Truing MV vs. BC

[Nik H]

Standard conditions. 29.92 59 degrees 0 RH

Figured...

Something is odd for certain. I need to get my own Chronograph versus always borrowing a bud's. I don't reload so I always thought it would be a waste of money but hell, even if shoot nothing but factory, you could collect a shit ton of data that would help with these matters

 

[Sheldon N]

Okay, here's a first hand experience from last weekends PRS match.

Did all my load development at home, have chrono'd multiple times and have a solid velocity number from a magnetospeed. Zero is spot on and from general shooting the load hits small targets no problem inside of 600 yards. The week before the match I go out to 1280 yards and I'm hitting 0.2 mils low consistently, good waterline of impacts. I was using the BC from a previous lot of bullets. I went home and ran the numbers on JBM and by adjusting my G7 BC from .292 down to .285 the data from 1280 yards matches up and fixed my 0.2 mil low impacts.

Get on a plane, fly to Utah and shoot in DA that's double what I normally see. Dope was solid all weekend, perfect vertical no matter the yardage. Had one target at 1379 yards and the dope worked spot on, first round impact with two more follow up hits.

 

[Lowlight]

Fixing the BC is the key ...

 

[tea&jam]

tag

 

[B-P-UU]

Guys with a 5700, please walk me through your process regarding using the velocity cal at 5-600 yds and BC truing at extended ranges (1100-1200 or 1.2 Mach)

I'm aware of the velocity calibration function on the 5700 and have used it to true dope. Has worked extremely well for me except very far targets seems like I'm adding a few tenths. Is there a BC truing function or are you just over-riding the value until actual drops jive with SW? I'm assuming you cannot use the AB custom drag model when doing this?

Thanks

 

[Sheldon N]

Is there a BC truing function or are you just over-riding the value until actual drops jive with SW?

This is all I'm doing. Leave velocity alone as long as you've got a good chrono and nothing wonky is happening inside 600 yards. Reverse engineer the BC by trying different numbers until your solver matches the actual observed drops from shooting at long range.

 

[Lowlight]

JBM is one of the tools you have and it's free

JBM BC Calculator

If you take the true MV and then look at your drop at 800, you can then bring that data into JBM and it will adjust the BC then you can go back and adjust it

Funny, I can take real word data and JBM is usually pretty close to spot on, even guessing I am super close to right

But try gathering all your data, and then using the ballistic features in JBM to fine tune them for other software.

 

[Lowlight]

Using the above I shot the new Kelbly rifle the NYX this weekend and here is the actual vs the corrected JBM

Here is the actual recorded data

Using JBM 100% you can see the modified data, BC changed from .583 to .595

 

[B4Maz]

How about banded G7 BCs from Hornady using Strelok Pro? Once you put in a banded BC, it wont let you true your BC.

 

[Deleted member 113831]

How about banded G7 BCs from Hornady using Strelok Pro? Once you put in a banded BC, it wont let you true your BC.

No, you have to true your initial BC before you start inputting the banded BC's. Gotta have a solid starting point first. The subsequent BC's are adjusted manually until they match actual drop.

 

[300ATT]

Lowlight,

Don't want to be that guy but ... The JBM link that you provide puts you onto a BC calculator based on a downrange velocity differential. Apparently you need to shoot through two chrono's to provide the needed data. (My chrono has already been hit once or twice at close range ... can't imagine not hitting one farther out ... )

Is there another JBM calculator link that you use to "put your 800 drop in so that it will adjust the BC"?

- or -

Is this more of a process of "hunt and peck" trying out different BC's in JBM in order for it to produce a table that more or less matches what you observe in the field?

 

[PhilM]

I am just about to start using my new Kestrel elite 5700 in anger.

Can I ask why people are trueing BCs when all the AB literature I can find just says use CDMs and don’t bother with trueing BC, only true the MV.

Any thoughts?

My 100m zero is spot on, I know it’s said that atmospherics are irrelevent at that distance, but I can’t find anywhere to enter them anyway?

Thanks!

