Explaining Velocity Variation

[260 Rem]

Just wondering if anybody would like to explain the reason that some loads show low extreme spread and others do not?

[DiabeticKripple]

I’m having issues with my velocities as well.

They seem to be pretty high in ES and SD. I’m thinking it’s because I am using a chargemaster.

I shouldn’t increase powder charge and drop in velocity. Should go up a little or a lot depending on the node.

[Salavee]

Originally Posted by DiabeticKripple

I’m having issues with my velocities as well.

They seem to be pretty high in ES and SD. I’m thinking it’s because I am using a chargemaster.

I shouldn’t increase powder charge and drop in velocity. Should go up a little or a lot depending on the node.

It's quite possible that your pressures are abnormally high if an increase in charge weight yields a lower velocity. If your average velocity was over normal, that should tell you. I find it best to trickle up to your desired charge weight. That way there should be zero error. How many rounds were involved in the string ?

[Lefty-Canuck]

My charge master is very consistent. I checking it with Lyman check weights and cross reference to an RCBS balance scale.

LC

[260 Rem]

No doubt that inconsistent charge weights would result in inconsistent velocities ... that makes sense. But what needs explanation is why ... when every cartridge is loaded exactly the same ... some strings (in load development) show wide variation, while others show very low spreads?

[Salavee]

Originally Posted by 260 Rem

No doubt that inconsistent charge weights would result in inconsistent velocities ... that makes sense. But what needs explanation is why ... when every cartridge is loaded exactly the same ... some strings (in load development) show wide variation, while others show very low spreads?

If the cartridges were all absolutely identical I'm thinking it would have something to do with barrel fouling, chrony, or temperatures (barrel or ambient).Other than that, no idea. I can't recall ever seeing that happen.

[DiabeticKripple]

I was shooting 3 shot groups with the 7mmRM and 5 shot groups with the 223 Rem. I don’t have my data in front of me but I had some ES in the triple digits, and some single digits with the 7mm.

I also had terrible groups with it and wasted a bunch of $$$$. Was averaging 1.75 moa. 223 wasn’t shooting great either but I might be able to find a load in the bunch to work with.

[260 Rem]

Originally Posted by Salavee

If the cartridges were all absolutely identical I'm thinking it would have something to do with barrel fouling, chrony, or temperatures (barrel or ambient).Other than that, no idea. I can't recall ever seeing that happen.

If that is the case every different string loaded in load development would have low ES.
EDIT::: Perhaps I am not framing the issue with clarity. No, it is not a problem I am having, but rather a search for an explanation. Here goes - -
Load development for a 308 >all same brass, trim length, primer, powder, bullet, seating. 5 rnds charged with “X” grs of Varget / 5 rnds with “X” plus 0.5grs. / and so on. (Only difference is charge weight).
Each string is chrony’d and typical ES is about 20FPS
But, one string has ES of only 5FPS and groups well so it is declared “the load”.
The question is, why doesn’t every string (5 shots all loaded identically) have small ES?

[hogie]

Frequency. Everything is just right.

Experience it with vibrations in vehicles. 105 kph there is a vibration 2 or3 kph one way or the other it's gone. Something is not as it should be. Nothing is really wrong but enough to cause an issue. Small issue usually.

No different for load development.

Stars align and good groups.

I have a semi automatic rifle that is consistent 1.5 moa out to 300m. Poor ES and SD but consistently grouping 1.5moa. Have not went longer than that.

I go by groups not chronograph .

So far I have been lucky with load development going just by groups. I do use a chronograph but not what I depend on. Great numbers with poor groups don't work.

[Xbolt7mm]

Originally Posted by 260 Rem

If that is the case every different string loaded in load development would have low ES.
EDIT::: Perhaps I am not framing the issue with clarity. No, it is not a problem I am having, but rather a search for an explanation. Here goes - -
Load development for a 308 >all same brass, trim length, primer, powder, bullet, seating. 5 rnds charged with “X” grs of Varget / 5 rnds with “X” plus 0.5grs. / and so on. (Only difference is charge weight).
Each string is chrony’d and typical ES is about 20FPS
But, one string has ES of only 5FPS and groups well so it is declared “the load”.
The question is, why doesn’t every string (5 shots all loaded identically) have small ES?

