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A tale of two chrony's

 
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Old 01-02-2010, 08:13 PM
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A tale of two chrony's

I hauled my trusty old Shooting Chrony Beta and my shiny new CED M2 out to my brother-in-law's place along with a Sendero SFII in 7 mag. The Sendero has a Jewell trigger set at 1# 5oz, a KDF Accuracy muzzle break, and a Harris BR-2 bipod outfitted with a Pod-Lok (the Pod-Lok is a nice little accessory). Other than that it's dead stock. I had 10 rounds loaded .010 off of the lands with RE-22. The load data came from the Sierra reloading manual but the bullets were 168 gr Berger VLD's. Per the manual, velocity (for a 168 gr SMK) should have been 2,950 fps. Range was 600 yards with a slight (4-5 mph) 45 degree l-r tailwind. I was using LB 3.0 for initial and followup settings for the scope. The scope is a Leupold Mark IV 8.5-25 x 50mm LR/T. I think that covers the setup.

I set both chrony's up in series, SCB first, at 10' - 12' from the muzzle. I don't have a good spotting scope so I was shooting three and walking down to the target to see how things looked (I got my exercise today). The first three shots were calculated using the book value of 2,950 fps in LoadBase 3.0 (along with actual wind, baro, etc). Going from memory I believe my initial value was 11.75 moa up (100 yard zero)

The chrony's disagreed with one another by about 100 fps. I've always trusted my good old, solid metal Shooting Chrony and it gave me velocities just under the book value of 2,950 (that rarely happens to me btw) so my initial thought concerning the CED M2 wasn't too good. I walked down to the target to mark the hits. They were low by a good bit. 2 MOA-ish. Ruh roh... it was looking like my SCB was wrong and the new plastic el cheapo looking CED M2 was right or at least closer to right. So... I plugged in the lower velocity info from the CED M2, dialed the scope up 2 MOA per LB3's results, and fired 3 more. This time I was about 2 cllcks (1/2 MOA) high and I needed some wind correction. About 3 clicks if I remember right. Winds were kind of tough (I'm just starting to deal with reading wind seriously) because of a row of trees parallel and upwind of the bullet path plus they were variable from direction and velocity (0 - 5 mph). Downrange the mirage would lay flat at times... others it would be nearly vertical. Follow up shots were within an inch or less of the aim point.

This is my viewpoint of the chrony situation. I think they are both good pieces of gear considering what they cost. I prefer the construction of the Shooting Chrony. I like the big readout of the CED M2. Obviously I prefer the better data that comes from the CED M2. However- based on bullet drop, I don't think I would bet any money on the validity of it or the SCB's data. Chrony's, imho, at least the lower end ones like these two models, are good for determining SD and ES but the velocity data doesn't seem to me to be on. That said I also have to throw in the possibility that the scope calibration is off. Ie 52 clicks not being equal to 13 moa. So if the scope is off a little and I didn't input something just right in the software, especially considering I had a slight, variable, quartering crosswind, some of, if not all of, the "error" could be attributed to the software and scope rather than the chrony. My guess is that they are all off to some degree.

On another note, the worst group that I had at 600 yards was 6.63" and the best was 2.8". The first group was the worst one and the last group was the best one. The middle group was about 4". I think that, as evening came, the winds died down and that helped the groups out a lot. That and the shooter (me) finally got in the groove and started using good trigger control.

The bottom line for me is that, just like in my work, if you have two measuring devices that don't agree then you have to use an outside method to determine which one is right and which one is wrong. An outside method in the case of my two chrony's was the bullet drop vs velocity data that I had available to use.
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Old 01-02-2010, 09:49 PM
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Re: A tale of two chrony's

I think you need to determine your scope's click values before using the number of clicks required to match LoadBase predictions as an effort to determine MV or which Chronograph is reading correctly or incorrectly. Even then, chronographed velocities are the way to determine MV. Not measured bullet drops. Scope click values are something that can be precisely determined. Once known, you can enter the exact value into LoadBase - which removes one source of potential error from your analysis of the problem.