 

[_Windrider_]

I am just about to start using my new Kestrel elite 5700 in anger.

Can I ask why people are trueing BCs when all the AB literature I can find just says use CDMs and don’t bother with trueing BC, only true the MV.

Any thoughts?

My 100m zero is spot on, I know it’s said that atmospherics are irrelevent at that distance, but I can’t find anywhere to enter them anyway?

Thanks!

Because they want you to buy their CDM?

 

[dghboy315]

I am just about to start using my new Kestrel elite 5700 in anger.

Can I ask why people are trueing BCs when all the AB literature I can find just says use CDMs and don’t bother with trueing BC, only true the MV.

Any thoughts?

My 100m zero is spot on, I know it’s said that atmospherics are irrelevent at that distance, but I can’t find anywhere to enter them anyway?

Thanks!

I was wondering the same thing about AB's CDM? I'm curious about people's results with the CDMs.

 

[Deleted member 113831]

Lowlight,

Don't want to be that guy but ... The JBM link that you provide puts you onto a BC calculator based on a downrange velocity differential. Apparently you need to shoot through two chrono's to provide the needed data. (My chrono has already been hit once or twice at close range ... can't imagine not hitting one farther out ... )

Is there another JBM calculator link that you use to "put your 800 drop in so that it will adjust the BC"?

- or -

Is this more of a process of "hunt and peck" trying out different BC's in JBM in order for it to produce a table that more or less matches what you observe in the field?

This one will work:
http://www.jbmballistics.com/cgi-bin/jbmdrag-5.1.cgi

 

[Sheldon N]

Is this more of a process of "hunt and peck" trying out different BC's in JBM in order for it to produce a table that more or less matches what you observe in the field?

That's what I do. Just click the back button, type in a new BC, repeat until it matches your actual drop. Took 60 seconds.

 

[DP425]

I've had a Kestrel 5700 Elite for a while now, and I'm just now considering truing BC and velocity; I really am not too sure where to start... fortunately, this thread has had great info in it and I believe I have a solid idea. My main question is, should SD and CE be turned "off" when doing this, or left on?

Otherwise, let me make sure I have this straight-
1- zero at 100
2- dial on predicted dope for 500; engage target
3- record vertical POA-POI deviation
4- run velocity correction
5- dial on predicted dope (with corrected velocity) for 800, 900, or 1000 (or ideally, 1200 if your range goes that far); engage target
6- record vertical POA-POI deviation... ---OR--- dial on until POA matches POI
7- screw with BC until Kestrel predicted dope matches observed true dope.

If adjustments are to be made off-site or at a later time, freeze environmentals after shooting.

This sound about right?

 

[Deleted member 113831]

I've had a Kestrel 5700 Elite for a while now, and I'm just now considering truing BC and velocity; I really am not too sure where to start... fortunately, this thread has had great info in it and I believe I have a solid idea. My main question is, should SD and CE be turned "off" when doing this, or left on?

Otherwise, let me make sure I have this straight-
1- zero at 100
2- dial on predicted dope for 500; engage target
3- record vertical POA-POI deviation
4- run velocity correction
5- dial on predicted dope (with corrected velocity) for 800, 900, or 1000 (or ideally, 1200 if your range goes that far); engage target
6- record vertical POA-POI deviation... ---OR--- dial on until POA matches POI
7- screw with BC until Kestrel predicted dope matches observed true dope.

If adjustments are to be made off-site or at a later time, freeze environmentals after shooting.

This sound about right?

Looks like you've got it. The only thing I would add is, be alert to your environmentals in the process. Don't let mirage or a change in lighting conditions, or something else throw you off when you move out to the further ranges.

 

[oakfig]

The BC does not remain constant, from any rifle, ever. What all of these BC banding programs do is break up the flight into segments and give corrections to keep things true. It's like the corrections you make to the steering wheel while driving down the road. The corrections are small enough and frequent enough to give the appearance of driving straight. Same thing here, some bullets need more frequent correction than others and at different speeds.