I trickle every round and actually use tweezers to put in single grains ( i use a tubular powder so some of the grains are large or small) to get as close to exact measurements as i can get. My over all length is within half a thousand.My fps will still vary but around 20- 50 fps. Short of measuring all my lead and separating into batch’s, i dont know what else to do. Factory ammo that I have chron’ed varyed by over 200fps. Pulling the lead and measuring powder on factory stuff showed over 1.0 difference between rounds and I would say mine are less than.01 difference or less. It’s a bit of a mystery to me but how perfect can it get, im not sure. There are others here way better at reloading than I am and im sure they will all chime in to help but I know if you dont find a level that is acceptable to you then it may drive you nuts trying to achieve perfection.

[Salavee]

In that case I would be inclined to shoot a larger string ... 15-20 rnds, of each load to verify. I don't think the results of only five shots are enough to base a decision on.

[Dean2]

First to Diabetic cripple - Increasing load velocity drops, back way off and start again. you are FAR over pressure. When developing any load you will get to a spot where velocity plateaus, 2 or 3 loads a half a grain apart have almost exactly the same velocity, already likely over pressure, then starts to drop, FAR over pressure.

Rem 260. ES is a function of primer, powder, case capacity, seating depth, concentricity and neck tension. Primer brissiance can cause wide variations is SD, particularity in smaller capacity cases. Too hot a primer, or large variations in the force a primer produces can both cause high SD, ES. Try Benchrest Rifle primers in two or three makes and make sure the primer pocket and hole have been uniformed. This is the reason many of us use Small Pistol Primers in 22 Hornet to reduce ES, while others crimp the hornet while using the stronger Small Rifle primers, to ensure the bullet has not left the mouth of the brass till the primer is completely lit off.

Case capacity should be obvious as should seating depth. BR shooters tend to shoot with bullets just touching the lands as a way to even out pressure. It also aids in coecentricity.

Load density. The closer to a full case one gets the lower the SD should be, all other things being equal. The more uniform the powder kernels the lower the SD typically should be. Some of the better powders for low SD is 8208, Benchmark etc, due to their consistent kernel size but easy to ignite extruded makeup.

Neck tension is less well understood, Long Range BR shooters turn necks to ensure identical neck tension on annealed brass as this will affect ES. Hunting loads are often crimped as a simple form of making neck tension the same.

If you are loading all of your ammo to exactly the same specs, except for powder charge, next time you shoot a group with an extremely low SD, go back and load 40 cases with the same load. Shoot these and see if the low SD holds. If it does you have found the perfect balance of the above factors. My bet however is the low SD will not be as tight as it was for 3-5 rounds. The primary difference will be the cases, not variations in powered charges. Compare real measured capacity variations even if the same weight for every case along with primer hole differences and neck tension/annealing.

[DiabeticKripple]

Loading 308 Winchester in my BCL 102 semi auto, Varget 42.0-45.5 with a 168gr SMK velocities were as follows

2438 42.0
2495
2438
2515
2489
2547
2567
2511
2538
2587
2561
2530
2593
-Error-
2633 45.5

And I stopped there as pressures were high and brass was starting to show pressure signs.

[West O'5]

Sounds like possibly a variation in actual ammo temperature,or in other words "cooking your load" prior to firing.
If you are firing a 5 shot string and closing your bolt prematurely on a hot chamber(say 30 seconds prior to firing for example) by the 5th shot you will easily be raising the ammo temp substantially,and enough to create triple digit deviations in many loads.
Try NOT chambering a round in a warm chamber until you are as close as possible to ready to fire,and/or try to develop some consistency in that respect,ie;10 seconds max from closing the bolt to firing.

[Salavee]

Originally Posted by DiabeticKripple

Loading 308 Winchester in my BCL 102 semi auto, Varget 42.0-45.5 with a 168gr SMK velocities were as follows

2438 42.0
2495
2438
2515
2489
2547
2567
2511
2538
2587
2561
2530
2593
-Error-
2633 45.5

And I stopped there as pressures were high and brass was starting to show pressure signs.

Those loads are well within the normal .308 Win /168 pressure/velocity range.
My guess, and it's only a guess, is that your chronograph readings could be a little erratic. The pressure indicators regarding your brass are what you have to go by. Back off to where they are normal and call it a day for that rifle..

