Extreme spread question

[Scottish Highlander]

Good evening everyone, hope you are all well. How important is the ES of a load while developing? I know someone who seem's to think it is extremely important and I would just like to learn about it. I know it is the difference between velocity's ...like this example: shot 1: 3124 velocity....shot 2: 3129 velocity...this would give an ES of 5. How important is the ES on an accuracy factor ??

Thanks everyone

[Unclenick]

It depends on the range. As the bullet goes downrange, the time it takes to cross each additional 100 yards gets longer and longer because it is slowing down. In the meanwhile, gravity is accelerating its fall so that each hundredth of a second difference in time of flight to those longer and longer ranges means a bigger and bigger vertical difference in bullet position.

As a result of the above, a velocity difference that might only be 1/20th of an inch difference in bullet height at 100 yards could become a foot of difference at 1000 yards. The difference between the two depends on the ballistic coefficient of the bullet (lower means it slows down sooner) and the specific bullet drag curve shape at the actual velocities it sees.

The other thing velocity variation does is suggest inconsistent ignition. Such inconsistency can mean small ignition delays are occurring that give the muzzle more time to move off point of aim if the shooter's follow-through isn't dead perfect. Velocity variation is actually a good indicator of which primer and powder combinations you rifle likes better among those that produce about the same velocity. However, the proof is in the pudding. I've seen, for example, in the .222 Remington that IMR 4198 does not produce the most consistent velocities, but did produce the best 100 yard groups. This has to do with the timing with which the bullet gets to the muzzle. In the right spot, the muzzle may deflect a little higher by the time a slower bullet gets there, thus tending to compensate for the velocity error at least at one range where the arcs of the faster and slower bullet's trajectory curves cross.

Anyway, an ES of just 5 is very good. 50 and over is more where it usually gets be an issue, and 20 is considered excellent.

[Jay24bal]

It all depends on the distance you are shooting at. If you are shooting at 100 yards, a "normal" amount of spread is not really going to manifest much.

If you are shooting at 1000 yards, it will manifest itself a lot more.

For example, according to my ballistic calculator, a muzzle velocity of 2600 fps causes a 175 grain SMK bullet to drop an additional 7.6 inches at 1000 yards compared to a shot with muzzle velocity of 2620., while that same change only moves the bullet .1 inch at 100 yards.

Also, a faster bullet moves less in the wind, so windage adjustments would be more consistent as well the smaller the spread.

*edit* sorry to duplicate what was said, I was running the calculations while he responded.

[Scottish Highlander]

Thanks for the quick response !! So an ES is quite important depending on your distance your shooting out to. What can improve the ES ? Is it primerally neck tension or OAL of the cartridge ? I'd have thought powder and primers would have affected it considerably or are those components so finely tuned its nothing to be concerned about. What sort of ES should I be looking for in my hunting load as acceptable ? I have a load in the 308 that is 3/8th inch grouping load @ 100 yards but never checked the ES on it....I know this guy is shooting F class competition and is getting below 10 ES in his loads I assume this is good !

[rclark]

Lots of factors can help ES. For example, in some powders, the higher the pressure, the lower the ES. Also, make sure your powder dump is very consistent. Crimp consistent. seating depth consistent, Etc... Oh, primer sometimes make a big difference too. Also you should not get the ES from just 2 shots. Shoot at least 10. I like to shoot 15. Then highest minus lowest gives you your ES with a high degree of confidence. I would not use a powder if the ES is above 50 fps for everything I try. I do a dance when the ES is sub 20 for my revolver cartridges. Have no experience with 'rifle' and what to expect there.