I can't speak to the difference in your solvers, other than to say go back and double check that every single input is identical. I have AB and 4DOF on my phone, but find myself using Strelok Pro and it's multi-BC function more and more. It isn't as sophisticated maybe, as the others, but I can see every single input. Nothing is "grabbed, populated, transmitted, blue toothed" for me.

With room for 5 BC inputs, I can correct the curve to match actual drop waaaaaaay out there. I'm only using 3 BC's to get my 338AX to 2500 yards, and it is accurate from the muzzle to steel. The user interface is way better also. Everything you want to see is either shown on the home screen or accessible from the home screen. Navigating AB and 4DOF just pisses me off.

Got the strelok pro, very user friendly! Wish I'd saved the $ on the rest and went with this one.

 

[B-P-UU]

I've had a Kestrel 5700 Elite for a while now, and I'm just now considering truing BC and velocity; I really am not too sure where to start... fortunately, this thread has had great info in it and I believe I have a solid idea. My main question is, should SD and CE be turned "off" when doing this, or left on?

Otherwise, let me make sure I have this straight-
1- zero at 100
2- dial on predicted dope for 500; engage target
3- record vertical POA-POI deviation
4- run velocity correction
5- dial on predicted dope (with corrected velocity) for 800, 900, or 1000 (or ideally, 1200 if your range goes that far); engage target
6- record vertical POA-POI deviation... ---OR--- dial on until POA matches POI
7- screw with BC until Kestrel predicted dope matches observed true dope.

If adjustments are to be made off-site or at a later time, freeze environmentals after shooting.

This sound about right?

I'm in the same boat as you.. I think what you laid out is correct and how I plan on doing it. Not sure on spin drift or CE.. some seem to think AB over-predicts SD. I would also think you want aero jump on to take crosswind jump into consideration as well.

Once you modify your BC at extended ranges, I think the only other thing I would do is confirm everything lines up at 500 yds or so again.

 

[Sheldon N]

Once you modify your BC at extended ranges, I think the only other thing I would do is confirm everything lines up at 500 yds or so again.

It takes a pretty massive change in BC to affect your 500 yard dope. For example with my match load at my range, everything from a .258 G7 up to a .295 G7 BC all come out of the solver at 2.4 mils of elevation.

 

[SageRatSafaris]

I can't speak to the difference in your solvers, other than to say go back and double check that every single input is identical. I have AB and 4DOF on my phone, but find myself using Strelok Pro and it's multi-BC function more and more. It isn't as sophisticated maybe, as the others, but I can see every single input. Nothing is "grabbed, populated, transmitted, blue toothed" for me.

With room for 5 BC inputs, I can correct the curve to match actual drop waaaaaaay out there. I'm only using 3 BC's to get my 338AX to 2500 yards, and it is accurate from the muzzle to steel. The user interface is way better also. Everything you want to see is either shown on the home screen or accessible from the home screen. Navigating AB and 4DOF just pisses me off.

Ballistic AE (uses JBM engine) also supports input of banded BC by velocity, albeit I find it awkward to experiment with. Auto loads it for Sierra bullets. I haven’t tried Strelok Pro, does it make working with banded BC easy?

 

[Deleted member 113831]

Ballistic AE (uses JBM engine) also supports input of banded BC by velocity, albeit I find it awkward to experiment with. Auto loads it for Sierra bullets. I haven’t tried Strelok Pro, does it make working with banded BC easy?

You just select the multi-BC option and put in a BC and a speed beside it to tell it when to start using that BC.

 

[Deleted member 113831]

I'm just putting this out for reference so all can see how much difference BC vs Velocity matters at 500 yards. There have been a couple of responses that have basically asked the question: "Why wouldn't you true BC at 500, if you know you have a good velocity?" That is a fair question.

I have said that out to 500 yards or so, velocity is the main component of trajectory. Here are some numbers from JBM:

Altitude of 2000ft (28.0Hg), 500 yards, 6.5 caliber bullet with .630 G1 BC.
At 2800 fps = 9.1 MOA or 2.6 MILS of elevation
At 2750 fps =9.5 MOA or 2.8 MILS of elevation

Raising the speed back to 2800 fps, we have to use a .540 BC to get the same amount of change at 500 yards.
So, it takes 90 G1 BC points to equal the same amount of change as 50 fps of velocity change at this distance.

