Help! Extreme spread and standard deviation loads

[bowhunthard88]

You're not happy with 1/4 MOA? Have you determined how accurate your chrony is yet? Seems to me the first thing I would do is rule out the possibility of instrument error before I started started burning up a 300 RUM barrel on a wild goose chase looking for a better ES that an inaccurate chrony will never give you.

Just my opinion

I completely agree with you. I am more than pleased with the accuracy at the ranges I have shot so far... but I don't have quick and easy access to much further, and would like to do as much load developement as possible before I set up a time/date to shoot that far. A 30 fps spread accounts to an 11" difference at 1000 yds. without factoring in actual accuracy and shooter error.

Thanks for the help and pointers, guys.

 

[MontanaRifleman]

I completely agree with you. I am more than pleased with the accuracy at the ranges I have shot so far... but I don't have quick and easy access to much further, and would like to do as much load developement as possible before I set up a time/date to shoot that far. A 30 fps spread accounts to an 11" difference at 1000 yds. without factoring in actual accuracy and shooter error.

Thanks for the help and pointers, guys.

Well first, I have to apologize because for some reason I thought this was a 300 RUM and it's a 308 Win. Barrel life shouldn't be a big issue.

Somethings to think about...

I can pretty much guarantee you that your chrony has some error. Just how much error is hard to know. If $100 Shooting Chrony's were perfect instruments, Oehler's wouldn't sell for multiple hundreds of dollars, and even they are not perfect. I have two chrony's and have shot them back to back and they have deviated from 5-60 fps on the same string.

When it comes to statistics, it always helps to have good sample sizes. SD really doesn't mean anything on a 5 shot string and not much on a 10 string.

As you are trying new loads, keep shooting this load and record the data. See how much variation you get from string to string in ES and Average velocity. shoot during different times of day, morning, midday, afternoon and evening. im my experiences, the sun's aspect has always had an effect on what my chrony's spit out.

For a 308 to be shooting as well as your's is to 350 yds, I really don't think you have an 80 FPS ES. According to the ballistic calc, an 80 ES would result in almost a 1 1/2" diff in POI @ 350 yds, not to mention the accuracy of the rifle and all the other factors. I think you have a pretty good load there, but I could think of worse ways to spend your time and $$$ trying to improve it

 

[Shootin4fun]

I usually put mine @ about 12 to 15' from the muzzle, but I've never replaced the batteries so far. Guess I'll have to do that.

Thanks alot for the help.

These are digital devices, folks. The detected speed will not vary based on battery strength. A quartz crystal generates the clocking pulses that drive the cpu, etc. The clock is either there, or its not. On the way out, you may see ERR and other strange behaviors but not variances in velocities or miscalculated ES, SD, etc.

This is not to say a device does not vary based on other things such as temperature.

Also, consider the angle of the bullet path over the sensors. If you shoot a path that is perfectly parallel to the plane of the sensors, and one that is 20 degrees up because the chrony is tilted, the bullet travels farther via the nonparallel path, therefore taking longer to get to the 2nd sensor. The parallel path is only 12", so a small tilt can have a measurable effect! 10% diff in path length = 10% diff in calculated velocity!!

I would like to run tests between the standard chronys we're talking about vs. the MagnetoSpeed which hangs on the barrel.

 

[Shootin4fun]

I completely agree with you. I am more than pleased with the accuracy at the ranges I have shot so far... but I don't have quick and easy access to much further, and would like to do as much load developement as possible before I set up a time/date to shoot that far. A 30 fps spread accounts to an 11" difference at 1000 yds. without factoring in actual accuracy and shooter error.

Thanks for the help and pointers, guys.

Bow- This thread is a few years old, so I'm wondering how it worked out in terms of narrowing your ES. A question I would ask is, do the lower POI shots always relate to a lower velocity and vice versa? If the answer is yes, then yes, it is the load. If no, then it may be something else. This is a comparison between real world physics & shooter with the chrono technology.

If I were shooting consistent .5" - 1" groups at 300, I would have just gone to the longer range to see what happens because the variance in chrono readings may be due to the device rather than the load itself.

 

[rcoody]

Hornady 168 gr. A-MAX and Varget, over CCI BR2 primers. .308 Win.

Unfortunately this is a factory gun, and I can't use the internal box if I seat the load to the rifling. The jump is 0.108" before I can do anything but single load.

I like Varget in my .308. seat 168gr amax or MK mag length with 44gr varget. one hole.

both amax and mk are very forgiving on COL not so the bergers

find your most accurate powder charge not worrying so much about ES and SD

after you find that then you can start playing with the COL

Personally neck tension is what dials in the ES. Neck sizing only is the best way to get there. I use the lee colllet neck sizer.

remember it is best to annel your brass every 3 firings if you are really trying to dial in a long range load. Also 168's are not for long range. You need 175's or heavier

 

[rcoody]

The furthest I've shot it so far was 350 yds., the groups averaged from 0.5" - just under an inch.

1/2" groups at 350yrds?

you mean 1/2 MOA?

 

[Bart B]

People that have shot their rifles chronographing loads the conventional way (resting on something atop a bench when held against their shoulder) then compared the same system shot in free recoil learned the following.

Average muzzle velocity is less; more recoil while the bullet goes through the barrel. Newton's law.

