Chronographs

[farmboy]

I bought a cheap chronograph yesterday wondering if I am doing something wrong or if I got what I paid for. Lol. I bought a crony Alfa. I shot five rounds of 22 long rifle ammo over it. Only three did I get a reading from . One was 1114 fps the next one was 1116 fps and the last one was 1618. I am think that was a miss read reading. I would guess that if it was that much faster  it should have sounded different etc or maybe not.  I had a cloudy day so I didn't setup the skyscreans. Maybe I needed them. Have I set it up with out enough care or do the just miss read once in a while or do you think that all those reading could be correct. It was Winchester  target ammo.

[j0e_bl0ggs (deceased)]

What make of chrono?

[gitano]

1) You probably need the sky screens. I believe that is the problem.
2) Those of us in the Great White North have a bigger problem with the angle of the sun and getting our chronographs to 'catch' our bullets. Cloudy day and low angle sun means missed bullets.
3) I agree that the third bullet should have sounded different and probably felt different too if it was truly that much faster.

That chrony should give you pretty good readings except for fast, small, bullets like those from a .17 Rem or the like.

I would mention to be careful not to be too close to the chrony with your muzzle. The gasses that drive the bullet are moving WAY faster than the bullet, and when the bullet pops out of the muzzle, the gasses scream past it. If the muzzle is too close to the chronograph, those HOT gasses will distort the air and create shadows which the chronograph WILL respond to. That is often the cause of spurious high velocity readings. I would be 10 feet away from the chronograph as a minimum.

Paul

The model is an "Alpha" j0e_bl0ggs. "Shooting Chrony Alpha".

Paul

[farmboy]

What make of chrono?

Sorry spelling miistakes.

[j0e_bl0ggs (deceased)]

What Paul said...

[farmboy]

Yes that is correct.

[farmboy]

By they way I misspelled mistakes for a laugh.

[gitano]

By they way I misspelled mistakes for a laugh.

Well then, you succeeded, because I laughed at the above!

Paul

[Jamie.270]

Not cast aspersions or anything, but this is a hunting forum.
If "mistakes" is to be misspelled, it should reflect a hunting theme.

As in "miss-steaks" or "missteaks."

:D

Otherwise known as "tag soup."

But hey, I'm only here for the fun of it!

[gitano]

I like miss-steaks... Take that any way 'you' care to. :D

Paul

[farmboy]

Not cast aspersions or anything, but this is a hunting forum.
If "mistakes" is to be misspelled, it should reflect a hunting theme.

As in "miss-steaks" or "missteaks."

:D

Otherwise known as "tag soup."

But hey, I'm only here for the fun of it!

The farmboy got out iq ed with that post! Lol that is truly funny!

[farmboy]

Did some sighting in and chronograph work. A custom 358 Norma loaded with225 grain bullet accubond read 2787. 270 wsm loaded with a factory federal 150 grain hollow point 3248 out of a Browning a bolt. And my Parker hale made 3187 with a bonded loc core 100 grain Remington bullet.

[gitano]

2787 f/s with a 225-grain bullet should have 'gotten your attention' I would think. That's almost 3900 ft-lbs at the muzzle.

And 3250 f/s from a .270 Win with 150-grain bullets ain't too shabby either!

Paul

[farmboy]

The 6 mm is the only one that showed any pressure signs and not to bad I will post a picture of the primers

[gitano]

That's great!

How'd they group?

Paul

[Jamie.270]

I dunno, maybe there's a miss-steak,  but if you got 3200+ out of a .270 win shooting factory 150gr bullet ammo, I'd be double checking things.  
I have a friend with a .270 Ackley and a 26" barrel and he can't come close to that with his best handloads.
That's the high end of .270 WSM territory

[farmboy]

Sorry a 270 wsm auto correct problem. I dislike auto correct a great deal.

[farmboy]

Both the short mag and the six mm three shots under a 25 cent piece at one hundred yards the 358 we just got on paper.

[farmboy]

[/IMG]

[Jamie.270]

Sorry a 270 wsm auto correct problem. I dislike auto correct a great deal.

Whew!  
(I kinda figured that was the case)

[davidlt89]

but if you got 3200+ out of a .270 win shooting factory 150gr bullet ammo, I'd be double checking things.

I can't get that out of my 7mm mag! Almost seems "high" even for a WSM.
I have the same chronograph, has always worked well. God Bless.

[farmboy]

Well I saw it the only things that could be wrong are maybe my friend got mixed up on the bullet weight or the chronograph is reading high. The 6 mm load measured about 125 fps above what hodgdon manuel said it should. Although 1114 fps out of a 22 lr seems about right.

