Chrono results

[bgary]

Out of curiosity, I ran some ammo thru two chronographs today, back to back (a ProChrono and an M2)

The ProChrono gave readings that were consistently about 1.5% lower than the readings on the M2.

With my pistol ammo I wouldn't care. But given that I really want to know where this rifle ammo will hit at distance, and that the 1.5% represents enough velocity (about 50fps at the muzzle) to make a difference in my drop tables.... now it has me wondering.

Is there a good way to determine which velocity is "right"?

And, yeah, I know I could shoot it for groups at 500 yards and use the actual drop figures to work backwards to a velocity, but... I was really hoping that the chrono would give me good usable info.

Any hints?

[h2osport]

Split the difference. I do not see 25fps either way making much difference on you load out to 1000 yds depending on caliber.

Randy

[bgary]

Split the difference. I do not see 25fps either way making much difference on you load out to 1000 yds depending on caliber.

Thanks for the reply. I'd probably do that, except for two things:

1) I really want to understand this stuff, and velocity is a pretty key input. Not looking for a "good enough" answer, I kinda want to know how to figure out what the "right answer" is. And

2) It does make *some* difference. With my load, 50fps difference is 2" at 500y, 13" at 1000y. That's easily enough to make the difference between a "hit" or a "miss". I want to really "know" why I missed, when I do, and can't really do that if I don't really know the velocity of the round. Can I?

Put it a different way... what's the point of working up a consistent (single-digit SD) load, if at the end of the day I *still* don't know whether or not the round is leaving my muzzle within a known range of the velocity I'm using for my dope?

Edited April 9, 2011 by jakers

[45 Raven]

Are you positive that the distance from the muzzle to the center of the chrono screens was exactly the same in you back to back tests? Did any environmental conditions change during your tests? For example, did cloud cover move in or out during or between your chrono tests?

I would suggest that you repeat your tests on another day and see if the differences remain constant. I suspect that if you used five different brand chronographs, you might see some differences across the chronograph brands on a reasonably consistent basis.

I think you will have to actually shoot at desired distances to see the point of bullet impact. Chronographs make it "close enough" for most work, but I understand you want to be more specific than that.

Elevation, relative humidity, barrel length, barrel machining differences, ambient temperature, barrel harmonics, wind direction and speed, etc. will all have variable affects on the actual bullet flight path. And the chances that your load in your rifle at your location will exactly match the equipment and conditions that produced the bullet drop and drift data you are currently looking at are very, very slim.

The chronograph information gets you in the "neighborhood". The only way to determine with 100% confidence where YOUR rifle shoots YOUR ammunition at YOUR location is to actually shoot it and see. The knowledge you have gained with your "pre-work" with the chronograph should position you so you don't have to spend an inordinate amount of time confirming point of impact.

Good luck.

[pjb45]

I just wasted twenty minutes looking for a review of chronographs I had seen recently. It looked at all the major brands and did a comparison. If I remember correctly they all were pretty close.

That said, at Nationals and Area 2 they use two chronographs. I have watched my results closely for the last six years or so. The interesting part is rarely do the two numbers every come up close to each other.

I have seen too much variations in both readings to every believe one is the absolute truth.

I know my CED M2 comes pretty close to matching the lines on my IOR scope and ballistic software available on the web.

http://www.jbmballistics.com/ballistics/calculators/calculators.shtml

[markcic]

Since you have them front to back swap them and compare the results to see if the variance is related to position.

[VegasSean]

Here is my opinion, so take it for what it's worth. Don't use the chrono results to build your ballistics chart. Use your chrono results to build your load and check your work. It may not be accurate from an actual fps standpoint, but it should be accurate enough for you to measure extreme spread and standard deviation. Once you get done with your load development, then do out and build your ballistics chart based on known distance shooting. It's easy enough to reverse engineer a ballistics calculator once you know how many clicks (or whatever) you need at 100, 200, 300 and 400 yards.

[Singlestack]

1) Chronos are voodoo

2) There is no way to tell which one is right. They might both be wrong.

3) Chronos are voodoo

[fastmtnbiker33w]

A chrono is like any measurement device where it's validity can be debated. If this is bugging you, don't get a graduate degree in any sort of science. More than half the research in science is about the validity of a certain measurement device. I hated it.

Vegas Seans advice is solid. It's kind of like your scale at home. It may read differently from your doctor's scale or your gym's scale. It doesn't matter as it's all relative. Use your scale and only care about what it says.

Don't use a different chrono every time you go to the range. Use the same one...then the only variable you have to deal with are temp and air pressure.

[bgary]

Good advice, all (especially singlestack ) Thanks!

