Chronograph integrity Questions

[Dave S]

1. Distance from the muzzle to Start sensor?
2. Which is faster from the Muzzle....gas or the projectile?
3. At what trigger point does  the projectile velocity begin to deteriorate?

Reading through the various replies on the 12 fpe Co2 post. I see many discrepancies in FPE. and mass V. clocking measurements. It would be nice to have a universal  "Standard" when clocking.  Thought I'd throw out a few of these questions. dave

[Ribbonstone]

Might have an issue with a cranked up snow-ejector and 1 foot from the first sensor.

Spend most of my shooting life using firearms....you lean not to get too close to a chronograph,so normally set up airguns about a yard away (which means your readings are slower than MV, but not enought to get me upset).

Wonder about the little clamp-to-the-muzzle type chronographs.....don't have one,so have no idea if they'd clock a co2 gas cloud (like shooting a blank) or not.  I can't get  my old Prochono to noice a co2 blank at 2 feet, but none of mine are spitting clouds of snow.

[KnifeMaker]

3. At what trigger point does  the projectile velocity begin to deteriorate

The instant it leaves the barrel.

[ballisticboy]

1. Distance from the muzzle to Start sensor?
2. Which is faster from the Muzzle....gas or the projectile?
3. At what trigger point does  the projectile velocity begin to deteriorate?

Reading through the various replies on the 12 fpe Co2 post. I see many discrepancies in FPE. and mass V. clocking measurements. It would be nice to have a universal  "Standard" when clocking.  Thought I'd throw out a few of these questions. dave

Taking each question one at a time.

1. If the sensor is about a foot away from the muzzle you should be ok. However, it will depend to some extent on the gas pressure behind the projectile when it leaves the barrel and thus the gun design.

2. The gas will always leave the gun faster than the projectile unless you have a very long barrel and very low gas pressure. The gas velocity will slow down very quickly and the projectile will soon leave the gas cloud. However, while it is being overtaken by the gas the projectile is effectively flying backwards. This is usually called the intermediate ballistics area and can be vital for group size.

3. The projectile will continue to accelerate while the gas from the gun is overtaking it. I have never looked at the length of the effect on guns over 12FPE but it should only occur over a few inches and makes little difference to the projectile speed. A simulation of a 12FPE springer suggested the effect may last for about 3 inches from the muzzle but there could easily be large errors in that figure as the simulation was simple. The pellet speed only increased by about .2 ft/sec. A PCP can be expected to have a greater muzzle blast due to the higher pressures usually remaining in the barrel when the projectile reaches the muzzle, but it will depend on the gun design. Gerald Cardew photographed the effect back in the 1980's.

The presence of anything attached to the muzzle such as moderators, air splitters etc. will modify these effects.

[Ribbonstone]

Thinking about it...if the gas cloud tripped the first sensor BEFORE the pellet....and the pellet tripped the last sensor....wouldn't the read-out's velocity be LESS, not more?

[anti-squirrel]

Inconsistencies in several models of the same gun shooting the same pellet at the same pressure and temperature are very real.

Regarding the other items:

1: a barrel mount chrono has a different start sensor.  Does this really matter?  We're talking a foot or three for stuff moving hundreds to a thousand or more feet per second.  How long does it take that projectile to move a foot or so?  Not long at all so velocity drop is a non-starter.

2: The projectile is going to exit "in front of" the propellant- meaning CO2 or air- by and large.  Sure, in those tiny grooves you'll have a smidjen of fluiid out-accelerate the projectile... but really?  Does this even matter?  Maybe if you're shooting a .177 or .20 pellet in a .22...  Otherwise, pellet leaves the bore first.

3: the moment the pellet or projectile exits the barrel, it is decelerating.

I love standards, but unless somebody buys every shooter on the planet the same gear with the same measuring capacity, we will have different standards.  Just about every thread I take seriously that reports on speed/velocity includes some info on the chrono and how far it was.  In fact, most shooters will preempt any attempt at standardization by shooting their first chrono to reinforce proper practice.

[A moron]

Might have an issue with a cranked up snow-ejector and 1 foot from the first sensor.

Spend most of my shooting life using firearms....you lean not to get too close to a chronograph,so normally set up airguns about a yard away (which means your readings are slower than MV, but not enought to get me upset).

Wonder about the little clamp-to-the-muzzle type chronographs.....don't have one,so have no idea if they'd clock a co2 gas cloud (like shooting a blank) or not.  I can't get  my old Prochono to noice a co2 blank at 2 feet, but none of mine are spitting clouds of snow.

There's like a 21 page thread here on them cheap little chins chronograph

Seems there liked  I'm going to order one and find out  . Heck, for $20 or so why not?

https://www.gatewaytoairguns.org/GTA/index.php?topic=151661.0

[mobilehomer]

Considering tolerances, no two chronographs will rarely, if ever, give the same velocities. A variance of only a couple of thousandths between screens will cause a difference of possibly hundreds of fps. The thing a chronograph is best for, is comparing YOUR loads/guns to each other, NOT to someone else's chronograph. As long as you keep your muzzle at the same distance from the start screen, your measurements will be valid. And the very best chronographs use screens that are several FEET apart, not the one piece units. 

[Thane]

... and even if you had identical chronos, with matched operation distances, lighting... the atmospheric conditions would introduce another variable.

I often wonder if some modifications get the benefit of changing conditions over actual performance increase.