[Jay24bal]

Several Factors are commonly attributed to velocity spreads:

1. Neck tension on the bullet, consistent pressure ensures consistent release of the bullet from the case.
2. Powder - some are more consistent burners than others, and they all vary a little but lot to lot. Benchrest shooters work up a new load with every lot of powder. What was 45.1 grains of powder XX with lot ZZZZ-ZZ might be 45.2 grains of the same powder with lot ZZZZ-YY to get the same muzzle velocity.
3. Primer - same as above, but they are generally affected less between different lots.
4. Atmospheric conditions. Temperature and humidity can have an affect on the powder, thus causing the velocity to change.
5. OAL - just like with neck tension, if one round is seated longer than the next it affects how long after ignition the bullet seats in the bore. This variable also affects how much unused space there is in the case and how the powder reacts in the case and how much pressure builds up.
6. The brass itself can have a bit of an impact. If one piece of brass is thicker than the rest, and you resized them all to the same EXTERNAL dimensions, this means there is less INTERNAL space in the casing. This reduced case volume translates to change in pressure when the powder ignites. This is why competition shooters use the same headstamp and weight their cases.

I am sure there are other variables, but these are the main ones people attempt to eliminate.

As for its impact on you, for hunting rounds where you will not be shooting at long range, I do not pay much attention to it. As long as you are shooting acceptable groups at the ranges you intend to shoot game, I call it good to go.

For my competition guns, I aim for an ES of 1% or less. So for a load shooting at 3,000 fps, I want a spread of 30 fps or less. Usually I can get it much lower in the .5% area. All just a matter of how much time and care you devote to the reloading process (particularly brass preparation).

Your friend who is getting a spread of 10 fps in his loads is doing well and probably in the .3-.5% range.

[Catfish]

I have a friend that had a 243 that was his go to gun for groundhogs. He thought it was the best shooting,at longer range, past 250 yards, that he ever saw. One day he cronographed it, SD was 43. Varation in velocity does`t mean much until you get out to longer ranges.

[Unclenick]

Originally Posted by Scottish Highlander

So an ES is quite important depending on your distance your shooting out to. What can improve the ES ? Is it primerally neck tension or OAL of the cartridge ? I'd have thought powder and primers would have affected it considerably or are those components so finely tuned its nothing to be concerned about. What sort of ES should I be looking for in my hunting load as acceptable ? I have a load in the 308 that is 3/8th inch grouping load @ 100 yards but never checked the ES on it....I know this guy is shooting F class competition and is getting below 10 ES in his loads I assume this is good !

This gets a bit technical, but ES is usually not as useful as standard deviation (SD). The reason is that ES depends only on the two most different velocities you get out of however many shots you took in your group. A sample of two is not statistically very reliable, so you might have to shoot ten groups and average the ES result from the ten ES's to get a better sense of its normal value. That said, for a group of 9 or 10 shots, ES, on average, is very close to 3×SD. So, you could shoot a single 9 or 10 round group, find the SD and multiply it by 3 to get the average ES 9 shots would be expected to produce. If you shot ten, ten shot groups, you would find the SD's don't vary as much as their ES does because the SD formula includes the influence of every shot in the group, and not just the two shots whose velocities are furthest apart.

In general, high power rifle shooters working up to 600 yards strive to get SD below about 20 fps. Folks shooting 1000 yards are usually working at getting it below 15 fps.

To minimize SD, there are many steps you can take. Some are a waste of time in some guns but not in others. The best thing to try to do is to experiment with them one-at-a-time to see what actually makes a difference for you. For example, I had the experience that deburring case flash holes made a significant difference with a spherical powder load that didn't fill the case too well in .308 Win, but found it made little if any difference with single-base stick powders. This was for 600 yard shooting. So, depending on the components, something may matter in one combination that doesn't in another.

[rclark]

One other thing, a good ES (or SD) doesn't guarantee an accurate load. The gun itself may hate it , but normally, an accurate load will have a good ES. I do calculate SD... but I find that usually the SD is within a few fps of ES div 3, so I rely more just on ES. I do throw out a flyer (if there is one) from the final data. For example if my velocities are all about 20fps apart from the mean, but one of the 15-20 shots is 70fps out, I toss. I do my calc in a spreadsheet at home (copy from notebook to LibreOffice Calc) for analysis.