Muzzle velocity standard deviation is 1/4 to 1/3 as much. More consistent resistance to recoil. Newton's law all over again.

Shooting slung up in prone resting the rifle on bags typically produces smaller groups than the conventional way as well as lower standard deviations. At least that's what I and many others have observed.

Those tiny benchrest groups are shot typically in free recoil untouched by humans except for a finger on a few-ounce trigger. Only after the bullet's left does the rifle finally stop moving as it's against ones shoulder.

Us humans don't hold centerfire rifles exactly the same from shot to shot. Gets worse with more recoil.

 

[bowhunthard88]

1/2" groups at 350yrds?

you mean 1/2 MOA?

Ok all, this is an old post, and I've come a loooong way since then...

First of all, NOT 1/2 MOA, but legitimate 1/2-1" groups between 3-350 yds. I have legitimate witnessed proof, of 800 yd. groups in the 3-4" range now also... But this post is not to brag or really even worry solely about group size. And groups like this do NOT happen every day, but are certainly possible from time to time.

Now I believe that brass prep is certainly important (i.e. length, I weigh and measure internal volume, anneal, prep pockets, neck size, etc. now). All that being said, this is all a lot of work for minimal difference... Powder and bullet combination was fine (I did tweak the OAL), BUT I did switch to Fed GMM Lg primers and noticed an EXTREME difference. Now almost all of my .308 loads are in the mid single digits. Out of all 3 of them: cheap, factory modified, and a custom rifle.

All in all, I'm very satisfied now, and I have a much great understanding of how/what/etc, but the more that I understand and become involved in this whole procedure, the more I realize how much more that I have to understand and learn. IT'S AWESOME!!!

Thank you all for your help!!!

 

[dunbar1471]

So what do you think helped the most?

 

[918v]

I achieved single digit ES by eliminating variables:

1. Sorting brass by headstamp into 1gr lots to maintain consistent case volume. Not sorting by weight typically results in a 1 to 2gr H2O capacity variance in the 308.

2. Sizing with appropriate dies to achieve a certain neck tension which results in similar bullet pull to new factory ammo. This means annealing is also required. Tip- if your seater die is mangling your bullet nose and your powder charge isn't compressed you are running too much neck tension and your ES will suffer.

3. Weighing powder on a scale accurate to one sixth of one kernel.

4. Finding the right jump length.

5. Matching the primer to the powder. Certain combinations work better than others.

6. Finding the correct powder charge. There are accuracy nodes and there are ES nodes and the two don't always coincide. Do you want a quarter-inch group with an ES of 30 FPS or a half-inch group with an ES of 10? Sometimes you adjust the powder charge and/or jump to compromise.

7. Testing in a dirty barrel. Squeaky clean barrels deliver poor ES. Seeing these people cleaning between groups makes me laugh. I dry brush to maintain a reasonable amount of carbon fouling in the bore. I seldom use solvent. Maybe once in a blue moon.

 

[FEENIX]

First of all, NOT 1/2 MOA, but legitimate 1/2-1" groups between 3-350 yds. I have legitimate witnessed proof, of 800 yd. groups in the 3-4" range now also... But this post is not to brag or really even worry solely about group size. And groups like this do NOT happen every day, but are certainly possible from time to time.

Perhaps I'm missing something but those numbers are 1/2 MOA and better.

 

[Bart B]

All loads shoot group sizes somewhere betwen zero and some bigger number. The odds of one being the smallest are the same as one being the biggest. Both extremes are shot at the same frequency.

Smallest ones happen when one of two things occur; everything is perfect and all the variables are at zero, or, all the variables at different amounts tend to cancel each other out. Largest groups show what happens when some of those variables add up in the same direction. Which one happens most often?

It's popular to mention only the smallest ones and keep the biggest ones a secret.

 

[Shootin4fun]

It's popular to mention only the smallest ones and keep the biggest ones a secret.

Ain't that the truth! And sometimes we like to fool ourselves too. Sometimes I keep my 300 yard groups to 2 or 3 shots in order to not spoil the target with a "flier". But then I sometimes end up with 3 targets, each a 2 or 3 shot group, each < 1" with same relative centerpoint, and I'm ****ed that I didn't end up with 1 target of 6 shots < 1". LOL, it backfires sometimes.

Then there's the times when your 1st 3 are < 1" and you decide to push it to 5+ shots and end up with a 3" group!

At 300 yards, it seems to me that myself and environmental conditions matter a lot more than a single digit ES compared with 30 fps ES assuming we're at around a good node.

 

[918v]

Small groups are achieved through 1. Good shooting technique and 2. Mechanical accuracy of your firearm. The best rifle will shoot poorly if shot with bad technique. The bullet takes a while (relatively speaking) to get out the barrel during which time the rifle is recoiling. This induces aiming error and opens up groups. The goal is not only to keep the rifle steady but also to make sure it recoils exactly the same from shot to shot. If your rear bag moves from one shot to the next, the point of impact will also change because the rifle will recoil differently based on the placement of the bag. It s not unusual to see the rear bag move two inches to the rear in a five shot string. I make sure the rear bag is in the exact same spot from shot to shot by indexing it off the edge of the shooting bench. I adjust the height of the front rest so the rifle is solidly riding both the front rest and the rear bag for every shot, like it was the first shot every time. Otherwise the rear bag has to be manipulated to a greater degree for each succeeding shot and the group opens up.