How do I use crony data?

[Zig308]

Hello, I was thinking about getting a crony Beta. Since I reload I'm wondering how I use the info from it to work up and get better/optimum loads. Particularly:

Will speed give an indication of pressure and therefor max load?

How far apart can a group be speed wise and still be a 'good' load?

Does a small SD relate to small groups?

TIA for your answers and suggestions.

Zig

[six_gun]

Yes and No to all of the above questions.

Speed does give an indication of excessive pressure, if you are getting a whole bunch more than you should be getting.

Here is what speed will tell you:
If you know the velocity you can figure out energy and downrange ballistics, using various formulas. With that you can determin what point blank ranges you should count on, or if you have the energy or momentum at a desired distance.

Your second question:
As you shoot over a crony you will be able to determine what erratic velocities will mean to your particular gun. I had a friend that was shooting cheap Remington ammo in his 30-06 and accuracy was nonexistant. We shot it over my chrony and there was a 150fps difference in his extreme spread. That makes a difference. I started loading for him and we discovered that his chamber was streached when we fired once-fired-brass from his rifle in his rifle again.

A small SD is a good indicator that you have an accurate load, but not all conclusive. It does help in load development though. It will point you in the right direction in your load development.

Sixgun

[versifier]

Excellent summation, six_gun. I would add this about extreme spread to flesh it out a bit. If you have a ballistic table (I'm looking at the one in an old Sierra manual right now) you can see the difference in bullet drop at longer ranges. At 50yds, a big ES won't be as noticable, but as the range increases, it becomes a major factor.
Let's say you have a .30cal 150grSBT, and the book says you will get a MV 2700fps for your chosen charge. Your initial tests at 50yds look promising, but when you move out to longer ranges the shots are all over the paper. When you chrono the load, however, you find that the velocities are erratic from shot to shot. You see speeds from 2580 to 2810, an ES of 230fps. (This is an extreme example to illustrate the point.) Assuming a 100yd zero, the optimum trajectory for the bullet at 2700fps goes like this: bullet drop @ 200 - 2.05", 300 - 4.95", 400 - 8.36".
At 2600fps we get 200 - 2.28", 300 - 5.44", 400 - 9.16".
At 2800fps we get 200 - 1.85", 300 - 4.51", 400 - 7.66".
At each different velocity, the bullet has a different trajectory, so, practically speaking, if we have a hunting load that is capable of 2MOA, at 400yds we have an 8" group if the load is perfect, but with a 200fps+ ES, we add in the bullet drop difference of another 2" (but it is random and unpredictable from shot to shot), giving us a minimum of a 10" group. THEN, we add in the human factors of breathing, flinching, jerking, changing light, paralax error of the scope, unstable rest, mosquitos and deer flies chewing on exposed flesh, etc., we are lucky if we are hitting the paper at all beyond 200yds.
With a low ES, at the very least we have a similar trajectory for all the rounds fired, then we only have to worry about everything else. [smilie=1:

[Jammer Six]

What led you to discover the streached chamber?

How did the chroney show that it was a chambe problem?

I love theads like this... It's why I'm here. :drinks:

[jhalcott]

the chamber probably didn't! The RELOADING dye did.

[montana_charlie]

What led you to discover the streached chamber?

How did the chroney show that it was a chambe problem?

I love theads like this... It's why I'm here. :drinks:
The chrony showed the accuracy-killing velocity inconsistency.
Reloading (instead of continuing with factory ammo) should have made for better consistency.
Trying to chamber fireformed brass revealed the stretched chamber.

A logical deduction...
CM

[wills]

Here are some thoughts from Mr. Danielson though they may not directly answer your question.
http://www.public.iastate.edu/~jessie/PPB/Stats/Testing%20loads.htm

[KYCaster]

Welcome, Zig. You're brand new here and already trying to make us think!!

I was ready to come up with a list of good reasons to have a chrono and after much pondering I've come to the conclusion that there's only one situation where it is absolutly necessary. That's in competition that requires a minimum power factor (IPSC, IDPA). Fail to make the PF (bullet weight X velocity) and your score suffers, and the only way to get that info is with a scale and a chrono.

Like Six Gun and Versifier pointed out, there are lots of situations where the info can be very helpful, but in actual practice, its pretty much redundant. I don't doubt that Six Gun would have found the chamber problem without the chrono data, but it probably saved him a couple of trips to the range because of the additional info.

If you're concerned about mid range trajectory and point blank range, there are many sources that have the BC, trajectory tables and velocity you can expect from your loads, but I wouldn't bet my elk tag on the results without checking it on the range, and the chrono data is another tool that will help you get your answers a little quicker.

I seldom go to the range without my Chrony. It often tells me some very interesting things. For example, I load 303 Brit. with mil surp WC860 powder. Its very slow burning and a case full will not cause an over pressure situation so I'm confident that I won't damage anything unless I do something really stupid. However, I've been told that using dacron filler will reduce the velocity variation and make it burn much cleaner so I had to try it. I shot them over the chrono and sure enough, not nearly as much garbage left in the bore and the extreme spread was reduced by about 50%, except for one round that was more than 300fps slower than the others. The group size wasn't any different than my control group except for one round that was four inches outside the group. That was the one that was slower.

Without the chrono, I wouldn't have known that there was a reason for the flier. The data told me the reason for the flier, but I still don't know what caused it to be slower.

Bottom line.....its just another tool that can be very helpful at times, and like any other tool, sometimes it does the job better than anything else in the tool box. The Beta Chrony probably costs less than you'll spend on powder in a year, so there's no excuse to do without one.

Enjoy
Jerry

[six_gun]

I had to cut my initial response short because my boss stuck his head in my office and needed me in a meeting. I wanted to expand the answer on how to use the SD but that is already done now. A Chronograph really expands your horizons when it comes to loading your own ammo. It gives you a good solid velocity on which to base all of your ballistic calculations on. Without that, it just a guessing game.

Montana Charlie got the right answer about the streached chamber. Actually when we loaded the next batch of ammo, cases started seperating and then checking the once fired brass showed that it was almost seperating on the initial fireing. It was a model 700 ADL that my friend had bought new as a boy and had never had anything but factory ammo shot through it. Seeing the 150 fps extreeme spread lead to trying to work up a good load for the rifle which helped discover the streached chamber.

Sixgun

[7br]

Another thing to consider is that your barrel is going to be vibrating. If the velocity happens to put the bullet at the time the it is at maximum applitude, a low es and sd won't be worth much.

That said, I think I am going to ask for a chronograph for christmas. Might come in handy for figuring out how far out I can take a deer etc.

[Wayne Smith]

Another appropriate use is finding the most effecient high velocity load for your rifle/cartridge combination. Load up a series of loads, say in .5 grain increments. You shoot these over the chronograph, and see a gradually rising velocity at a relatively steady rate. Suddenly you realize that the powder has increased perhaps a full grain, but the velocity isn't moving much. That is the most effecient load for that combination. Back off the to point where you got the most velocity for the least powder and shoot for accuracy.