Calculating B.C. with a box stock chrony?

[wogpotter]

I saw this posted elswhere & I'm missing something. Pleas ehelp me unnerstann.
I have a bullet of unknown B.C.
Its a flat base FMJ.
I have a chronograph.
I have an app that lets me calculate B.C. but it needs a velocity drop from the muzzle to the 100yd mark to establish a drag coefficient.
How do I do that with a chronograph, specifically one that is designed to operate at 5~15' from the muzzle?

[g.willikers]

Eh?
Set your sights or hold for close, shoot at the target at 100 yds, measure the drop.
Yes?
It doesn't have to be the same round that passes over the chrony, if that's the problem.

[mehavey]

You either need two chronies -- one at muzzle and one at 100 yards -- or you need to fire several (let's say 10) bullets w/ the chrony at the muzzle and then several more with the chrony at 100.

Take the average velocity difference and enter that into the program (along with the velocity error estimate) to get the calculated BC for that velocity range. (BC's will change for different velocity ranges)


post: If you put the muzzle chrony at 15 feet, then put the far chrony at 103 yards for that 100 yard delta-V

[g.willikers]

Isn't that kind of risky for the second chrony, the one at 100 yds?
Considering the small change in velocity at 100 yds, is the second one necessary?
Wouldn't bullet drop do as well?
Or is it going to be too small to be useful at that distance.
Assuming this is about a rifle round.

[Sport45]

How do I do that with a chronograph, specifically one that is designed to operate at 5~15' from the muzzle?

Chronographs don't care how far they are from the muzzle. If you can reasonably shoot a 2" group at 100 yards your chronograph should be safe.

As mehavey mentioned, just use a proven load to establish an average velocity at the muzzle (15') and an average velocity at 100 yards.

To reduce risk to the expensive parts I'd set my aim point as high as I could above the electronics at 100 yards.

Or I'd just try to look up the BC of a bullet of similar weight and shape and call it good. I mean really, you're talking about fine tuning the ballistics of a FMJ bullet?

[g.willikers]

Who's going to loan wogpotter their chrony for the 100 yd shot?

[Jim Watson]

I've done it.
Chronograph the bullet and load of interest at 4 yards, while zeroing at 100 yards; then chronograph out of the same box of ammo again at 97 yards.
(Had to come out in front of the 100 yd target frames to get on flat ground.)
I then calculated BC from velocity loss. Not perfect but it let me calculate come-ups well enough to get on the paper at longer ranges.
Two chronographs would have been more precise, but I didn't have them.

[Bart B.]

Better yet, borrow another chrony for use at 15 feet then use yours at 100 yards. Shoot bullets through each at the same time so each bullet's velocity loss is more accurately measured.

[Sport45]

Isn't that kind of risky for the second chrony, the one at 100 yds?

Nah, the average shooter here puts everything into a 2" group at 25 yards offhand with a revolver shooting DA. Keeping them between the uprights of a chrony with a rifle at 100 yards should be a piece of cake.

[HiBC]

I was gathering data to have a Kenton knob made for a Leupold scope,I put my CED chronograph at 10 yds,averaged velocities,then lasered 310 yds and fired a series.Not a problem.

I also did a 300 yd zero and then counted clicks to get zero on a 700 yd target.

Gave them my numbers

The knob works good.

[Mike / Tx]

As mentioned there are several methods to come to the same end result.

What is the longest range you have to shoot? If you have to 300yds simply zero at 100, then using the same aiming point shoot a group at 200 and then 300, and measure the actual drops. From that you can usually reverse engineer the BC if you know the velocity.

If you only have a 100yds you can zero at a closer distance and measure from there, it will however be more of a pain to get as close.

Shooting through the chrony is best, as mentioned above. Shoot a batch at 10yds then another at 100yds and average.

As for simply getting pretty close, you can look online on most commercial manufacturers sites and get real close using similar bullets BC. A point or ten difference, here or there, isn't going to make a lot of difference unless your shooting to 1K anyway. There will be more variables in your rifle, load, and shooting ability, that what little your off will be lost in the noise.

[wogpotter]

Who's going to loan wogpotter their chrony for the 100 yd shot?

Yeah, who has one with 100yd cables so I don't have to walk a mile by the end of the day?

Great Idea, borrowing a LR chrony! Why didn't I think of that?

The issue is the app I have uses velocity change to calculate B.C. so entering bullet drop won't be usefull for it.

[Sport45]

Got a spotting scope? Use it to read the display.

[SSA]

You'll need a way to precisely measure 100 yards.
What's the bullet?

[wogpotter]

Measuring 100yds isn't an issue, a laser r/f does nicely.

