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Truing MV vs. BC
- Thread starter Ppierceall
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Deleted member 10043
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Deleted member 113831
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I've personally experienced the need to true at 1k. I trued out to 800, every thing was spot on other than 1k. I think I was .2 or .4 MIL low at 1k where everything else lined up great. I then trued at 1k, got it nailed, and wawas only off by .1 on shorter ranges. Instead of truing at 500 and truing bc at 800, I just used my actual velocity and trued bc, then changed my velocity a hair to get the final touch up.
Im doping out some new ammo on thur. I'm going to try the 500 then 800 method.
Im doping out some new ammo on thur. I'm going to try the 500 then 800 method.
I'm not sure where .595 G1 came from but this was what I got using other software and the inputs supplied
Cold Bore Desktop *AJ off *no azimuth supplied
AB G7 .294
800=4.8 1000 = 6.9 1350=11.7 1500 =14.2
AB G1 .575
800=4.9 1000 = 7.1 1350=12.0 1500 =14.6
Prime G7 .289
800=4.9 1000 = 7.0 1350=11.8 1500 =14.4
Prime G1 .583
800=4.9 1000 =7.0 1350=11.9 1500 =14.5
using the modified G1 BC .595 lined up at 800
800=(4.8) 1000 =(7.0) 1350=11.7 1500 =14.2
using modified G1 BC .610 lined up at 1350
800=(4.8) 1000 =(6.9) 1350=11.5 1500 =13.9
using modified G7 BC .302 lined up at 1350
800=(4.8) 1000 =(6.9) 1350=11.5 1500 =13.9
TRASOL (Cold Bore Engine) *no azimuth supplied
AB G7 .294
800=4.82 (4.8) 1000 = 6.93 (6.9) 1350=11.71 (11.7) 1500 =14.17 (14.2)
AB G1 .575 lined up at
800=4.88 (4.9) 1000 = 7.06 (7.1) 1350=12.0 (12.0) 1500 =14.6 (14.6)
Prime G7 .289
800=4.85 (4.9) 1000 = 6.99 (7.0) 1350=11.84 (11.8) 1500 =14.37 (14.4)
Prime G1 .583
800=4.86 (4.9) 1000 =7.01 (7.0) 1350=11.89 (11.9) 1500 =14.43 (14.5)
using the modified G1 BC .595 lined up at
800=4.82(4.8) 1000 =6.94 (6.9) 1350=11.73 (11.7) 1500 =14.19 (14.2)
modified G1 BC .612 lined up at 1350
800=4.77 (4.8) 1000 =6.85 (6.9) 1350=11.53 (11.5) 1500 =13.88 (13.9)
modified G7 BC .302 lined up at 1350
800=4.77 (4.8) 1000 =6.85 (6.9) 1350=11.52 (11.5) 1500 =13.87 (13.9)
Applied Ballistics (Kestrel 5700 Elite) *AJ off (WS2) *no azimuth supplied DOF=000
AB CDM (Norma 130 Diamond)
800=4.80 (4.8) 1000 = 6.89 (6.9) 1350=11.50 (11.5) 1500 =13.95 (14.0)
AB G7 .294
800=4.81 (4.8) 1000 = 6.90 (6.9) 1350=11.48 (11.5) 1500 =13.91 (13.9)
AB G1 .575
800=4.85 (4.9) 1000 = 6.96 (7.0) 1350=11.60 (11.6) 1500 =14.04 (14.0)
Prime G7 .289
800=4.84 (4.8) 1000 =6.95 (7.0) 1350=11.61 (11.6) 1500 =14.09 (14.1)
Prime G1 .583
800=4.82 (4.8) 1000 =6.92 (6.9) 1350=11.49 (11.5) 1500 =13.89 (13.9)
*using modified G1 BC .595 lined up at 800
800=4.78 (4.8) 1000 =6.85 (6.9) 1350=11.34 (11.4) 1500 =13.68 (13.7)
Ballistic AE (JBM engine) *no azimuth supplied
AB G7 .294
800=4.81 (4.8) 1000 = 6.91 (6.9) 1350=11.51 (11.5) 1500 =13.95 (14.0)
AB G1 .575
800=4.85 (4.9) 1000 = 6.97 (7.0) 1350=11.62 (11.6) 1500 =14.07 (14.1)
Prime G7 .289
800=4.85 (4.9) 1000 =6.97 (7.0) 1350=11.64 (11.6) 1500 =14.14 (14.1)
Prime G1 .583
800=4.82 (4.8) 1000 =6.92 (6.9) 1350=11.52 (11.5) 1500 =13.93 (13.9)
*using the modified G1 BC .595 lined up at 800
800=4.79 (4.8) 1000 =6.86 (6.9) 1350=11.36 (11.4) 1500 =13.68 (13.72)
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.583 came from Frank's written dope sheet in post #58, .595 came from the BC Frank put into JBM. It is listed clearly at the top of the data.
