Hi,
I developed some "personal guides" while chronographing shotshells at the range for several years. I've found some of them work reasonably well with my metallic ammo, too, so here they are. Maybe they'll help, maybe not? Get your cup of coffee, this will be wordy:
First thing is to get the figures my old Oehler 33 gives, whether your chrono gives 'em to you, they come from a spreadsheet like Open Office or Excel can do, or you're more of a whiz with your scientific/biz calculator than some of us and derive them yourself. These are Hi, Lo, ES (extreme spread, or the highest figure minus the lowest in the string), Avg (the average of all shots in the string), and Sd (standard deviation.)
It's also helpful to have a thermometer with you while chronoing your loads. It's my understanding that most lab chrono/pressure testing is done at 70 deg F. You'll find SOME loads are very temperature sensitive, some are quite temperature tolerant. In plain English, that means some of my loads work well from the low 40s up to over 100 deg, while I have to tone some of 'em back a touch for summer use in the desert. Barring better data, a couple of us figured a change of 2fps/degree, up or down from 70 deg, was a "workable" rule of thumb for shotshells. Warmer temps meant higher velocities, lower ones slowed things down. I haven't come up with a similar number for metallic ammo, but perhaps some of you might be able to find one.
Ok, we've got some numbers. Now we need to look a touch further than just raw numbers and look at percentages. The average number is my starting point. From there, we look at the ES and Sd figures. Again, these figures come from shotshell work, so they might not be as good for your metallics. What we found was the very best we saw over the chrono (on average) was an ES of 2% and Sd of 1%. So, using a target average velocity of 1200 fps, which was the "legal" competition limit for both American trap and skeet at the time, if the ES was 48-50 fps, and the Sd was 12 fps, we considered that to be a "perfect" load.
Reality was the "real" numbers were generally in the neighborhood of twice those figures! So we allowed as how a guy's reloads that matched factory target ammo was probably pretty good. The rulebooks at the time allowed us 1200 fps +/- 25 fps for an "average." That often translated into an ES figure of 75-100 fps, and an Sd of 15-20 fps. Since good shooters could regularly shoot 100 straight scores using factory ammo, and often much higher (750 plus was the highest I've watched in a single string over a couple of days), those numbers must work ok, right?
So if one's handloads matched factory specs they were generally good, if they beat the factory, perhaps excellent, provided the nut behind the butt wasn't loose! In other words, misses really weren't a function of ammo problems...
What I'd suggest before someone drives themselves to the brink of madness with their new chrono is to establish a baseline figure for a particular gun, using decent FACTORY ammo first. As much as handloaders tell us they can beat the factory, the reality is it's like a pretty good shooter going up in competition against a world champion: on any given day the "good" guy might beat the champ, but going head to head for a week, the champ will prevail. The factories build millions to billions of rounds per month--I figure they've learned something along the way doing that! Let's call them the champ in this scenario.
Ok, once that baseline's been established (and it must be repeated for a change of guns), one has something to shoot for. Are your handloads as good as the factory's loads? If so, you're loading practices are probably pretty good, and now you can see if you CAN beat the factory. (If they're not, it's time to clean up the loading bench and pay more attention to your loading regimen until you can match the big boys.) You may well be able to do that, in a given gun with a given load. If so, and the load works as well on paper as it does over the chrono, you've achieved something! Congratulations--make sure you make several copies of your log notes so you don't lose that load data!
I've gone on too long here, so let's close with a reminder the chrono is but one of many tools in the handloader's tool box. How it's used depends to some extent on what the goal is. For example, there are 1000s of hunters out there who've never even seen a chrono, yet have a favorite load that puts venison in their freezers year after year. Those guys may use a chrono more out of curiosity than anything else. Others shoot competition, where maybe they need to meet certain specs with their ammo per the rules. They'll use their chronos frequently. Some shooters are blessed with more innate curiosity than others, and love to experiment, so they'll put their chronos to use accordingly!
In the end, though, the goal is to put a bullet in the same place time after time. The numbers may or may not be big players in achieving that end with your particular gun and load combo, so keep that in mind with your own chrono use! Test, test, test, as they say...
Rick C