[Road_Clam]

When I first started chrono testing I would get all worked up over an es over 60fps. Now that I've accumulated a ton of varied 223, 308 and 300wm data , I've come to realize there is no strong correlation between high precision and low es from my amatuer loading level of experience. I've shot sub moa with an es of 70fps, an adversly shot 3 moa with an es of <20fps. My focus right now is keep it simple, minimise the variables, load , shoot, document and learn.

[Metal god]

I've done a fare amount of chronographing my loads . I can say with out a doubt how I hold my rifle to my shoulder has the biggest impact on ES/SD . That is assuming you are loading pretty consistent ammo in the first place . If I want a true value as to my ES/SD I barely hold the rifle and let it almost completely free recoil .

I did some limited test on this and posted them here at TFL . Problem is the test were done a year or two ago and the post were in another thread titled something else and I can't find the post . I will how ever tell you what I did .

This was all when I was having issues with my first shot being slower then the rest of a string . I was actually at the range and doing a semi live chat here at TFL while I was there . I was there shooting and testing my velocity of the same load . I was updating the thread after each string and applying and or trying the suggestions I was getting back at the time in real time .

BartB was one of the posters helping me at the time . He suggested The consistency of how I hold the rifle can effect the velocity from shot to shot . Now I had heard him and others say this could be an issue but never gave it any real thought . How ever this time I was at the range with a very nice load I had developed . Why not give it a try and test it out .

Unfortunately I was at the end of the rounds I brought and only had 10 left to shoot . So I did two 5 shot strings starting each string with the same barrel temp and shooting each shot at the same intervals loading one shot at a time .

308 / 40.7gr IMR 4895 / 175gr smk / WLR primer / WCC-06 cases
Savage FCP-K shot from a bench with front and rear bags

Keep in mind all these rounds were loaded at the same time with all the same lot components and all were handle from my reloading bench to the rifle in the same manner

First string was shot with the rifle pulled in hard and tight to my shoulder for each shot .
Results = 2517 / 2582 / 2568 / 2552 / 2568 / ES-65.52 / SD-28.60

20min later with a cooled barrel I shot the second 5 shot string . This time I barely even touched the rifle and let it freely recoil for the most part . I did have to keep in control of the rifle but in comparison to the hard tight hold it was like not even touching the rifle .

Results = 2551 / 2555 / 2559 / 2552 / 2565 / ES-14.50 / SD-5.74

That test was a huge eye opener to me and changed how I hold my rifle . Now that was a small sample but I believe it to be accurate based on all the other testing I had been doing at the time with that load .

Hope that helps

[Sevens]

^That post is awesome.^

I have no doubt that dealing with a chronograph opens yourself up to pitfalls like the one you detailed extremely well in that post. Most of my work is with handguns and I'm sure the same thing is at work with them also.

I like what a chrono can do, but I think too many folks get FAR too wrapped around the axle with using them. I also firmly believe that the chrono can royally suck the fun out of an otherwise very nice range day.

[rclark]

I also firmly believe that the chrono can royally suck the fun out of an otherwise very nice range day.

Naw ... Just don't don't spend all day with it . Like this last go around with CFE Pistol, I had 5 tests of 15 to chrono on Saturday, then had some 'fun'. And then 1 test on Sunday afternoon, and then more fun . When shooting a new powder a chronograph is very nice to see where your at.

[Jay24bal]

Chronos also come in handy when switching lots of powder.

I have found Varget (or maybe my gun is just super picky) to be very finicky from lot to lot. My "standard load" for my 223 bolt gun is 25.4 grains under a 69 grain SMK. But I have also have used 25.3 and 25.5 depending on the lot to get the same velocity. I would switch to a different powder if it weren't for the load working so darn well (and I have tired IMR 4064 in substitute, along with a couple others).

I find it easier to match my recipe to the lot I am using at the moment rather than get new dope. This is for a varmint match gun so by keeping the velocity the same, I know my drops are always going to the same at all distances.

I learned this lesson pretty early on in my reloading career, and now only buy powder in 8 lbs quantities from the same lot. During the shortage I was forced to buy 3-4 lbs at a time, but try not to go lower than 8 lbs at a time from the same lot.