You can run your own numbers at longer distances if you choose to do it that way, and make your own decisions. This is purely for information purposes.

If you are seeing a significant difference from projected dope at 500 yards, neither velocity nor BC are likely to blame. There is likely something going on somewhere else with the rifle / shooter system. Or, there has been a drastic change in the environmentals, to include lighting conditions.

 

[Freediver111]

Tagging. I listened to Frnak's podcast regarding this but wasn't sure just how you go about adjusting BC. The JBM calc helps. Just got my new build set-up and need to do this.

 

[B-P-UU]

You want to test for MV Truing at Mach 1.2 +/-.

I actually tried this earlier this week and everything lined up really well. 1150-ish yards with my dasher. Used the custom drag in the program, AJ was on as well as SD.

 

[Freediver111]

So clear up the BC side of things for me.
I zero at 100, check my velocity at 500, then modify my BC at 800-1000. So say I’m low at 1,000. I assume I would lower my BC, but by how much in increments? How much do you have to modify BC to make a difference at those yardages?

 

[spife7980]

So clear up the BC side of things for me.
I zero at 100, check my velocity at 500, then modify my BC at 800-1000. So say I’m low at 1,000. I assume I would lower my BC, but by how much in increments? How much do you have to modify BC to make a difference at those yardages?

I zero at 100. I have a magneato for the velocity so I stick with that velocity, I did verify it at 600 yards and it was good. I found the actual amount of elevation by shooting. 7.3 mils dialed for 1050 yards.

.280 g7 is the 105 rdfs bc via nosler and with that number the calculator was calling 7.7 mils for 1050 yards.
So if I had dialed what it called I would have been shooting over the plate. That tells me the bullet is more slippery than claimed and so I need to increase its bc number.

I went in .01 increments until my calculator reflected the 7.3 mils with a g7 of .310
It still works well at 600, the difference was in between a clicks worth, instead of group centers shooting a hair over the waterline Im shooting a hair under, just an ever so slight shift. My groups are big enough to make up the difference anyways of that one click anyways.

 

[Deleted member 113831]

So clear up the BC side of things for me.
I zero at 100, check my velocity at 500, then modify my BC at 800-1000. So say I’m low at 1,000. I assume I would lower my BC, but by how much in increments? How much do you have to modify BC to make a difference at those yardages?

It doesn't take much at those ranges. It also depends if you are using G1 or G7. Using G7 I try it in 5 point increments.

 

[_Windrider_]

It doesn't take much at those ranges. It also depends if you are using G1 or G7. Using G7 I try it in 5 point increments.

We could apply some maths here as well if one needs a place to start. The ratio formula can be applied.

A/B = X/Y

Where:

A is manufacturer BC
B is manufacturer velocity (used to determine A

X is trued BC (unknown)
Y is known MV from Chrono

Solve using criss cross method

AY = BX

Example

.308 with BC of .496 at 2600 FPS. What is BC if my actual velocity is 2800?

.496/2600 = X/2800
Using cross method, equation becomes

.496(2800) = 2600x
1394 = 2600x

Now divide both sides of equation by 2600 to determine new BC (x)

.536 = X

This is not perfect as the change in BC is not linear with a change in velocity but it’ll get you close.

 

[Sheldon N]

I just poke BC numbers into the JBM calculator until my drops match up.

http://www.jbmballistics.com/cgi-bin/jbmtraj_simp-5.1.cgi

JBM uses the same engine as my phone app (Ballistic AE) so the data works across both platforms.

 

[_Windrider_]

I just poke BC numbers into the JBM calculator until my drops match up.

http://www.jbmballistics.com/cgi-bin/jbmtraj_simp-5.1.cgi

JBM uses the same engine as my phone app (Ballistic AE) so the data works across both platforms.

