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How to use a chronograph to work up loads?

 
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Old 08-17-2009, 03:30 AM
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Location: Olympia Washington
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How to use a chronograph to work up loads?

I just got a chronograph, now how do I use it to work up loads? Can anyone explain it to me or recommend some good literature. I have determined bullet seating depth and now want to work on the optimum powder load.
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Old 08-17-2009, 07:47 AM
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Join Date: Jul 2009
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Re: How to use a chronograph to work up loads?

If you search to load development there's a couple good threads that'll help. I use mine for more data in addition to the targets. If all the groups are similar between several settings (ie. powder charge), than I'll choose the one with the best data from the chronograph.

I think most people use Standard Deviation (SD) as the most important factor to determine an accurate load. You can also use Extreme Spread (ES) to help determine the best charge, or other setting.

You can also examine the FPS data to try and perfect your loading procedure. Certain additional steps or performing an operation a certain way can help you SD and ES numbers shrink.

For example when I started load development on my rifle I was getting 30 FPS of SD and 60 FPS of ES. Now that I've finished my load it averages around 7 FPS of SD and 13 FPS of ES. My best 5 shot string was 4.5 (SD) and 9 (ES).

Good luck

Stalker
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Old 08-17-2009, 10:13 AM
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Re: How to use a chronograph to work up loads?

The chronograph is a useful tool for much more that just measuring velocity.

SD and ES are the most obvious results that can be measured.

What about increase in velocity per increase in powder?

I start with a low end amount of powder and go up 1 grain at a time, in an '06 size case, to well above max load in the book. I then shoot each load from the least powder to the max charge. I record each velocity and plot the "hit". As each subsequent shot it made the velocity increases a pretty much constant amount. By the way I never get to shoot the max loaded cartridges. I end up pulling them.)

If velocity isn't where I desire, I go to a different powder.

Also, the chrono will be quite revealing regarding primers. For example when I switched from 30 years of using CCI-250s in a 270 Win to using CCI-200s it was as though I spend a thousand bucks on an accurizing job including new barrel.

At some point, well below the max load cartridge the velocity increase will start to drop off. This is the point where pressure in increasing but velocity is not increasing proportionally. Pretty much the law of diminishing returns. Things are getting inefficient.

Ladder/OCW methods. The chrono is vital when doing either of these or a combination.

Velocity with temperature change. The idea of LRH is to have the first cold bore shot go where you desire. Shoot one shot per day, at the same target, over the chrono, noting the temperature difference and plotting the hit. This will be very revealing. Your measurements will will tell you several things. If your velocity and vertical impact change proportionally with temperture and the temperture swing is small you may wish to finde another "note" as in OCW. Or you could adjust a click or so for temperature. (not recommended - a memory thing).

The chrono used in conjuction with a ballistics program will show you your vertical dispersion at distance without having to shoot that distance.

An example of good luck was when I stumbled on to a load using US 869 in a 338 RUM with 300 SMKs at a velocity of 2735 FPS MV with a consistent ES of +.5 over many shots I stopped dinking around with loading and only pull the trigger when there is hair in the reticle. Additionally as temperature changes, velocity changes as much as 15 FPS but ES remains the same. Out of five shots one of the shots will read 1 fps high. Never low.

Not as good as a book and a bit of a rample but
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Old 08-17-2009, 05:13 PM
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Re: How to use a chronograph to work up loads?

I've never found a chronograph to be any help at all while working up an accurate load, as such. Effective load work-up is read at the target, not the sky screens.

I've never seen any correlation between accuracy and SD/ES. In fact, in my limited experiece and all I've read on the topic suggest the best shooting load is rarely the one with the lowest such numbers but it certainly could happen.

The best use I have for clocking my developement rounds is watching the speed against the book projected results, right up to the listed max speed. Muzzle speed is not a precise measure of chamber pressure but it is a good, general indication that I'm at, or below, or above the commonly accepted pressure limits for any given bullet weight with a given powder. Meaning, mostly, I shoot for groups but limit the upper speed to what my book suggests is right for that combonation. Only one of my several rifles gets higher velocity for a charge than the books suggest.

Now, for long range shooting - beyond 300-400 yards - ES/SD does become significant and it shows in the vertical spread. A chronograph will allow you to see how changes in crimps, primers, brands of brass, etc. can reduce or worsen the velocity extremes.
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