[WhatUPSbox?]

2. The gas will always leave the gun faster than the projectile unless you have a very long barrel and very low gas pressure. The gas velocity will slow down very quickly and the projectile will soon leave the gas cloud. However, while it is being overtaken by the gas the projectile is effectively flying backwards. This is usually called the intermediate ballistics area and can be vital for group size.

Interesting, I would think that until the back of the pellet leaves the muzzle, the gas behind the pellet is traveling at the pellet velocity. Once the pellet leaves the muzzle, it still obstructs the axial direction and the gas expansion would primarily be radial. This seems to be illustrated by this Schlieren video (best viewed at .25 speed). The gas does appear to move about half way up the pellet (at 6 sec) but does not appear to overtake the pellet significantly. I wonder if that initial pulse seen in the video can trigger a chrony sensor. Note that the video does not say what gun/pellet is used.

[Ribbonstone]

Thane:
 
HAven't found that much variation to be true...so long as you shoot level across the screen....at least not a major difference compared to the long screen spaced "big units".

On like Crosnograph #8 over the years....some just died of old age,some got rained on,some got shot....worst I found was like 2%.

I could have got luck....could have taken real care to get the muzzle as dead-center-square to the chronograph.

Would often have two functional cheap/short ones and one long/expensive one I could borrow. HAd two as I wanted some real world down range comparritive data for BC calculations.

(World before radar units).

Of course,at the long range cheap ones would get rained on, cow-stomped, or shot.


The really old printed screen/6v.dry cell/ no digital read out units were pretty unreliable....won't think popping though two sheets of wire laced paper would make it that hard on a pellet....but it really did.


Comparing now to way back then...we're in chronograph hog heaven.


WhatUPbox:

But they don't always seal the bore....and some times do eject a small plume of co2/fog ahead of the pellet if really cranked up.

Don't currently have one cranked up to the level that it that makes that fog...which is also temp/humidity dependent....did have a Philippine .32 that would always spit out a visible plume of gasthat MIGHT have tripped the chrono sensor if shot close enough.

2 Feet seems safe enough....if paranoid, set up a sheet of cardboard with a hole in it as a "blast shield". That's something I'd do with some of the black powder muzzle loaders that seemed to be clocking the sonic blast wave rather than the slug.

[triggertreat]

I agree that with all types of chronographs being used, you gotta wonder about them being consistent between them.  Now there is a new one out using radar and clipped to the barrel/shroud/LDC in some fashion.  Initial reports are iffy at best, but this one has to be held still to get a good read.  Another factor added in the quest for consistency.

For me, I have used the same Pro Chrono digital since the beginning of air gunning for me, and at a measured distance from the tip of the gun to the face of the chrono regardless of the gun.  I have a thick-cut yardstick I use.  In addition, for the last couple of years, I have done all of my tuning regardless of gun indoors using the OEM light kit installed.  With that, regardless if off, all results are coming from the same setup.  I even included a chrono distance area in my Free Spreadsheet offering so anyone can keep up with their chrono distances as well. In the end the groups count the most.  The data is fun while not shooting groups, and is helpful in seeing how the gun/projectiles are performing on paper, but in front of the gun while shooting groups can give you good info as well.

[Rohmpm]

My Prochrono has been the been the recipient of a few "impacts" over the last 20+ years. taped and glued together. I have a new Caldwell and have shot across them back to back with the Caldwell after the Pro. shooting from about two feet, The Caldwell consistently reads a few fps faster.

[Ribbonstone]

So just like pressure gauges....which to you believe?

If it's just a few FPS different...does it really matter.

[Dave S]

I've come to the conclusion that distance from muzzle to start screen isn't all that critical on air guns. Concerning Co2, air density (altitude and humidity) does play a major factor. Temperature is probably the most critical factor with Co2.  As far as Chronographing goes, I started clocking bullets 30 years ago when I was building loads for ,223 and 7.62 x 39
 bullets on my Dillon 550 B. Rule was minimum 5 feet minimum muzzle to start sensor on a PACT 2 Chrony. Old habits die hard. I still follow the rules. Bottom line...whatever works for comparisons sake for a decent reference is good enough! Clocking bullets is FUN! Have at it! dave

[Deckard1973]

I mount mine up against the fill cap.  Same distance every time. 
No need for light kits, no missed shots.  Still on the same 9v battery as when I bought it, 2 years ago. 

[A moron]

So I guess in the end there too many factors to affect any reading outside what the controlled test and chronograph the guns manufacturer uses and how they set up.   Best you can hope for is a +/- with in reason 

Then have 2 chronographs to prove each other to be correct under your conditions in the field.

[SC_RV6A_Pilot]

My FX and my Caldwell are within 2-3fps of each other when the gun is tuned.

[A moron]

My FX and my Caldwell are within 2-3fps of each other when the gun is tuned.

Ya, I'd call that good but say they were 20. Then you'd have to wonder what's up and what the correct one . Now your looking for a 3ed to prove one of them with in a fair +/-

How much +/- between 2 would you feel ok with? 

[rsterne]

I've shot just air though a Chrony at a foot or so, and never had a velocity reading.... not saying it can't happen, but I haven't seen it.... CO2 is another story, if there is liquid being discharged from the muzzle, but I would venture to guess it is always behind the projectile....

Shoot too close to the Chrony with a high powered Big Bore, and you can blow the diffusers off it, though.... 

Bob