very close to the furthest distances.

I think a lot of people build in errors like, not a perfect zero, wrong DOF, not accounting for wind for AJ, misranged targets, incorrect HOB, not updated atmospherics, wrong BC inputted, shitty scope, bad hold, too close distance to true…

If all that’s correct and my MV doesn’t match my chrono after I CalMV, then it’s probably a BC variation and weather I correct that with adjusting the BC or the MV, it’s the same outcome. From my experience.

I agree

 

[dk17hmr]

Possibly an issue but the PVA Cayuga isn't in the AB library. I manually entered the data, g7, length, diameter.

I haven't been keeping up with it but is the only way to get a CDM through AB mobile lab?

 

[JustSendit]

Possibly an issue but the PVA Cayuga isn't in the AB library. I manually entered the data, g7, length, diameter.

I haven't been keeping up with it but is the only way to get a CDM through AB mobile lab?

I’m not familiar with that bullet but in all likelihood the G7 should generally track when you true in transonic. Also assuming the bullet is stable which in all likelihood it is as well.

 

[6.5SH]

Possibly an issue but the PVA Cayuga isn't in the AB library. I manually entered the data, g7, length, diameter.

I haven't been keeping up with it but is the only way to get a CDM through AB mobile lab?

For the heck of it upload a zip of your Labradar .trk files from your chrono session here:

Geladen | Labrabaco

Fill out the environment data and see what it gives you for a G7.

 

[dk17hmr]

I'll give it a try. Thank you.

Love velocity for hunting but my range is at about 6500' asl so getting to transonic takes some commitment of time.

 

[Huskydriver]

Possibly an issue but the PVA Cayuga isn't in the AB library. I manually entered the data, g7, length, diameter.

I haven't been keeping up with it but is the only way to get a CDM through AB mobile lab?

I would email AB and see if that one is in the works or if they have a starting point for you.

In that case yes the more real data points at distance you have, the more accurate the bc you tweak in-between those points will be whether that's the g7 or g1.

I would argue that one data point at 700ish yards is not enough to accuratly tweak your curve to reality. I bet you are off at 300 and 900 with that estimated mv vs what your Chrono is telling you but would be an interesting experiment for you to try

 

[Taylorbok]

I'd say the BC of the bullet is off. If they aren't in the AB library then it is likely that they were not Doppler tested. What kind of correction on the BC does it require if you use your measured MV?

I have heard from people saying that PVA's BC numbers are inaccurate. Could be a combination of both.

 

[dk17hmr]

I essentially have mapped random distances from 300 to 750. I think I have 15 data points on that curve and 25 data points from my Chrono.

Deer season starts Saturday and I'm limited on time. If I'm .1-.2 mil off even at the far end of my kill distance I should be ok.

I ran the zip file of my Chrono data last night and it gave me a g7 slightly lower than published.

Huge fan of the accuracy, velocity, and effect on game animals with this combo. I killed a couple animals with it last season just finally got serious about testing it.

 

[Taylorbok]

@dk17hmr see my above post. how does it look if you adjust BC and use measured MV

 

[dk17hmr]

@dk17hmr see my above post. how does it look if you adjust BC and use measured MV

Honestly didn't check.

 

[Taylorbok]

Honestly didn't check.

If you have all those data points shouldn't be tough to punch into your calculator now to see the difference.

 

[6.5SH]

I essentially have mapped random distances from 300 to 750. I think I have 15 data points on that curve and 25 data points from my Chrono.

Deer season starts Saturday and I'm limited on time. If I'm .1-.2 mil off even at the far end of my kill distance I should be ok.

I ran the zip file of my Chrono data last night and it gave me a g7 slightly lower than published.

Huge fan of the accuracy, velocity, and effect on game animals with this combo. I killed a couple animals with it last season just finally got serious about testing it.

So the published BC is likely too high.
Given the relatively short range a LabRadar can track (100ish yards) it should give a number that is .0x0 slightly high from a good published average.

Make a new cartidge in Strelok with your measured velocity and this new BC. Do the predictions line up closer with your recorded real world DOPE at various distances? If still overestimating adjust the middle number down a point or two and compare again.

 

[dk17hmr]

I'm going to need more range time to test this out. I appreciate the help. Definitely closing the gap on the discrepancy.

 

[Taylorbok]

Glad you are figuring it out. ballistic calculators are really finicky, If your calculated solution and real dope don’t match up (or really close) it’s usually a bad input. The more you use it the more proficient you will become. I would suggest picking one and sticking with it. I like strelok pro it’s super easy to use and very accurate. I do understand using AB because they have all the custom curves. In strelok you can build your own if you have the range and time.

 

[6.5SH]

Another article well worth reading (and re-reading) here is this one:

Going Inside the Numbers

Follow Sniper's Hide and pick up the latest information on all things precision rifle related. Sniper's Hide Training is for the serious tactical marksman.

www.snipershide.com

Sometimes the bad inputs are something we are doing fundamental wise at distance.

 

[dk17hmr]

I have Shooter on my phone and AB on my Kestrel.