[ignoring, for the moment, all the other variables such as baro, temp, humidity, wind), I well-understand that velocity and ballistic path are intricately linked.

The challenge I'm dealing with is that at my home range, I only have 200 yards, so I only have the ability to work with velocity - I don't have the range to see what the rounds are actually doing at 300/400/500/etc. With a good ballistic calculatory I can model what they "should" be doing based on velocity, but that depends on a good velocity. So, in preparing for a match where I'll be shooting at 400-500 yards, the only prep I can do is theoretical... and uncertainty with the velocity makes that hard.

I'm thinking at this point my next step is to find a place where I can shoot some groups out to 500 yards. *IF* I can get good data about what the rounds actually do at various distances, I should be able to

-- back into a velocity figure that matches the actual point(s) of impacts, and

-- maybe even use that velocity to "correct" the readings I get off my chrono.

If I can get to a "correction" that allows me to model different ammo with my ballistic calculator... that'd be a win.

[kamikaze1a]

I'd say that if you shot across those same two chronos again you would get different readings. Light conditions, how square you shot across the sensors and maybe even temp or battery voltage could make a difference. For a couple hundred bucks you get pretty close but not exact science...

[SHinck82]

I would highly recommend using an IR sensor type chrono like the M2, this will remove the light issue. I have used my M2 to great effect for getting loads developed and getting ballistics tables built. One thing to keep in mind though is that when you start talking longer distances that things such as pressure, humidity and numerous other factors can effect the drop of the round greatly. THe best thing you can do is find a place to verify your chats and do the old school method of the data book. Ballistics programs are great but having a data book is even better. With a .308 just a 2000ft change in elevation can be the difference between a hit and a miss at 1000 meters on a 20x40in target

[Ken Kwiat]

I could write a book about my chrono experiences.

I have a PACT (IV x2 timers/computers) and a CED (w/o IR). The CED would always read higher than my PACT and almost led me to going minor at Natls. In my case, the PACT seemed to track with results at major matches. So, for IPSC/3-gun loads, I've been content for years with what I have.

Then, I got into Tac LR rifle. In this game, you really spend an inordinate amount of time with load development, dope sheet creation, and validation.

While I was struggling with inconsistent results using my PACT (across multiple Density Altitudes), I noticed that just about all the serious rifle shooters were using Oehlers. Primary reason? They have a Proof channel.

So, I bit the bullet while Oehler is making them again and have been very pleased with the results. You get instant verification with the proof channel.

I'm not saying that a $100 Chrony can't get the job done just that it should be viewed wrt a portion of the budget for LR rifle. If you're just a dabbler in the sport, spend accordingly. If you're already spending $3-6K on a gun + $2-3K on an optic + all the accessories, $575 for the gold standard chrono is part of the kit (imo).

Btw: most LR guys these days focus on Density Altitude for dope (so you have one number to factor vs. six+ variables) but that's a far longer discussion that's covered in detail on other LR forums.

[TimBoettcher]

I could write a book about my chrono experiences.

I have a PACT (IV x2 timers/computers) and a CED (w/o IR). The CED would always read higher than my PACT and almost led me to going minor at Natls. In my case, the PACT seemed to track with results at major matches. So, for IPSC/3-gun loads, I've been content for years with what I have.

Then, I got into Tac LR rifle. In this game, you really spend an inordinate amount of time with load development, dope sheet creation, and validation.

While I was struggling with inconsistent results using my PACT (across multiple Density Altitudes), I noticed that just about all the serious rifle shooters were using Oehlers. Primary reason? They have a Proof channel.

So, I bit the bullet while Oehler is making them again and have been very pleased with the results. You get instant verification with the proof channel.

I'm not saying that a $100 Chrony can't get the job done just that it should be viewed wrt a portion of the budget for LR rifle. If you're just a dabbler in the sport, spend accordingly. If you're already spending $3-6K on a gun + $2-3K on an optic + all the accessories, $575 for the gold standard chrono is part of the kit (imo).

Btw: most LR guys these days focus on Density Altitude for dope (so you have one number to factor vs. six+ variables) but that's a far longer discussion that's covered in detail on other LR forums.

+1

[Bear23]

I would use two of the same chronos. At least from a data set you will eliminate one variable. I would assume most chronos are +/- 1% for each brand. Chrono multiple rounds and give yourself more data (N values)and see if you fall within a standard of deviation.

[Mr.Kirk]

I go back and forth between my shooting chrony and my buddy's pact. I hate them both. I've put mine next to an identical one and had different results.

Until I develop the nerve to buy an Oehler, I'll just have to Endeavor to persevere.