Chronograph accuracy

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Colorado Redneck

New member
In working up loads for a 222 Remington the third trip to the range higher velocities started showing up. The Caldwell chrony was set up as near to level and aligned straight between muzzle and the target as this old geezer could make it. One of the loads is plenty accurate for prairie dogs, but it was concerning that my readings were about 200 fps fast. To my way of thinking, accuracy, consistency, and correct setting on the elevation are the most important factors. So, after making a few more rounds it was back to the range. I knew exactly the point of impact is a half inch high at 100 yards. Shot 3 rounds at 300 with the scope set at zero....which is a half inch high as stated. The group was 8.30 inches low. Pulled up the JBM Tracectory calculator, and fiddled with the velocity number till the calculator showed 8.3 inches of drop at 300. As suspected, instead of 3575 fps, it figured out at 3360 fps. Big difference! Adjusted the elevation on the scope, shot three rounds, and was pretty close at 300. Went up 2 more clicks and dead on.

I'm bumfuzzled as to the problem with the chrony. I have had this unit a couple of years, and it never gave any reason not to trust it. A couple months ago, it showed very accurately the velocity of a .204 Ruger load I have used for 3 years. So, it appears to me, a chronograph might not be the end all do all for scope settings for going down range a ways.
 
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Unclenick

Staff
Details. What load? Height of scope above bore axis? How far from the muzzle are you setting up the chrony? A lot of people find moving it back to 15 feet or even 18 feet will help get rid of false velocities. Is the battery fresh? If you are using it in cold weather, keep in mind a lot of consumer electronics are only rated down the water freezing point. Also, the Energizer lithium batteries function much better in cold than alkaline or carbon-zinc batteries do.
 
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MarkCO

New member
Agree with Unclenick. Lithium batteries are a benefit in an optical chrono. Also, especially with higher pressure loads, sometimes the burned, or burning gases get the the first sensor before the bullet. Using the Magneto Speed, the Caldwell and a Labradar all at the same time, they all were in closer agreement (making the calculations for distance form the muzzle) when the Caldwell was further away than the cord would reach when shooting some loads. Not saying it is the chrono, but it could be. The BC of the bullet might not be what the catalog says. I have to tweak my BC on most of my bullets being a mile high. :)
 
C

Colorado Redneck

New member
Hi Uncle Nick
Load is RL 7, 22.0 grs, CCI 400, 40 gr. Sierra Blitz King.
It was about 50 degrees last time it was used. I set the chrony up 6 paces from the bench, which is probably 15 ft. New battery afew weeks ago. Height of scope above the bore, center to center is 1.6 inches. Unit has been in the truck in the garage, so it never gets too cold.

I was just reviewing the data from a few weeks ago, and the .204 Ruger velocities were around 3550 for 4 rounds. After shooting other guns, returned to that load and the velocities were about 100 fps less....3450. Evidently that disparity didn't register at the time. 3450 has always been what that load produced. It hadn't been touched or moved during the entire shoot. Looks like my chronograph could be a politician
 
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C

Colorado Redneck

New member
MarkCo
I'm thinking of getting a magneto speed. Some of my rifles have a Witt Clamp On brake. Do ya think the lower priced Magnetospeed Sporter unit would accommodate that large of a brake?
 
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totaldla

New member
I always bring a 22lr along for a sanity check.
I've used an optical chronograph for a long time and found various setup mistakes that can cause errors. And I've also found powder/primer/bullet combinations that don't give results desired. I wouldn't ditch your Chrono for another unless you're looking for an excuse to buy a magnetospeed.
 
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TX Nimrod

New member
I would be more inclined to suspect the bullet or the ballistic program. My Labradar has been an eye opener regarding bullet BCs. Tracked over 100 yards, some bullets come very close to the advertised BC while others do not. Some of the difference is due to the changing BC - most bullet makers only list one BC value when we know that the BC changes with velocity. So, we are stuck with the velocity the bullet maker uses and the inaccuracies that can cause.

Next is the ballistics program. Garbage in garbage out, again how many shots at 300 yards were fired to determine the true group center? Three isn’t likely to be enough, so the true drop may be off a bit. Too, ballistic programs are mathematical models, not reality. It wouldn’t surprise me to find some are off by several inches at 300 yards, particularly using an inaccurate BC.




.
 
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MarkCO

New member
I have a V3 MS, so I don't know for sure Colorado Redneck. I'd guess no. I am sure if you email them a photo of your muzzle, with a ruler, they will tell you.
 
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Unclenick

Staff
Colorado Redneck,

Something is wrong with your setup of the ballistics program. Using Jeff Cooper's rule thumb that about an inch high at 100 yards gives most high-power rifles about a 200-yard zero, I knew your ½-inch high at 100 number couldn't be right. I ran this in QuickTARGET Unlimited, which is a three-degrees-of-freedom point mass solver. Using the Sierra BC's with velocity breaks as shown below and your 1.6-inch scope height, it says a bullet 0.5-inch high at 100 yards will be zeroed at between 158 and 164 yards and impact between 8.7 and 8.4 inches low at 300 yards for the two ends of that zero range, respectively. This matches your measurements quite well, suggesting your chronograph is right. Dropping 200 fps I get 10.4 inches low at 300. The velocities I am getting in GRT are lower than yours by about 100 fps, but this is for a 24-inch barrel and I forgot to ask your barrel length. Also, even if it is the same length, if the bore is a little tight you can get added velocity from it, too.