A Mk VII ball round of .303 British. flat base FMJ with an over-long body due to it's alloy nose filler. 174 Gr with a .311" diameter & a length of 1.2951"

[jmr40]

The chrony at close range is in more danger than the one at 100 yards. It is easy to forget that he bore is close to 1.5-2" lower than the line of sight up close.

[Mike / Tx]

The issue is the app I have uses velocity change to calculate B.C. so entering bullet drop won't be usefull for it.

Ooops, sorry about that, what was I thinking, should have considered the "APP". What in the world did folks do before the internet or APP's?

You might try a bit older approach and do a search for the formulas or data that you need, or you could simply try out one of the tables listed here.

BC Calcs

There is plenty more great info at this link as well,

JBM Ballistics

[wogpotter]

I don't get tables when I open that link, just screen shots.

[math teacher]

If you are willing to take all the fun out of it, go to nikonsportoptics.com on the Spot On program. You will probably find your bullet listed there with the ballistic coefficient listed.

[Unclenick]

Picking out bullet drop accurately at 100 yards is tough, as the drop resolution is poor and, for some military bullets, is smaller than the size of the groups they shoot at that range. To get the drop to be a more significant number, you need to go out further, as HiBC did, but then you'll be getting the BC for a lower velocity than you would earlier. That's only OK if you are using a drag model whose standard projectile shape matches you bullet's shape well.

Follow Mike / Tx's recommendation to use the JBM free online site. It has a calculator that doesn't care what range you actually have, as long as you know what it is. I keep one of those 100 ft tape measures on a wind-up spool for this kind of thing. Obviously, if you pick some distance that's too short, you will lack adequate resolution. But assuming you have worked around those issues, you just enter the starting and stopping velocities, the drag function you want the BC for, and the distance over which the velocity difference was measured.

Figure that having one chronograph at 3 yards (15 feet) and one at 28 yards will give you about 50 fps of velocity loss with the bullet you describe. That's enough difference if you manage things well. Make the ammo to minimize muzzle velocity variation. Do any kind of case prep and uniforming you have the tools to do, including neck annealing or other such steps you have the ability to perform. Seat the primers fairly hard and use the most consistent ones you can get. I currently use TulAmmo primers for this. Scuff the insides of the case necks up by turning a bronze bore brush (or special case neck brush if you have one) inside them a couple or three turns, which seems to make bullet pull more consistent. I get past the argument about weighing vs. volumetric charging by zeroing a scale with my primed case on it, then dispensing by volume and checking to see if the weight has hit the average value. If not, I dump the charge and repeat until it does.

At the range, set the two chronographs up. Make the 25 yard endpoints at the centers between screens if the two units don't have the same screen spacing. Fire fouling shots with the same load you will use so you don't have different bore conditions for the two halves of the the shooting. Then fire eight to ten rounds over both chronographs, recording both results for each round. Then go out and reverse which unit is closer to the gun and which is further out. Fire a second 8 to ten. This is to neutralize any tendency of one machine to err higher or lower than the other. Average all the starting velocity readings and all the ending velocity readings and use these averages with the JBM calculator.

I believe you'll discover with that bullet what I did with the M2 Ball bullet, and that is that the G6 drag function matches it best. G6 is actually for a flat base bullet with a 6 caliber secant ogive, which the M2 has a 7 caliber tangent ogive, but the match I got was good. It was slightly better than the G8 drag function which is for a flat base 10 caliber tangent ogive projectile. When you have a perfect match, the BC does not change with velocity, so that is the reason you look for one. If the match is poor, it can change significantly with velocity. By picking one that's a good match to the bullet shape, you can accurately project performance over the whole expected velocity range with it, but you do need to use a ballistics program that works with your selected drag function. The ones at the JBM site will.

On the assumption your bullet nose shape is the same as that of the M2 bullet, I expect your result to be a G6 BC of about 0.240. The nose probably isn't an exact match, but figure something on the order of 0.23-0.25 anyway. At 2500 fps, that works out to a G1 BC of .423. At 2000 fps it becomes a G1 BC of .393. At 1500 fps it drops to a G1 BC of .366. I'm not saying these are your numbers, but it gives you a little bit of an idea what to look for.

[Mike / Tx]

I don't get tables when I open that link, just screen shots.

The first link was simply explanations of the calculators which are listed in the program and also in the second link I provided, you simply have to study what is written there.

That said, Unclenick has about nailed this one shut. With using the JBM calculator(s), a set of calipers to get reasonably close bullet dimensions, and a chronograph to get your velocity, you should be able to narrow things down to a VERY usable number pretty quickly.