Chronograph Accuracy & Bullet Velocity

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LloydM

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How accurate is a chronograph?
Are some models more accurate than others?

I picked up a Beta Chrony and have shot some 5 round groups with very carefully weighed charges and had velocities all within about 4-6fps so I think they must be fairly accurate.

Another related question... are some powders known for producing more or less consistent velocities than others? The groups spoken of above were with IMR 4350. I just tried some US869 for the first time and my velocities have spreads of 16, 62, 80, and 53fps (most accurate group by far was the one with a spread of 53fps at 111.5 grains of US869). I use a lab scale that is accurate to 0.02 grains so I am confident my charge weight is consistent. I have also noticed that the pellets of powder in the US869 are not nearly as consistent in size, shape, or weight as the IMR powders.
 
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The Chrony people test their equipment on my range. I have a 200 yard range at he back of the house, with a heated/air-conditioned firing point building. The quick answer is "very accurate".

"Accuracy" means two entirely different things.

The first element of accuracy relates to the clock speed of the unit. If clock speed is very high (or if the two screens are far apart) succesive shots of identical speed (in theory) would have almost identical readings.

The second element is how close is the reading to the actual speed? This relates to how exactly is the distance between screens the same as the distance assumed in the software. Since the screens look 'up", accuracy also requires that both screens are exactly parallel to each other..

Chrony use a very high clock speed. This tends to mitigate all the other error sources. The reading you see is within 25 fps of the actual velocity, and the shot to shot accuracy is almost bang on. yes, it can see a 5 fps variation in velocity.

One source of error is the shooter. If you fire your shots at various angles though the screens, you can induce a 15 fps error. It is best to shoot off a bench at some kind of target in the distance. This will put the bullet through the screens at the same place and angle.

The Chrony test rig is a plank with a number of Chronys mounted on it. A shot fired across will read on all the units, including a very expensive big name mounted in the middle with double the usual spacing.

From this testing, one can see that a given shot is about 12fps slower than the last shot, on all the units. That would be plus or minus 1 fps accuracy.

But each unit is a certain fixed amount different than the "master" unit. Typically within 15 fps of it. That kind of error means nothing.
 
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Wow - I nominate #2 for post of the year. That is great info.

Not keeping the chronograph exactly horizontal (sensors not looking directly up) would throw the speed displayed by the chrony off from the true velocity but the speeds displayed should remain fairly consistent - meaning that if you were using near perfect ammo over a slanted chrony, all your shots would display within a few fps of each other but all would be slower than the actual bullets speed. Sound right?

So what about powders? Do some have a harder time burning more consistently than others? I don't know why I had such large variations in velocity yesterday. I am very suspicious that the US869 (either in general, or at least my batch) does not burn as consistently as the IMR powders I am used to. Like I said above there is a lot of variation in pellet size, and some are very misshapen. This is my first experience with US869 so I don't know if this is typical or if I got a bad batch. This is starting to be more fitting for the 'reloading' section, but I suspect bench shooters would have more insight.
 
I was using some W231 with a new bullet recently. I wanted to create some accurate test loads. I used a powder trickle & a Dillion digital scale. I scaled every powder load in these test loads and after shooting over my Beta Master. I had fps spreads as high as 80-90 fps. I was hoping to create some sort of accurate powder charges. Didn't seem to matter. Mayby this batch of W231 was of poor quality.
 
From what I have been told by a member here is that there seems to be an average spread of velocity in the 40 to 60 ft per second. For the average reloader.

If you are really anal about loading, the spreads can be in the 10 to 25 ft per second spread.

I can see where different powders will vary.
 
"Not keeping the chronograph exactly horizontal (sensors not looking directly up) would throw the speed displayed by the chrony off from the true velocity but the speeds displayed should remain fairly consistent - meaning that if you were using near perfect ammo over a slanted chrony, all your shots would display within a few fps of each other but all would be slower than the actual bullets speed. Sound right?"

Funny I was going to ask what causes chrony inaccuracys last night.
I looked up their sight last night. I have been having some high readings on mine occasionally.
Sounds like the light above the screens can affect how well it works. To much or not enough will cause bad readings.
They state you can actually point the shades at the sun if its is bright out by tilting the chrony so they are shading the screens.
Stephen
 
best light is on a light overcast or cloudy day.

I am not familiar with US869. Sounds like a ball powder. Are you using a Magnum primer? That might help.

Powders generally purn more consistently as pressure rises. A max load might be best.

However, becasue of barrel harmonics, the rifle also has a favoured velocity, which might mean a lower velocity that has more variation. Low Velocity SD does not mean best accuracy.

My 308 match ammo has a SD under 10.

My SD standards are under 50 for hunting ammo, under 25 for match ammo (rifle).

5 shots groups are not statisticlly valid, and can lead you astray.
 
Us 869

111.5 gr of 869.... Sounds like a .338 Lapua AI...... 300 grainers.....

I use 869, but i dont find that much of an extreme spread, i just find it is temp sensitive..... My load is like yours, but at say -10 or so i have to up my charge to maintain accuracy/velocity........
 
Bradtothebone is right - the load is for a .338 Lapua AI with 111.5gr US869 for 300gr SMK's
Brad, What does your charge weight get up to in the cold?


I am pretty sure that some of the other variables mentioned which can affect velocity are not my problem...
-Neck tension was precisely set with a Redding Type-S Bushing Neck Sizing Die
-Bullets were seated with a Redding Competition Seating Die
-Case volume should be very constant since I am using expensive Lapua Brass
-Primers are Federal Large Magnum Rifle Match
-Bore is almost brand new
and as mentioned before the scale I use is an Acculab which displays accuracy to 0.02 grains
 
Max Owner said:
From what I have been told by a member here is that there seems to be an average spread of velocity in the 40 to 60 ft per second. For the average reloader.

If you are really anal about loading, the spreads can be in the 10 to 25 ft per second spread.
Click to expand...


A spread of 40-60fps for the average reloader sounds reasonable to me. I have a 7mm Rem Mag, and it is the only other rifle I have done chrony work with - using IMR 4350 most of my shots only had a spread of about 8fps
 
In handguns, I have found that quick burning powders tend to have less deviation - likely I suppose because the peak pressures are not happening as the bullet leaves the barrel, but occur sooner. This is likely true in rifles also - a consistent speed aiding accuracy. I do remember one of the old masters saying that if it is a choice between consistent speed and accuracy, take the accurate load - even if it is not as consistent in fps.

Some powders definitely have more consistent speeds - with cast bullets, I expect Clays to give 12 or less SD in an eight shot string while other powders can be considerably higher in .45acp. With a quality jacketed, under 6 fps SD is common.

An amazing thing was shooting some .40 cals with a SD of just over 2fps in a 10 shot string. That seemed hardly possible, but it was repeatable.

I use the Chrony with the separate display, and I notice that if shooting .454 over it, the cord and display need to be protected from the blast or readouts will go crazy.

It is always fun to try factory ammo - the end of the box tells one story... which just occasionally can be the same one your chrony tells.

I have wondered about dismantling my chrony and building a box twice as long - and double the numbers - not sure why, but just considered it.

In the winter, a better battery pack helps - I use a rechargable toy 12 volt pack, and it works great in the sub-zero temps.

Uh - what was the topic again?
 
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