I'll clarify. I meant I didn't know how he came up with .595 (I thought he might have been setting coldbore to = 4.8 at 800)
you had pointed out that both .583 and .595 = 4.8 at 800 in JBM so I had included both in my examples
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My contention was that Frank wasn't truing his data at 800 at all but at some further distance, and .595 accomplished that. Regardless, truing at 800 makes no sense if both BC's give the same result.
I confined myself to JBM since that is what was used in his example.
I agree. Truing at 800 yards makes no sense if both BC’s give the same result
If “800 yards” is the set “calibration point” then that presupposes the shooter hasn’t got access to ranges beyond the “calibration point” to even know if the trajectory is off at further distances and if he (the shooter) has “1350 yards” set as the “calibration point” then the shooter has access to the range necessary (based on the current example given) to true both the BC and back-extrapolate the MV
So I was stressing over dope for an upcoming match. Literally only got to put 2 rounds on steel at 993 yards before a downpour the last chance I had to collect data. Following the advice here, I input my MV from my Magnetospeed and tweaked the BC until my 500, 800, and 1,000 (993) yard dope matches what I observed from the 1,000 yard shot.
Using Shooter I put together a dope card that allowed me to get first round hits out to 1,207, along with my fiancé who shot her first match and did excellent. End result was a solid 2nd in my 2nd match.
Thank you all a lot for the help.
Using Shooter I put together a dope card that allowed me to get first round hits out to 1,207, along with my fiancé who shot her first match and did excellent. End result was a solid 2nd in my 2nd match.
Thank you all a lot for the help.
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If you own a 5700 Elite you don't have to buy a CDM. They are already included.
Yes, wind speed and direction do make a difference. So does a lot of other things. Take a look:
This is incorrect information. At least on our systems. You can do a MV Cal, or a Ballistics Calibration on any CDM.
It is actually more complicated than that. This is why our AB Analytics allows for wind in 3D in 10 different places. We can place special monitors that measure the wind in all directions and report in real time. This is how we validate the accuracy of the solver. By using a dozen or sometimes more instruments at one time to remove the variables. The wind everywhere technically matters. But by how much can differ greatly.
http://appliedballisticsllc.com/Articles/NVDOC1403-Wind.pdf
MV CAL, yes. Don’t CDM change the BC to 1.00? Unfortunately I don’t have my kestrel on me right now to try and see if the 1.00 will change
With a CDM their is no BC. When you use a BC you are essentially picking one of a dozen standards (Gsp, GL, G1.......) then you are saying "my bullet performs at this percentage of the standard I chose, so you should adjust....).
When you use a CDM instead of using a BC and the Drag Model of a standard, you are using the Drag Model of the actual bullet as measured in flight.
BC changes to 1 because you are comparing the bullet to itself, and not to another bullet so it is being zeroed out. (when you multiple something by 1 you get itself).
But in doing so, you are not actually tweaking the CDM itself, just the usual truing we all are aware of, while the original question is about tweaking the CDM, unless I got it wrong.
