LabRadar Benefit: Estimating BCs

[mcb]

While both give accurate results the magnetospeed has the following advantages

1. it’s lightweight and easy to pack to the range or wherever. Has a small case but you don’t need it. Just throw it in your backpack or range bag

2. Battery is just a couple Aa or aaa I can’t remember and life is usually 2 years or so with fairly heavy use. Compared to the labradar which sucks juice so bad it will last a few hours unless you buy an external battery pack to schlep around. Preferably rechargeable.

3. I Typically shoot several different target arrays that require traversing horizontally and vertically between berms and I can do that with magnetospeed attached. Don’t have to stop and point my chronograph towards a different target.

4. and of course there’s the price.

On the other hand, as I’ve said, the one thing the labradar could possibly do that the magnetospeed can’t is calculate bc. But to do that in a useful way it would have to track the bullet out to the distances you want to shoot at. Since the labradar can’t read your velocity at 1600 yards or even 500 yards, it won’t give you a useful number. I don’t know any serious shooter that bases their calc on a bc they derived from the 100 yard velocity they measured with their labradar.

So since the labradar won’t actually do anything useful that the magnetospeed doesn’t, it should really be incumbent on the labradar proponents to explain why one should pay 2-3x the price for the privilege of carrying a heavy bulky device to the line and burning through batteries. The only legit reason is just because you want to. And if that’s the case, rock on. I’m a gadget and technology geek too. It’s a cool device. I get it.


As was explained above, it can change barrel harmonics.
However, on my heavy contour barrels, with a suppressor on the end already, I very rarely get a measurable shift in POI or group size. And even then we’re talking .1 mil.
Ymmv if you’re using a pencil profile but is it really that much of an inconvenience to work up your load and then chrono it?

If you bullet fits one of the existing Gx models (G1 or G7 in particular) well than velocity changes from muzzle to ~100 yards is more than sufficient data to correctly estimate the BC for the appropriate Gx model and use it out as far as your gun will shoot.

If on the other hand you are using a bullet that does not fit the existing Gx models well (ie some of the new VLD bullet) and you are looking to create your own drag model or piece wise fit multiple Gx BC's to existing models for this bullet then the LabRadar would be harder but not impossible to use. If you really had your mind set on it you could work up a reverse ladder with some ammo loaded at you normal velocity and a few loads at lower muzzle velocities. Ideally you would down load so you had velocity spanning or at least bracketing the velocities you expect to see at the muzzle down to your maximum range. Above transonic the change in drag is fairly smooth so it would not have to be continuous, you could interpolate fairly well between subsets of velocity ranges, The transonic range it would need good to be continuous if your expecting to engage targets that far out as the coefficient of drag changes very rapid in the transonic velocity region. Once you had all this change in velocity data at different velocities you could use that to create a new drag model or piece wise fit multiple BCs using existing drag models. It would be a bit tedious but doable.

 

[taliv]

It's a heck of a lot easier to measure velocities from a handgun using a LabRadar. The MagnetoSpeed worked fine with my rifles; handguns, not so much.

That’s true. Maybe it’s just my perspective but I’ve around 50k pistol rounds past 6-7 years or so and never once felt the need to chrono them.

 

[taliv]

Mcb I really don’t think it works like that. Either your theory is incorrect (which is what I believe) or literally no bullets match the G profiles, which may also be true.

In either event, you aren’t going to calculate a usable long range bc with a magnetospeed unless as you said you went to the extreme trouble of loading and shooting bullets at every velocity down to transonic and then doing your own blended rate. And why would you do that when litz and hornady have measured it with Doppler and You can buy all of their bc for 1/20th the price of the labradar

 

[lysanderxiii]

With most chronographs there is no difference but with a Magnetospeed, that hangs on the muzzle end of the barrel to take its measurement, it might have an effect. Anytime you hang a weight on the end of the barrel (brake, flash-hider, suppressor, Magnetospeed, etc) it is likely to effect barrel harmonics. Testing is as simple as shooting a few groups without the Magnetospeed, mount it on the barrel, and shoot a few more groups with it and compare. Barrel harmonics being what they are, group size is as likely to get tighter as looser. Point of impact might also move depending on barrel length and contour.