[farmboy]

[/IMG]

[gitano]

If you got more than one reading in that 'zone' it's probably right. If that is only one measurement, then it warrants some more shots before you start calculating trajectories. The fact that the .22 RF numbers were 'right' suggests that your CF numbers are too.

I can't see those primers well enough to see anything to comment on. I don't worry about how primers look. The only thing I care about with primers is loose pockets. If I'm loosening primer pockets, that's "bad". I don't "worry" about pressure until I start having 'stiff' bolts. One measurement that I take if I am thinking my loads are a bit 'hot' is the head diameter. I measure the heads right in front of the web BEFORE shooting, then again after shooting. Any expansion over 0.010" is TOO MUCH. MOST of my loads usually expand the head about 0.003" and occasionally less. The chamber on the AR-10 8-08 is large, and my resizing dies are "small", so I'm getting about 0.017" expansion on those cases, but I know that the cause isn't excessive pressure. Still, those cases aren't going to last long.

Three shots covered by a quarter is great shootin'!

Paul

WOW! That's some FLAT country!

Paul

[davidlt89]

I'm sure its possible farmboy, I am pretty sure I could get a 150gr bullet going 3200fps in my 7mm mag, not sure how precise it would be though. I have a 160gr accubond going 2888 and that is probably a little slow, QL predicts my load to be going over 100fps faster, but its shooting tacks! God Bless.

[farmboy]

In the end group sizes is more important than the last few fps. I am at around 3300 feet above sea level but that just makes bullets fly flatter because of less air resistance. I don't think that has a thing to do with mussel velocity.
The primers look flattened but not like they are welded to the case. Bolt lift was normal. To me it is about maximum that I want to load. They are new cases I will measure them latter tonight.

On the hunting after the sighting in. I think I need to learn how to shoot a shotgun again I could not hit a partridge for love not money. Had a chance at some geese. Thaat went bad they were about sixty feet above me I pulled the trigger and a few pellets rolled out of the barrel. No powder I guess the was was still in the shell. I took the barrel off to make sure it was clear took to long and the next batch flew by. Saw a whitetail spike buck left him alone. Did not see a whitetail doe. I have a doe tag this year as well. Lots of decent mule deer bucks but the area is by draw only. So the hunting can only get better .

One other fun things we were shooting at a big round bake with the target pinned to it. When my friend shot his 358 at it a cloud of dust came out the back side of the bale. Not sure if they went threw the bale or just shook it hard enough to make the dust storm. The others did not do that. It it went threw that is impressive as that would be five feet of penetration.

[gitano]

Bummer about all the hunting snafus, but the sighting in sounds great.

That .358 Norma Mag is a great cartridge!

Paul

[farmboy]

That is my friends gun it is a stainless 700 Remington with a break and a laminate stock. Looks quite nice.

[sakorick]

Those speeds with a 150 gr bullet are way too fast w/7MM Mag. Current data shows IMR7828 as top speed for a 7MM Rem Mag w/ 150 gr bullet at 3100'/sec. and 59,200 PSI. QL shows I'm at the top end of pressure curve at 3070'sec with my new 270 with a 130 grain bullet!

I am not familiar with that chrony but you should be about 10 feet back and firing the bullet level with the earth through the middle of the sunshades. I would immediately cease firing those loads until I resized and deprimed several cases and checked the primer pockets. Clean, then reprime. If you feel little resistance seating the primer you have a problem. If you are using published data and the primers seat normally, you have a bad chrony IMHO.

[gitano]

It's difficult - but not impossible - for a chronograph to be "bad". Given the way they work, and I have actually built a few, they are either "on" or "dead". It's possible that the screens are too close together. Small errors in the distance between sensors make big errors in measures of time. "Doing the math" I get the following:

Sensor spacing = 1 foot
Time to travel 1 foot if going 3000 f/s = 1/3000 (0.0003 sec)
Therefore, in order to measure with a precision of 1 f/s, the device must have a sample rate of at least 3000 times per second. If the device is expected to 'catch' bullets going 5000 f/s then it needs a minimum sample rate of 0.0002 Hz or 5000 times per second. Of course that's just barely fast enough, AND (and this is the most important "and"), digital clocks that are a 'zillion' times faster are VERY cheap. I would be stunned to find out that the clock on a modern shooting chronograph had a sampling rate of less than 1 megaherz or a million times a second or one sample every 0.000001 seconds.

Therefore the question is; how far off would the sensors have to be to produce a muzzle velocity error of 150 f/s if the bullet was actually going 3000 f/s?