Yes. Agreed best way to do it after you have fired rounds.

 

[OzzyO20]

Gonna preface this with I'm using Shooter on iOS...… Besides the obvious advice to upgrade to a better solver, what I have done so far is chrono my handloads, and plugged that into Shooter. It spit out 2.5 Mils at 500, and it was dead on. Cleaned a KYL rack. Got out last weekend to 1,000 and was shooting high by a decent amount. I went into the bullet profile and increased the BC slowly until the numbers matched up. My 500 yard data was still the same 2.5 MILs. In my head, taking the BC up should of made the 500 yard shot require less elevation, but it didn't. Reading this it sounds like I half way did the right thing. My fiancé and I are shooting a match this weekend, and I want her to have a good experience so I'm looking for input as to if this makes sense to everyone?

Before I was just using the G7 off the Berger box, and it was shooting high. Now I'm within .1 mils at 1,000. I wasn't able to shoot further or closer since it started to pour the rain. Was hitting a 66% IPSC at 800, but that's an awfully generous target and the rain wasn't allowing me to get any feedback other than I was hitting it.

 

[Deleted member 113831]

Gonna preface this with I'm using Shooter on iOS...… Besides the obvious advice to upgrade to a better solver, what I have done so far is chrono my handloads, and plugged that into Shooter. It spit out 2.5 Mils at 500, and it was dead on. Cleaned a KYL rack. Got out last weekend to 1,000 and was shooting high by a decent amount. I went into the bullet profile and increased the BC slowly until the numbers matched up. My 500 yard data was still the same 2.5 MILs. In my head, taking the BC up should of made the 500 yard shot require less elevation, but it didn't. Reading this it sounds like I half way did the right thing. My fiancé and I are shooting a match this weekend, and I want her to have a good experience so I'm looking for input as to if this makes sense to everyone?

Before I was just using the G7 off the Berger box, and it was shooting high. Now I'm within .1 mils at 1,000. I wasn't able to shoot further or closer since it started to pour the rain. Was hitting a 66% IPSC at 800, but that's an awfully generous target and the rain wasn't allowing me to get any feedback other than I was hitting it.

Changing the BC by reasonably small amounts has very little effect at 500 yards. Out to 500 yards the largest determiner of trajectory, by a wide margin, is speed.

You would need almost a 100 G1 BC (50 G7) point change to make a 0.2 mil difference in trajectory with most "long range" rounds at 500 yards.

Edit: Just to put a finer point on it...a .400 G1 BC bullet at 2800 fps drops 3 mils at 500 yards from a 100 yard zero.

A .700 G1 BC bullet at the same speed and conditions drops 2.5 mils. This is why I true velocity at 500 yards and not BC.

 

[Timetoshoot]

My limited and at times frustrating experience aligns with skookum

 

[Timetoshoot]

What the hell is a skookum?

 

[Deleted member 113831]

What the hell is a skookum?

I asked the same thing the first time I heard it. It can mean "strong", "large", "intelligent or wise".....it is also a euphamism for being fat. My father-in-law is Canadian and gave me the nickname. His family is big on nicknames.

He of course told me it was because I am smart and strong...but I'm not too convinced.

https://en.wikipedia.org/wiki/Skookum

 

[Timetoshoot]

I’ll file that in the” learn something new everyday file “

Thought it might be a line from a Chevy chase movie.....

 

[_Windrider_]

I’ll file that in the” learn something new everyday file “

That it was might be a line from a Chevy chase movie.....

We need to come up with an Urban dictionary meaning.

 

[Mt_Hunt]

Wow... How did I miss this thread... Very helpful.. I have been trying something similar but I think I was missing something... Thank you...

 

[Deleted member 10043]

I've had a Kestrel 5700 Elite for a while now, and I'm just now considering truing BC and velocity; I really am not too sure where to start... fortunately, this thread has had great info in it and I believe I have a solid idea. My main question is, should SD and CE be turned "off" when doing this, or left on?

