Why so much variation with my Chronograph?

[TargetTerror]

I've noticed that depending on the exact angle the bullet travels through my chronograph's screens, the velocity can change significantly. For example, I setup my .308 to shoot at 100 and 200 yards, and place the chronograph ~10 feet in front of the rifle. The 200 yard target is above the 100 yard target, so the bullet would be traveling at a different angle when shooting at it (not as parallel to the ground). The calculated velocity different was quite dramatic - the average difference from shooting at 100 and 200 yards for the 3 loads I tried ranged between 60fps and over 100fps, for the same load. (200 yard loads were clocked slower, and had less parallel trajectory). That is a pretty big difference, especially when trying to calculate drop out to 600 yards and beyond.

How common is this phenomenon? I'd imagine that whats happening is the bullet is taking slightly longer to go through the screens because of the steeper angle when shooting at longer distances. Is this something I should worry about, or should I get a rough estimated of fps and just worry about standard deviation?

 

[floydster]

Chrono's are a joke, you don't need one, go by loading data, you will be better off, thats what it's for.
Just my opinion, and I have been at it for 54 years.

 

[ranger335v]

"I setup my .308 to shoot at 100 and 200 yards, and place the chronograph ~10 feet in front of the rifle. "

Obviously, none of us can tell you why that is happeing but I would suspect your " ~ 10 feet" may account for at least part of your difference. Consistant chronograph readings demand consistant spaceing from your muzzle, not approximate.

I pack a ten foot lenght of bright colored 1/2" ribbon with loops on each end with my chronograph. I attach one loop to my first screen and the other to the bench at the muzzle. That way, I know exactly how far it is to my screens, every time!

With respects to Floydster, my nearly 50 years of experience makes me appriciate the value of a chronograph more each day. I have NEVER found a factory load nor a manual estimate of velocity to be as high as predicted. Sometimes the predicted speed is off by as much a 350 fps for a high velocity rifle round.

Most, if no all, factory rifles have overly large chambers that reduce the pressure and therefore the velocity of our loads and before getting a chronograph some 12 years ago I never had had any idea of that loss.

Maybe that much velocity loss doesn't matter inside a couple of nundred yards when deer hunting but it does matter for my 3-4 hundred yard .22-250 & .243 varmint loads. And I really like to know what my loads are doing anyway.

 

[USSR]

TT,

First, 12' to 15' is a better distance to set your Chrony at. Second, make sure you set your Chrony up so that regardless of what distance you are shooting at, your bullet traverses your Chrony at the "sweet spot" above the sensors. I do this with my boltguns by removing the bolt and looking down the bore at a black dot that I draw on a piece of masking tape that I tape between the two front metal diffuser rods at the proper height. Takes a little while to initially set up, but no bullets thru the Chrony case and no problems with erratic readings.

Don

 

[The Bushmaster]

A chronograph is a tool. It must be set up the same way everytime you use it. A chronograph is a tool. You use it in conjunction with other tools to determine what your load is doing. It was not meant as a cure all for shooting and reloading...Just a tool to help with the other information you gleen...

 

[cliffy]

Chronographs don't vary standard deviation as much as . . .

loading practices do! When I see just one fps deviations develop on chronograph readings, I feel I did something right, somehow. One extra kernel can often create a fifty pfs spread depending upon ambient temperature. Using the same brand of brass and same primer can reduce these abberrant readings like you wouldn't believe. Yet a mere fifty fps variation will NOT change the point of impact significantly in most applications. Factory-Loaded ammo varies velocitywise more than my handloads. Go figure . . . cliffy

 

[Slamfire]

Instrumentation setup and calibration is important on all pieces of test equipment. Slight misalignment, off angle shots , sunlight on the sensors, powder blowing over the sensors, will cause errors in velocity readings.

I set my chronograph up with a level, and visually check that I am shooting over the center of the sensors.

I typically start off with a "known" load and if velocities are where they should be, I am good to go. If velocities are too high or too low, I try to figure out what is wrong.

 

[Slamfire]

deleted, multiple posting

 

[Slamfire]

deleted, mulitple posting

 

[Steve C]

If you do the math a minor change in angle (say 5 degrees) will not change the distance the bullet travels enough to give you greater than a 1% variation in velocity, well below and within one standard deviation for most data samples.

However the change in angle results in change of distance between at least one of the sensors and the bullet which could change the way they read the shadow which is more likely to be the cause of any disparate average velocities.

 

[Tarvis]

Chrono's are a joke, you don't need one, go by loading data

No, going by the load fps in the book is a joke. Unless of course, you are shooting a 1-9.5 twist 22" test barrel at 1500 ft elevation, when the temperature 62 degrees. Yes, they also use rifles for their tests, but even two rifles from the same manufacturer with the same barrel shooting the same cartridge can shoot differently. Writing off a tool as a joke just because you don't use it is ignorance.