Clearly something is up with one or both of the chronographs if readings differ by 100 fps. Now you need a third one to add to the two so you can figure out which one gives the bogus readings. Well... seriously, maybe you know someone that has a chronograph that you could borrow for one shooting session. That's what I'd look at doing next. And if you buy a third one, look for a used Oehler model 33 or 35. My Oehler seems to be pretty reliable.
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Old 01-02-2010, 09:58 PM
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Re: A tale of two chrony's

Quote:
Originally Posted by phorwath View Post
I think you need to determine your scope's click values before using the number of clicks required to match LoadBase predictions as an effort to determine MV or which Chronograph is reading correctly or incorrectly. Even then, chronographed velocities are the way to determine MV. Not measured bullet drops. Scope click values are something that can be precisely determined. Once known, you can enter the exact value into LoadBase - which removes one source of potential error from your analysis of the problem.

Clearly something is up with one or both of the chronographs if readings differ by 100 fps. Now you need a third one to add to the two so you can figure out which one gives the bogus readings. Well... seriously, maybe you know someone that has a chronograph that you could borrow for one shooting session. That's what I'd look at doing next. And if you buy a third one, look for a used Oehler model 33 or 35. My Oehler seems to be pretty reliable.
After a while I could accumulate enough chronographs to cover a full 1,000 yards

My Shooting Chrony Beta is at least 5 years old. Probably closer to 10. The CED M2 arrived on Santa's sleigh this year so it didn't really surprise me that it seems to be closer to correct. Oehler... I looked... and looked... and looked but none were to be had right now.

I plan to back up and "check my clicks" on the next go-round. The ES values on the loads that I've tried are pretty high. Certainly not single digit. 30-ish to 60-ish. So I still have a lot of work to do in that department. RE-22 looks promising. Onward through the fog.
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Old 01-02-2010, 10:30 PM
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Re: A tale of two chrony's

Interesting data. Thanks for posting. It's a good "reality check" for people. I think there are a lot of people out there who don't realize their chronograph is off by a pretty decent amount. Then when they find the ballistics programs don't match they go messing with the bullet's BC in the program when that should be the last thing on the list....
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Old 01-02-2010, 10:39 PM
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Re: A tale of two chrony's

I did the same thing about a week ago. I compared a Chroney Beta and a discontinued three screen Oehler, the old three screen model suppose to be very accurate. We set them up in series and shot five rounds. The SC was about 130 fps faster, that is what I was expecting from the cheep Chroney. I started looking at the Chroney and found that the front screen wasn’t all the way against the stops, I tightened the clamp that held the screen and verified exactly 1 foot spacing between the two eyes. We shot three more 5 shot groups and the average velocity for each group was within 2 FPS of each other. So I guess I better take back that comment about the Chroney being cheep.
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Old 01-02-2010, 10:44 PM
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Re: A tale of two chrony's

The Shooting Chrony is about 1/5 the cost of the Oehler... but I wish that I had bought the Oehler when it was available. They go "used" for over $600.00. At least that's what I've seen when I googled Oehler 35 and come across expired auctions.
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Old 01-03-2010, 01:41 AM
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Re: A tale of two chrony's

Interesting post Mike. I've run across the same thing. My two chrony's being off anywhere from 5 to 50 fps. I dont think they are even good for SD's or ES's. They might distinguish higher ES's from lower ES's to some degree, but I take them with a big grain of salt.

I'm not sure a third chrony would tell you which is right, unless it was a really good one like an Oehler. If you had three ordinary chrony's you would likely have three inaccurate chrony's. And I've seen them act different on different days.

How sure where you of your zero, which I'm assuming you based your drops off? I've read the Leupy's are IPHY and not true MOA, so as already sadi, you need to find out just what your turret is calibrated for.

Drops aren't the most accurate way of measuring, but if you shoot them at regular intervals and measure them, I think you can graph them and get a fairly accurate trajctory to calcualte a velocity off. Just thinking out loud...
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