However, on my heavy contour barrels, with a suppressor on the end already, I very rarely get a measurable shift in POI or group size. And even then we’re talking .1 mil.
Ymmv if you’re using a pencil profile but is it really that much of an inconvenience to work up your load and then chrono it?

I understand that hanging stuff off the end of the barrel affects group size, but why the need to try and measure both at the same time.

If you are trying to minimize group size, then velocity is largely irrelevant. Adjust the load to minimize size. I don't see any reason to try and do both (adjust velocity and group size) at the same time.

3. I Typically shoot several different target arrays that require traversing horizontally and vertically between berms and I can do that with magnetospeed attached. Don’t have to stop and point my chronograph towards a different target.

Labradar should be able to pick up bullets provided they are in a 5 degree wide and 7 degree vertical oval cone, that means they are within than 8 yards horizontal and 12 yards vertical of each other.

 

[mcb]

Mcb I really don’t think it works like that. Either your theory is incorrect (which is what I believe) or literally no bullets match the G profiles, which may also be true.

In either event, you aren’t going to calculate a usable long range bc with a magnetospeed unless as you said you went to the extreme trouble of loading and shooting bullets at every velocity down to transonic and then doing your own blended rate. And why would you do that when litz and hornady have measured it with Doppler and You can buy all of their bc for 1/20th the price of the labradar

I agree the data is already out there and it would be tedious to do it with a labradar but it could be done.

How it it different doing the change in velocity at a given velocity piece-wise with downloaded ammunition vs what the commercially loaders are doing with high power radar and getting the same measurement continuous at longer ranges? The drag on a bullet at a give velocity is going to be the same whether it's 10 yards from the muzzle or 1000 yards. The drag on a bullet is proportional to its velocity squared and a coefficient of drag that is a function of its shape and velocity. Range does not play a role in the measurement only the bullets shape and the velocity its going (and some atmospheric stuff).

And yes very few bullets fit the Gx models exact. A bullet that is an exact scale up or down shape of one of the Gx models use to create the drag model is a pretty good fit to the model. A lot of bullets that are similar enough in shape to the Gx model to also fit well enough for us to use out to long ranges. For that matter over the ranges most of us shoot we can shoe-horn a spitzer boat tail bullet into a G1 model (A spitzer boat tail looks nothing like the G1 model bullet) and kill deer out to 500 yard and never realize just how bad a fit that is, cause it works well enough over that range. But yes very few bullet fit the Gx model exactly.

Some of the more advanced shooting software is going away from the Gx models all together and and creating bullet specific drag models. As you point out Applied Ballistics offers custom drag model for a large selection of bullets. I believe Hornady's new ballistics app included bullet specific drag models for some of their bullets rather than use a G1 or G7 BC to scale one of the more generic Gx models to that bullet. Barnes also shares some of their radar data for some of their bullet and you can see how well or bad the G1 and G7 model capture the drag model of the bullet.

 

[lysanderxiii]

Mcb I really don’t think it works like that. Either your theory is incorrect (which is what I believe) or literally no bullets match the G profiles, which may also be true.

In either event, you aren’t going to calculate a usable long range bc with a magnetospeed unless as you said you went to the extreme trouble of loading and shooting bullets at every velocity down to transonic and then doing your own blended rate. And why would you do that when litz and hornady have measured it with Doppler and You can buy all of their bc for 1/20th the price of the labradar

The ballistic coefficient will vary with all velocities, unless the Drag Coefficient (Cd) is exactly parallel to the corresponding "G" Cd. In this example, the test bullet (black line) runs fairly parallel to the G7 bullet until you get to about Mach 1.6 (1,900 fps). So, for this particular bullet a single G7 BC would be accurate at all velocities greater than 1,900 fps.

In contrast the G1 BC will have a single BC with velocities in excess of 2,450 fps, but the curves do not match well under that speed, so many BCs are required if you want to try and fit it at these speeds.


In this example, Bullet "X" is a very good match for the G7.