If the physical separation of the sensors was actually 1.001 feet and the processor (brain) "thought" the sensors were 1.000 feet apart, the error could be calculated using the kinematic equations, the most common of which is D = V x T, or in words, distance equals velocity times time. Since we know the distance (at least we think we do), and what we want is the velocity, we algebraicly manipulate the equation to get the velocity alone on the left side of the equation which results in V = D/T or velocity equals distance divided by time.

Therefore, if our clock starts 'ticking' when the shadow of the bullet passes the first sensor, and stops when the bullet's shadow passes over the second sensor, and our clock ticks 1,000,000 times a second, and we know (for this arithmetic exercise) that the velocity is 3000 f/s, then the number of samples per foot should be 1000000 samples per second divided by 3000 feet per second: 333.

So now we have:
D = 1 foot
T = 0.0003333 seconds (333.33 samples at 1,000,000 samples per second)
V = 3000 feet per second

Filling in the variables of the kinematic equation we get: 3003 ft/sec = 1 sec / 0.000333 samples/sec.

What that means is that with a sampling rate (clock speed) of 1,000,000 samples/sec, a distance between sensors of 1 foot, and a velocity of 3000 f/s, the distance per sample is 3000/1000000 or 0.003 feet per sample.

That's an important number because for every 0.003 feet, (0.036"), that the chronograph sensor's TRUE separation is different from 1 foot, the calculation of the velocity will be in error by 1 f/s. So in order to be off by 150 f/s on the velocity, the sensors would have to be 150 x .003 = 0.45 FEET, or about 5.4 inches. That's A LOT in a chronograph that is only 1' between sensors. I kinda think the manufacturers would have noticed that even with very poor quality control. While it is also true that the clock could be "off", that would be extraordinary because it would have had to make it past TWO quality control inspections: First the clock-maker's QC then the chronograph maker's QC. That just ain't gonna happen. In my experience, ICs don't "fade" into failure. With few exceptions, either they are "perfect", or they are "dead".

Now it's true that the slower the clock speed (sampling rate), the bigger the error in MV with a smaller error in distance. However, while I don't know the actual clock speed of the processor on the chronograph, let me make an educated guess based on 'stuff' we do know. The clock speed of your Pentium D processor in your computer is - depending on the manufacturer - between 2.66 and 3.73 GIGAHZ. That's 2.66 THOUSAND MILLION and 3.73 THOUSAND MILLION times a second! Remember what I said about "fast" clocks being cheap. A 1 MEGA Hz clock - the sampling rate that I used above - is more than 2 THOUSAND times SLOWER than the clock on the processor in your computer.

One of the simplest IC "timers" ever made is called the "555". It was invented in 1971 if I remember correctly and was widely available to The Common Man by the mid '70s. It is a trivially simple device to set up, and ridiculously inexpensive. Even using this crude of a device, one can get a sampling rate of 500,000 times per second. In our above example, that would make the distance error of our chrony reduce to 2.7 inches instead of 5.4. STILL, I don't think the error in the device's sensor spacing is even close to that.

All of the above said, there are A LOT of 'things' that can 'go wrong' for any given INDIVIDUAL chronograph reading. It is VERY UNWISE to use a single measure to 'determine' the MV of a specific load in a specific rifle. The best OPTICAL chronograph makers use three sensors and calculate 3 velocities; one between sensor #1 and sensor #2; one between sensor #2 and sensor #3; and one between sensor #1 and sensor #3. They throw out the value that differs the most from the other two (if that difference is greater than a predetermined value).

Bullets move VERY fast. Clocks on chronographs have to go VERY fast to 'catch' bullets over a distance of only 1 foot. There is plenty of room for variability in the speed of the bullet and the ability of the chronograph to "see" the  shadow of that bullet. Sample size is CRITICALLY important. SMALL sample sizes like ONE are unwise in the extreme.

I'd shoot some more over the chronograph. If the numbers remain essentially the same AND there are no pressure signs, I'd not look back. You could always call the manufacturer and ask what they thought. If they have a large 'failure' rate, they might tell you to "send it in for calibration". They will probably tell you to buy/borrow another one and set it up right behind yours and "calibrate" that way. OR... you could part with $175 and get a chronograph that doesn't depend on "shadows" to 'catch' the bullet.

Paul

[farmboy]

I think after that explanation I should get a shell form him and pull the bullet and weigh it. The speed reading looks alright at a 130 or at least in the correct area and yes it seems way to fast for a 150. I looked up federal ammo online and I don't see a load with hollow point bullets loaded.