Technically speaking... a CDM by itself is not “adjustable” because it’s is not a BC.
(example: If the shooter is shooting a Lapua 136 Scenar-L and uses a Lapua 136 Scenar-L CDM supplied by a particular Vendor. The shooter is comparing the performance of His/Her Lapua 136 Scenar-L to another Lapua 136 Scenar-L supplied by the vendor shot from that Vendors’ test equipment. NOT a “G-Standard-Projectile”)
If the question is “How Do I adjust the trajectory output for the solver if it needs fine tuning?”
Then the shooter would use whatever tools the vendor has implemented to do the fine tuning
Applied Ballistics would adjust MV or use the DSF feature depending where the trajectory needs adjusting
Honrady 4DOF would adjust the Axial Form Factor
Lapua Ballistic 6DOF app doesn’t support a “drag-variable” adjustment so the shooter would have to manually adjust MV to adjust the trajectory
(This presupposes that the shooter did their due diligence and followed the vendors’ pre-requisite checklist/procedures to rule out CWAJ in their zero. Shooting Azimuth, Scope tracking errors... etc in order for the vendors' tools to remain a fine adjustment rather than a gross adjustment)
I think it all comes down to what the OP really meant in his question about the CDMs.
Actually you can tweak the "drag curve" in Hornady 4DOF, Coldbore and FFS. How we adjust the trajectory is an altogether different scenario, where the usual truing ways come in. It doesn't matter if the drag curve is not a BC, because you can always change its form, at least in the mentioned software.
If it is windy at all, I do NOT use it for my final truing number. If you do your base wind will always be in your predictions.
Unfortunately, to work BC correctly you need to work around image displacement and wind. It might mean the day you plan on “truing” doesn’t work.
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Could you clarify what you mean when you say "If you do your base wind will always be in your predictions."?
Wind depending on the direction could influence your bullet up or down. If you true on a windy day then your numbers could be slightly off when you shoot another day with different wind speed or direction.
The solver (presuming it supports AJ) would be accounting for that when the shooter inputs the wind speed and direction along with Latitude Azimuth and SD in order to isolate the secondary effects from being adjusted into the MV/BC.
If those inputs are left blank or turned off the solver will interpret the recorded drop that's being used to true the algorithm as being entirely the result of gravity instead of isolating the gravity component of the drop data from the secondary effects that were present in the environment when the drop data was recorded.
Let’s make this super simple-
Something I’ve seen, done over thousands of rounds and years of ELR shooting, is needing to really bend curves, or even use two profiles because the curve fit was to extreme.
Optical disturbances and high winds are almost impossible to estimate and cause very large result swings.
We see the effect, input the results. But because we do not have the actual optical or absolute wind data our inputs are flawed.
Just make sure to do your final tuning under ideal conditions..
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Deleted member 113831
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I was out shooting at 2,400 yards just a couple of weeks ago and had a common experience that I think illustrates your point.
I had gathered dope on this load at 60 degrees F in this exact location in heavy overcast and in winds that were from 1130 oclock at less than 3mph. There couldn't have been better conditions.
I go out again on this day, and the only difference (same batch of ammo, same temp, same wind) is that the sun is out and rising over the mountain to my 10 oclock. My group was 0.5MILS low and 1 MIL right.
When using a DSF you are essentially tweaking the Drag Curve. But you are doing it in a more advanced manner. You are adjusting the prediction at a specific Mach Number. This means the adjustments will carry over in all conditions. Bullet models are built off Mach/CD tables anyways, so this allows the prediction to be adjusted at a specific point, or points (up to 6).
Most of you are running at high MV. Does the 500/800 truing apply if your MV is closer to 2500, or do you adjust down the range based on the percentage slower the MV?
2500 is roughly 14% slower, so MV at 400 (430 based on %) and BC at 700 (690 based on %)?
2500 is roughly 14% slower, so MV at 400 (430 based on %) and BC at 700 (690 based on %)?