 

[lysanderxiii]

For those interested, the two primary reference projectiles:


(Dimensions in calibers)

 

[taliv]

agree the data is already out there and it would be tedious to do it with a labradar but it could be done.

Yes you can do it with calibrated hanging steel plates too. But even if you got a free labradar it’s not good enough to justify the time it would take when you could get a better bc for $30.

I understand what you guys are saying about drag profiles. I’m just saying I believe many of the ballistic calcs just use it as a coefficient, without being specific to a particular shape of bullet. So I don’t think it matters as much as the 100 year old ballisticians intended.
For practical purposes the bc remains velocity dependent on all the popular bullets

 

[MEHavey]

RIDDLE ME THIS: (i,e., where's my blunder)
(Velocity/Distance data from LABRADAR test yesterday of a handload dupe of M2 Ball for Garand)

 

[mcb]

Yes you can do it with calibrated hanging steel plates too. But even if you got a free labradar it’s not good enough to justify the time it would take when you could get a better bc for $30.

I understand what you guys are saying about drag profiles. I’m just saying I believe many of the ballistic calcs just use it as a coefficient, without being specific to a particular shape of bullet. So I don’t think it matters as much as the 100 year old ballisticians intended.
For practical purposes the bc remains velocity dependent on all the popular bullets

Ah but some of us like to do it the hard way and the LabRadar brings a capability to the hobbyist ballistician that was formerly unavailable. And it's not just bullets the LabRadar works for, arrows, popcans etc. It's just one more tool in the cabinet of fun gadgets to play with.

The drag coefficient is the velocity depend and empirically tabulated value needed to model a bullet trajectory. If you dig into the code running a ballistic model BC is never use to calculate anything other than scale the drag table during initialization. After the drag table is scaled the BC is never touched by the integration scheme in the modeler. The BC was a way to scale that table of data up or down for a bullet that was similar to the original model. No one use a velocity depended BC that is redundant. Sierra and a few other will try to shoe horn a "new" drag table by listing several G1 BC's for a several range of velocities. The ballistics software simply assembles a new table using segments of the scaled Gx model and again the BC is never touched again. Multiple BC if need are a kludged fixed for bullet that lie to far from existing models. Unless you are entering multiple BC with velocity ranges into your ballistics software you are either using a static BC or in the case of Applied Ballistics or Hornady's app no BC at all.


The above graph is the coefficient of drag vs velocity for the 230 gr Sierra Matchking modeled three ways.

First using Sierra's listed three velocity ranges. G1 BC plotted as blue dots G1 BC = .8 above 1880 fps, G1 BC = .78 between 1600 and 1880 fps and G1 BC = .74 below 1600 fps. I did nothing to smooth those data points, a good ballistic solver would smooth that data set so the drag coefficient did not have a step changes at the boundaries between velocity ranges.

The orange line is simply the G1 drag table scaled to a BC of .78.

Then the gray line is the G7 drag model, using a G1 BC of .78 at 1740 fps I use the JBM Ballistics calculator to convert the .78 G1 BC to a G7 BC and plotted that. G7 scaled to .403 BC.

With many bullet the old models work fine, I use Strelok and the G7 ballistic model and a single BC with my 6mm Creedmoor and get hits out to 1000 yards without issue. The bullet I am using matches G7 reasonallby well over the velocity ranges from muzzle to 1000 yards and it works. Similarly with my 22LR I am using a single G1 BC and have good results out to 250 yards. I suspect if I was shooting some of the new VLD bullets and shooting far enough that bullet is transition through transonic velocities (as you can see in the graph above their is lots of changes in coefficient of drag in the transonic velocity range) then something more sophisticated would be needed. But my 6mm is always super- sonic and my 22 LR is always subsonic.

-totally rambling at this point.

 

[lysanderxiii]

RIDDLE ME THIS: (i,e., where's my blunder)
(Velocity/Distance data from LABRADAR test yesterday of a handload dupe of M2 Ball for Garand)
View attachment 981228

Sierra's BC listings are always based on the G1 model, it might be calculating based on G7, but it is listing the G1 BC figure.