Otherwise, let me make sure I have this straight-
1- zero at 100
2- dial on predicted dope for 500; engage target
3- record vertical POA-POI deviation
4- run velocity correction
5- dial on predicted dope (with corrected velocity) for 800, 900, or 1000 (or ideally, 1200 if your range goes that far); engage target
6- record vertical POA-POI deviation... ---OR--- dial on until POA matches POI
7- screw with BC until Kestrel predicted dope matches observed true dope.

If adjustments are to be made off-site or at a later time, freeze environmentals after shooting.

This sound about right?

Shoot at 800 to tweek the unknown MV or BC. It shouldn't be much in the way of elevation on the turret.

 

[Rlovato]

This thread finally made everthing click people have told me this method before just seeing the steps made more sence

 

[Deleted member 113831]

Shoot at 800 to tweek the unknown MV or BC. It shouldn't be much in the way of elevation on the turret.

If you are going to do this at 800 yards, I would still use velocity rather than BC to begin with. Then I would use BC for the farther ranges. If you are having to tweak BC more than say 10 G1 points or 5 G7 points, you are probably barking up the wrong tree. Something else is going on.

In the example Frank gave above in post #58, he only had to tweak the BC by 12 G1 points...and he is at 5,000 ft.

Even at 800 yards and 2,000 ft elevation (28.0 inHG), a .630 G1 BC bullet going 2800 fps drops 5.4 mils.
Dropping the BC to .620, it still drops 5.4 mils.
However, drop the velocity 1% (28 fps) and the bullets drops 5.6 mils. So even out to 800 yards, velocity is still the main determining factor.

 

[Deleted member 10043]

I'm just trying to keep the process as simple as possible.

Also, I'm missing something on LL's. The first image is his data sheet. The JBM data with new BC is what?

 

[Deleted member 113831]

I'm just trying to keep the process as simple as possible.

Also, I'm missing something on LL's. The first image is his data sheet. The JBM data with new BC is what?

He adjusted the BC from .583 to .595. The actual dope at 1500 was 13.8 mils and JBM predicted 13.7.

Yeah, I get that it needs to be simple. The thing I keep seeing though, is that people keep saying "tweak BC and velocity at 800". Well, yes that works with a couple of caveats. The first being that the further you go out, the more likely some optical effect will introduce unaccounted for error and throw your data off.

The second being that most who advocate for truing at that range don't say how much velocity and how much BC to use. So what happens is that people end up tweaking BC to match at 800 yards and it is way, way too much. Because even at 800 yards, velocity is still a larger determiner of trajectory.

 

[300ATT]

So Skookum (Oh strong and wise ... never fat), is there a cross-over point where changes in the BC or velocity have an equal effect? My guess is that it will be way out there and in the sub-sonic velocity range ... time to play with numbers ...

Have you done this with a .22LR say at 200 yards?

 

[Deleted member 10043]

Actually, he adjusted his BC to line up with his 800 yard dope. But it is off reconciling to the rest of his data sheet.

 

[Deleted member 113831]

Actually, he adjusted his BC to line up with his 800 yard dope. But it is off reconciling to the rest of his data sheet.

I don't know how you would true at 800 with BC when it comes up 4.8 mils with either one listed. If they are the same, what are you truing?

I re-ran it with all the inputs on JBM, exactly as listed on the JBM table shown in post #58. At 800 yards, with the listed speed of 2900fps and all other inputs identical, It is 4.8 mils with a .583 G1 and 4.8 mils with a .595 G1.

So, near as I can figure, Frank had to be talking about the rest of the trajectory.

 

[Deleted member 113831]

So Skookum (Oh strong and wise ... never fat), is there a cross-over point where changes in the BC or velocity have an equal effect? My guess is that it will be way out there and in the sub-sonic velocity range ... time to play with numbers ...

Logic would dictate that there must be, but I haven't chased that one down yet.

 

[ViP]

Logic would dictate that there must be, but I haven't chased that one down yet.

Does the wind speed and direction need to be recorded in the ballistics app/software when truing the speed and the bc? Thanks.