To correct the zero to 300 yards for a bullet that is currently impacting ½-inch high at 100 yards requires an additional 2.68 moa of elevation be dialed in, according to my software. In other words, the bullet should be 3.4 inches high at 100 yards to be zeroed at 300 yards. At 300, it also suggests 0.06 moa of left windage to compensate for spin drift (assuming your triple deuce has the usual 14-inch twist and scope that can make adjustments that small; mine can't and I wouldn't want it to, given all the click counting that would entail).

Code: 
G1 B.C.s for the Sierra 0.224" 40-grain BK #1440
From Sierra:

  ↑        
3000 fps + 0.196 
  ↑
2750 fps   0.182 
  ↑
2250 fps   0.177 
  ↑
 0.0 fps   0.171
 
C

Colorado Redneck

New member
Thanks for the post, Nick. I zero a varmint rifle so the trajectory beyond about 40 yards is no more than about 1/2" above line of sight, and then start adjusting the elevation when the drop is more than 1/2" or so. This method saves me frustration. For me, trying to perform decapitation on prairie rats that only show their heads is best done from a bench. Deer rifles are zeroed at 200 yds.

A long range precision shooter told me that obseved bullet drop is the best indicator of velocity, and the most reliable means of determining scope settings. If I wouldn't have tested the round at 300 yards and assumed the chrony was accurate, the outcome would have been less than happy. I like trigger time, but ain't no fun if I'm just slinging lead.
 
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hounddawg

New member
Jim Watson nailed it in post #3

Or advertised vs real BC.
Click to expand...
advertised BC's should be prefaced with "once upon a time"


altitude, humidity, distance from muzzle are some other factors that come into play
 
U

Unclenick

Staff
Not for BC. BC stays the same in all conditions. When you change those other conditions you enter them in the ballistics calculator with that same BC, and the results are right.

All BC does is compare your bullet's behavior to a reference projectile's behavior in the same conditions, whatever those conditions are. If they are conditions in which air is less dense, both the reference projectile and yours will experience less drag force in proportional amounts, and vice-versa, so their flight times and rates of loss of velocity will remain proportional.


Colorado Redneck,

I had to reread your post to see I must have misread that you were thinking ½" high at 100 would zero at 300. But otherwise, the high velocity readings seem to be confirmed. Were you, perhaps, using a single BC number instead of the velocity-adjusted ones? If so, that might account for the velocity disagreement.
 
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MarkCO

New member
BC stays the same in all conditions.
Click to expand...
No, it does not.

In classical fluid dynamics, there are some references to BC as a "constant", but that term is really not used often. In fact "Ballistic Coefficient" is not in the commonly used engineering fluid dynamics, heck it is not even in the the specific texts on the subject. Also, that is not the BC used for objects in flight, both propelled and launched. The BC used in exterior ballistics is the inverse of the form factor, which is not a constant and is dependent on environmental and speed.
 
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Wildernesshunter

New member
This post begs the question.....

What is the best, chrono for working-class shooters. Not some $2k option.... Something reasonable.

My Chrono that is 20 years old needs replacing....


.......
 
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totaldla

New member
Wildernesshunter said:
This post begs the question.....

What is the best, chrono for working-class shooters. Not some $2k option.... Something reasonable.

My Chrono that is 20 years old needs replacing....


.......
Click to expand...
I'm interested in replacing my Crony as it is on it's last legs. I'll start a new thread.
 
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mehavey

New member
Oeher 35(P)m or LabRadar,.
Both lifetime instruments.
One bombproof old Tech.
One latest cat's meow.
Both ~$600 new
 
M

MarkCO

New member
Wildernesshunter said:
This post begs the question.....

What is the best, chrono for working-class shooters. Not some $2k option.... Something reasonable.

My Chrono that is 20 years old needs replacing....


.......
Click to expand...
I have owned a dozen Chronographs. I own two today.

A Caldwell, which is used rarely and only with handguns and lever actions if I just need a check to make sure all is well. If you are a handgun shooter and hunter, this is probably the best bet.

The MagnetoSpeed (Sporter and V3) are more $, but they have a lot of benefits to them for that coin. If you need very accurate data and or shoot past say 500 yards with rifles, then it is worth it. Most of the folks I shoot with have a MS or a Labradar. I tried a LR and sold it off for what I paid for it. Happy as a clam with the V3 for about 85% of my Chrono needs. 10% it is not perfect, but I make do. Only a few pistols do I NEED the Caldwell, but if I am only shooting pistols and lever actions, I take it over the V3.
 
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Unclenick

Staff
Unclenick said:
BC stays the same in all conditions.
Click to expand...

MarkCO said:
No, it does not.
Click to expand...

Yes, it does, in ballistics.

Ballistics handles ballistic coefficients differently and more simply from how they are computed and used in aerodynamics. Ballistics has adopted a number of simplifications and shortcuts that don't match with aerodynamics or physics definitions. This came about in the 1ate 19th and early 20th century because artillery officers needed to be able to make trajectory calculations in the field quickly without modern computers. For this reason, ballistic coefficients in ballistics are merely the ratio of a projectile's ability to maintain velocity against drag as compared to how well a reference projectile does it. Read the Wikipedia article on ballistic coefficient to see the difference in the aerodynamic and ballistic methods of determining what a ballistic coefficient is.
 
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