Sounds like 54 years of experience has taught you not to care.

 

[snuffy]

There's been an ongoing thread on TFL titled "do you consider a chronograph a necessity for a reloader?"
http://www.thefiringline.com/forums/showthread.php?t=310469

My take is; a necessity? No. Is it a valuable tool that gives you a lot of valuable info, YES! I've had 3 different chrono's over the last 25 years. The first was made by a small company called Schmidt&Weston in New York. It was huge, weighed 20 pounds and took ½ hour to set-up. One of the first electric eye/sky screen chronos available. It read out in FPS but that's ALL! If you wanted an average, extreme spread, or standard deviation, you had to write down the velocity each time, no memory for shot strings!

People that poo poo those of us that use one don't know what they're missing, or just don't care. They must go 65 in a thick fog and fly blind with no instrumentation

A recent adventure in reloading should be an eye opener for those that think chrono's are a waste of time/money. I was exploring .410 light game slugs as offered by ballistic products. They come in a plastic blister pack, the long double ended plastic wad and .375 round lead ball.
http://www.ballisticproducts.com/prodinfo.asp?number=0721641

I substituted a .395 home cast lead round ball, it fits the modified choke of my rossi single shot perfectly. Using BPI multi hull .410 new primed hulls, I loaded some to try out. New primed cases, 20.5 of 2400, average of 1656 fps. Once fired cases with a WW-209 primer, same powder charge and ball, average of 1877 fps!

Now how would I have known that was happening. The extra velocity was not dangerous, there was no high pressure signs, but it was 221 fps more speed!

I never test a new load work-up without the chrono set up. It just makes the load work up more valuable.

 

[TargetTerror]

Thanks for all of the responses. I thought this would make an interesting thread, and seems to be so far.

Just to clarify, my question involves getting different readings at the same chronograph distance, just with a different aiming point. So when I said "~10" feet from the rifle, I meant that for any given rifle-to-chrono distance, I generally get different readings depending on the POI (high or low). My average velocity changes as the distances between rifle and chrono changes, but that I would expect and am not concerned about.

I think I'll probably continue as I have been doing with my chrono, at least for load testing. Setting up my chrono precisely is a huge PITA at my 100/200 yard range because of how it is setup. We literally have a bench line, an offhand trough in front of that, a raised prone grassy area, then a giant cement wall with tunnels in it that we shoot through. (We're in a very populated area, by 2 major highways, in one of the most anti-gun states in the nations - lets just say we'll have a a major problem on our hands if any rounds escape.) I'm lucky to find a spot where I can place the chrono in a more-or-less usable position.

I do see the arguments for not bothering with a chrono at all, as group size is the ultimate objective anyways (for most applications). That said, I find the chrono to be valuable, as it can tell me what the standard deviation of my loads are. If one load is bigger than another and has a much wider spread of velocity, I can probably guess that that is whats causing the larger group.

 

[Vern Humphrey]

Can you post actual data?

What I am interested in is, what is the spread of your 100 yard shots? If they are all quite uniform in velocity, then we do have a puzzlement.

I would start by placing the chronograph farther from the bench (since you may be chronographing gas, not bullets sometimes), and making sure of the light -- the diffusers must be properly positioned.

 

[Eagle103]

My chrony doesn't work well unless I keep the shots within a couple of inches over the sensors. The closer the better.

 

[Walkalong]

My take is; a necessity? No. Is it a valuable tool that gives you a lot of valuable info, YES!

I agree 100%

 

[Slamfire]

If you do the math a minor change in angle (say 5 degrees) will not change the distance the bullet travels enough to give you greater than a 1% variation in velocity, well below and within one standard deviation for most data samples.

I have read a number of statements where someone said something like this. What is the math?

The bullet traveling over the screens is a timed event (assume). Are you sure that changing a tiny amount of distance between sensors won't make a big difference in velocity reading?

The chrony people told me that it was very important to shoot straight over the sensors and not have misalignment.

 

[Galil5.56]

My chrony doesn't work well unless I keep the shots within a couple of inches over the sensors. The closer the better.

Yep, had the same problem with my POS F1 Master and I'm getting a new chronograph; probably a Pro Chrono Digital. Schizo errors, waaaaay too picky in regard to lighting needing damn near perfect alignment of the stars/sun angle/planets to get it to work correctly, plus the tiny sweet spot that got smaller over time was enough. Got my first Shooting Chrony in 1993 which was great, low error messages, wider shooting area, reliable...

Believe me, it's MUCH worse having a cranky, unreliable chronograph that you don't trust, than none at all! BTW, even if you shoot the CE Pro Chrono, it's cheaper to replace the whole unit than just the screen box of the Chrony, so the remote control argument is null unless you have to have the corded remote. Final big plus is that unbelievably, the unit is US made to include the moldings and they even make the electronic boards in the US. SOLD!