Between those two I have had pretty decent solutions with minimal work. Just seems like my high velocity stuff 3400fps + always takes more work. My 260, 308, and 6creed have pretty much been plug and play.

 

[Lowlight]

I never chrono anymore., unless you are reloading

I tell everyone using software to just use place holders and let the software do it’s thing. There is really only two ways to move the number and most software only tells you to use one. MV

So use a place holder,

308 - 2650
6.5 (any) 2750
6mm 2850
Magnum 2850

it’s 25fps to move the needle .1 mils +/-

 

[1911hombre]

I never chrono anymore., unless you are reloading

I tell everyone using software to just use place holders and let the software do it’s thing. There is really only two ways to move the number and most software only tells you to use one. MV

So use a place holder,

308 - 2650
6.5 (any) 2750
6mm 2850
Magnum 2850

it’s 25fps to move the needle .1 mils +/-

I can't find my notes from class but I think I remember you said to true MV at 80% of distance that bullet goes transonic, and true BC for farther distance, right?

 

[Lowlight]

MV To 600

Bc after

 

[dk17hmr]

MV To 600

Bc after

So if you do that and you end up substantial different in velocity like I'm seeing, does the AB/Kestrel software track at different conditions? DA has been around 8500, if it goes down to 4500 or up to 12000 will it track? Or at least be close?

 

[Lowlight]

It works better than doing it any other way

 

[Ledzep]

Garbage in garbage out. VERIFY scope height over bore, it's critical. VERIFY 100yd zero (10-20 shots). Those two things are pivot points on the short side of a gigantic lever and if you have error there it will absolutely manifest itself downrange.

If those two things are correct, and your drag data (whether it's G1 or G7 BC, full drag profiles, 4DoF profiles, etc..) is remotely close, you should be very solid out to 600-700yd with absolutely correct MV inputs. There are errors present with chronographs so bear that in mind, but if you're bending stuff more than 30-40fps off of what the chrono says (Magneto or Labradar), I typically start looking elsewhere for something I've fucked up or where something has walked on me. Especially true with 4DoF.

700-800+ is where cumulative drag (BC) errors start to manifest as vertical misses big enough that you notice them. Even at distance, I don't change anything until I've put at least 10-20 rounds on big clean steel or paper to verify the MPOI of the group.

With BC solvers it can sometimes be useful to create banded profiles-- for example 0-600, 600-900, 900-1200, etc... to account for the fact that Cd vs. Mach information for your specific bullets might not match that of the scaled G1 or G7 standard.

Also, DA doesn't account for temperature and Mach-- thus BC-- changes with temperature. BC solver using DA is the biggest invite for creeping errors from season to season or location to location IMO.

 

[JustSendit]

Also, DA doesn't account for temperature and Mach-- thus BC-- changes with temperature. BC solver using DA is the biggest invite for creeping errors from season to season or location to location IMO.

I thought the density altitude did account for temp, it’s part of the equation to determine the alt. Are you referring to powder temp? Or maybe I’m wrong I am not a kestrel doctor.

 

[Ledzep]

I thought the density altitude did account for temp, it’s part of the equation to determine the alt. Are you referring to powder temp? Or maybe I’m wrong I am not a kestrel doctor.

DA is a single value that describes air density. The original use was for subsonic aircraft flight. I don't know how the Kestrel handles it either, but Mach (the speed of sound in air) shifts with temperature. The BC based solvers are pulling Cd vs. Mach data to create the solution. If the solver is referencing STP and you're shooting in 100 degree weather, there's chance for offset error. May or may not be the case with your Kestrel, may or may not be significant, but often enough several small errors compound to make a big one.

 

[Lowlight]

Garbage in, Garbage out doesn't for MV,

The software has to change, no matter what you use, everything has to be trued, you can buy a CDM and it still needs to be trued, so looking at a MV number as nothing more than a value is needed.

Who really cares what the actual is as long as it's "ballpark" where you want. That is the load development part, we need to shoot for a number, we load to that number, and then once it's determined your SDs and velocities are where you want them, throw that number away.

Computers have to adjust, the easiest way to adjust the curve inside 600 is MV. So it will change. We don't even MV in class anymore, we use Weaponized Math as it has no data inputs, no MV, no BC, no Sight Height, it's just gravity. Then once you have your data from Weaponized Math (which is pre-Trued) you go into your Ballistic Computer and line it up.

Line Drop at 600 with MV

Line Drop at 800 / 1000 with BC

Done...

This is an entire ballistic calculator for anything you zero at 100 yards. It's a simple flow chart, Use 1.0 at 300 try 1.7 at 400, needed 1.8 for a centerline hit, try 2.6 for 500, and so on, if you are more than .2 off any yard line, the ranges are wrong or you are not shooting correctly

 

[dk17hmr]

I essentially have plotted my drop data with this rifle in a similar way as the weaponized math, except it was at random distances. I adjusted my kestrel to match my data points out to 750.

As far as knowing my velocity is ballpark correct and tracking on a curve comparable to my kestrel, I like to have a rough idea what my impact velocities are since it is a hunting rifle.

That said I killed an elk at 543 last season with this load and had excellent results.