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I would adjust it if it were me. There is nothing magic about 500 or 800 yards. It is just that rounds leaving the barrel between 2700 and 2900 fps have such similar trajectories that they can generally be trued at the same distances.
@Skookum, thanks for you explanations; they make a lot of sense. Especially correcting MV at 500 and BC at much longer distances... I've been using Strelok and I have a couple of questions about it. Would you take a screen shot of your "multi BC" page on Strelok and post it? I want my BC curve to be accurate and I'm not sure hot to determine my BC and velocity drops that I need to enter on that page. If you could walk me through it, I would appreciate it.
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I'll be honest, I don't have the slightest idea how to take a screen shot.
I basically use 2500 fps and 1600 fps as my velocity bands. I put in published BC for the muzzle velocity, And 5% less BC for 2500 and 5% less again for 1600 fps. I shoot it at distance and adjust the BC for whatever velocity band that range falls into.
My 300 Win with 210 SMK @ 2899 fps:
.315 G7 @ 2899
.305 G7 @ 2500
.275 G7 @ 1600
These weren't my starting BC's. My starting BC's were: .315, .299, .284. My gun has an 11 twist, so the last BC suffered as the bullet lost stability.
I basically use 2500 fps and 1600 fps as my velocity bands. I put in published BC for the muzzle velocity, And 5% less BC for 2500 and 5% less again for 1600 fps. I shoot it at distance and adjust the BC for whatever velocity band that range falls into.
My 300 Win with 210 SMK @ 2899 fps:
.315 G7 @ 2899
.305 G7 @ 2500
.275 G7 @ 1600
These weren't my starting BC's. My starting BC's were: .315, .299, .284. My gun has an 11 twist, so the last BC suffered as the bullet lost stability.
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I copied Sierra.
2500 fps for a magnum cartridge is roughly equal to 300 yards. This is the distance that has been used in the past to calculate BC's, so it seemed a logical jump off point.
I use 1600 fps because that gives a little room to fine tune the last 300 yards before transonic. I don't usually shoot very far into subsonic maybe 100 or 200 yards at most. I haven't found that I needed a subsonic band at that range. If going further into subsonic I'm sure I would though.
I would never say that what I do is the only way, or even the best way, but it makes sense to me and has worked for me so far. Smarter people than me do it different. All it costs is a few bullets to try.
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I created another cartridge profile, copied my data from my current cartridge, and I used JBM to come up with two generic BCs for the 2500 and 1600 velocity bands for my 7mm RM, along with the one G7 BC that is listed for my cartridge. Interesting results, my mil holdovers increased from my single BC mode:
+.1 mil----500-600 yds
+.2 mils--600-700 yds
+.3 mils--700- 800yds
+.4mils--800-900 yds
+.5mils--900-950 yds
+.7mils--1000 yds
Granted, this is untested. But I think I will do some testing. I'll correct my MV through 500 yds and then I'll adjust the BC values from 500 yds on. The only limiting factor is I do not have a place where I can shoot past 750 yds.
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JBM used to have a calculator to determine the best single BC. G1 and G7 are not the only ones.
EDIT: Never mind. It still has that specific calculator. https://www.jbmballistics.com/cgi-bin/jbmgfa-5.1.cgi interesting in as much G1 may be the preferred BC over G7. Example, the .224 69gr SMK.
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The actual bands should be readily available for that bullet.
https://www.hornady.com/support/ballistic-coefficient
https://www.hornady.com/support/ballistic-coefficient
I saw this the other day but did not fully understand it i guess. Right now my Kestrel is set using the custom drag profile from the AB Library for this bullet and I'm not sure where I would enter in these "mach" values in my AB profile loader or how to make that work. Might have to reach out to Doc and see what he advises.
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If it is from AB then Litz either tested it and put it in there or it comes from Hornady. Anyway you can rely on it. The work is already done for you. You paid for it. You also paid for Doc's support so I would take advantage of that. BTW, Mach is a measure of the speed of sound that can be converted to fps as shown in that Hornady link.
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