 

[Ledzep]

If your stuff is off a constant angular value at all ranges (0.3 mils high at all ranges), it's a zero issue. If the errors get bigger with distance (more MOA or more Mils as distance increases), it's either a MV or drag/BC issue. The inherent limitations of BC calculators usually means that the solver won't generate a perfect output, even with perfect MV inputs. If real-world correct verified MV doesn't work, you have found the limitations of your solver's ability to handle drag or environmental conditions. (assuming no other inputs are wonky). This corners you in a place where you basically have to fudge MV to make the trajectory line up. IF that is the case, and you're concerned with down-range retained velocity, you need to understand that there's some error in what the calculator is spitting out. You're forcing the path/trajectory to line up, not the velocity/time based stuff if you fuck with MV. Cd based solvers (4DoF, 6DoF) typically handle the drag side of things much better and you're more liable to get correct outputs with correct MV inputs. It's not always exactly perfect but the "tune up" on drag offered by features like 4DoF's form factor are very small shifts in the Cd vs. Mach curves that the program uses.

I've used 4DoF as a chronograph a few times, successfully. Basically note the POI at 100yd, set zero angle, then see how much it takes to hit waterline at 500-700yd and adjust MV accordingly. This works if all other inputs are very solid/accurate... So yes, fudge MV to get you on at 500-600yd is a decent technique, HOWEVER... if you're off more than 30-40fps from what your chronograph is saying, I still maintain that something is likely fucked somewhere else. If nothing else, it's a good eyebrow raiser to go back and verify zero, sight over bore, and environmentals.

If things aren't making sense, IMO the best thing you can do is to shoot at 100, 400-600, and 800-1000yd. Note exactly the conditions, and exactly the dope that it took you to waterline at those distances. 10 rounds MINIMUM on clean steel at each distance. Write it down and this will allow you to go back later and verify if the changes you make to your solver/profile reflect reality. Do not omit wind inputs and horizontal POI.

 

[JustSendit]

DA is a single value that describes air density. The original use was for subsonic aircraft flight. I don't know how the Kestrel handles it either, but Mach (the speed of sound in air) shifts with temperature. The BC based solvers are pulling Cd vs. Mach data to create the solution. If the solver is referencing STP and you're shooting in 100 degree weather, there's chance for offset error. May or may not be the case with your Kestrel, may or may not be significant, but often enough several small errors compound to make a big one.

I am man enough to admit that I am confused by your posts in this thread. You’re definitely beyond my level and I’ve been known to know a thing or two about the AB kestrel at least. But I’m not exactly sure if I should relook at my systems or if you’re talking some nuances so in the noise I’ve never recognized their factor.

 

[Ledzep]

When you pick a BC, you're taking a scaled Cd vs Mach curve of the G1 or G7 standard projectile, and that's what the solver is using to determine the rate of velocity loss of the projectile as it goes along it's merry way. It's saying "Projectile go Mach 2.1, what is Cd?" and it pulls that value from the scaled Cd vs. Mach curve that was so-scaled by the BC you input into the calculator. It puts that Cd value in to the total drag equation and determines the drag force, then says "ahh, projectile slow down this much". It's a constant loop of numerical approximation for the differential equations involved. That's not important to explain the part I mentioned above. What is important is to recognize that the solver is pulling Cd values at various Mach values.

If your Mach values change with temperature, then on one day at 59 degrees, it's pulling data for Mach 2.4 at the muzzle and everything is cool. Another time it's much hotter out but at the same DA, and now all of a sudden you SHOULD be pulling data for Mach 2.2 at the muzzle, but the solver is still pulling Cd data from Mach 2.4. Thus, along the entire trajectory, the Cd values are skewed a little bit. Again, I'm not familiar with the inner workings of the kestrel and it may correct for temperature or it might not. Generally this is a small error.

Depending on the bullet and its drag properties, a much bigger problem is the fact that the BC is constantly changing with respect to the G1 or G7 standard along the mach regime, and those errors add up quicker. Hence the bracketed published BC's, CDM's, etc...

ETA: I'll try to do some digging if I remember. I have a trip this weekend out of town but I'll revisit Reynolds vs. mach vs. temp vs. altitude vs. pressure and see if I can come up with something a little better descriptive.

 

[dk17hmr]

All very interesting. I'll be doing more testing and proofing.

As for right now I have data that matches my software and works inside the kill range Im willing to shoot in the field. I know my bullets have sufficient impact velocity at the far end based on previous kills.

Work is finally slowing down enough I might a be able to get some hunting and range time in.

 

[Lowlight]

I would not pay any attention to most of that it’s Nonsense

It’s how software works, your MV being off is common and g1 vs G7 is talking point garbage that means nothing in the real world that is lots of words with no meaning or answers

 

[dk17hmr]

I killed two antelope today with this rifle 397 yards and 421 yards one shot each. Software appears to be good enough for my purposes right now.

 

[Lowlight]

If you measure of success is hitting an animal inside 500 yards, well we probably have nothing for you

Weather is barely doing anything at those ranges, so enjoy, let alone any other real factors