[32-40win]

It can be amazing how much difference just switching brands of primer can make. I had a few incidences where I did switch out primers just to check if anything would improve, and it did, and a few where nothing happened. After that, then it gets to the nitpick stuff like Dean outlined.

[Dean2]

Originally Posted by DiabeticKripple

Loading 308 Winchester in my BCL 102 semi auto, Varget 42.0-45.5 with a 168gr SMK velocities were as follows

2438 42.0
2495
2438
2515
2489
2547
2567
2511
2538
2587
2561
2530
2593
-Error-
2633 45.5

And I stopped there as pressures were high and brass was starting to show pressure signs.

Whatever load you were using when u first hit 2515 is the top load I would use in that gun, with that combo of components. The load that hit 2547 is the beginning of the plateau and what I would call max pressure. Reasons for max pressure at well below published loads, SMK is a long bullet and uses up a lot of case space, may even be compressing the load at 43.5 to 44. The Boat tail makes it even worse as far s being a long bullet. A 168 Flat Base would likely produce lower pressure and higher velocity in the same loadings.

Don't know what kind of brass u are using but my bet is it is one that has a lower internal volume, WW, federal, Lapua or Military (in descending order of capacity. Rem and Hornady Match usually has 3-4 grains more capacity than WW.). Semi autos tend to have short barrels, which reduces top vel, most test barrels are 24-26", and the nature of their action operation often results in hitting max pressure at lower velocities.

Especially with a semi you do not want to stick a case. They are a real problem to get out.Make sure the load is mild enough even in real hot weather. Hope this helps.

[DiabeticKripple]

Originally Posted by Dean2

Whatever load you were using when u first hit 2515 is the top load I would use in that gun, with that combo of components. The load that hit 2547 is the beginning of the plateau and what I would call max pressure. Reasons for max pressure at well below published loads, SMK is a long bullet and uses up a lot of case space, may even be compressing the load at 43.5 to 44. The Boat tail makes it even worse as far s being a long bullet. A 168 Flat Base would likely produce lower pressure and higher velocity in the same loadings.

Don't know what kind of brass u are using but my bet is it is one that has a lower internal volume, WW, federal, Lapua or Military (in descending order of capacity. Rem and Hornady Match usually has 3-4 grains more capacity than WW.). Semi autos tend to have short barrels, which reduces top vel, most test barrels are 24-26", and the nature of their action operation often results in hitting max pressure at lower velocities.

Especially with a semi you do not want to stick a case. They are a real problem to get out.Make sure the load is mild enough even in real hot weather. Hope this helps.

I was using 1x fired Hornady Match cases that I bought off CGN and FL sized and swaged the primer pockets. I also used Winchester Large Rifle Magnum primers.

Load was definitely compressed pretty good.

[Dean2]

Between the mag primer and the compressed load9No open case capacity) I think you have your answer as to why you reach max load so early.

[Salavee]

Originally Posted by Dean2

Between the mag primer and the compressed load9No open case capacity) I think you have your answer as to why you reach max load so early.

X2 . Varget is not your powder. Something like Blc2 would serve you much better.

[skywrench]

This is not likely but it did happen to me - my beam type powder scale was faulty. I had been experiencing some irregular velocities such as a random 80 - 100 FPS spikes. I was using the Ohaus equivalent to the RCBS 505 and trickling to the last granule of powder to try and be as consistent as possible as well as performing the remainder of the usual accuracy tasks in terms of brass prep, bullet seating etc. but still had those annoying velocity spikes causing flyers. I reviewed my process over and over to see what I was doing wrong and in doing so I took a look at the beam pivot and found that it was very rough (pitted) which could most definitely be an issue. I purchased a digital scale and borrowed another and then weighed out loads as accurately as possible on the Ohaus and then compared on the two digitals. The digitals read the Ohaus scaled loads very close to each other (less than 0.1 grain differential - both scales are advertised to have 0.05 gr resolution) however they both showed the same load from the Ohaus with as much as 1.1 gr differential high and 0.9 gr low from the Ohaus for 2.0 gr total variance. I expect this to be somewhat rare (scale is 40+ years old and well used) but it did help sort out my issues.

[Dick284]

How accurate is your chronograph?
Within 1%?
Within 2%?

What’s 1% of 2500fps? Yup 25fps!

Does the load shoot?
Can you get consistent groupings a variety of distances.
1/2”@100
1”@200
1.5”@300
And so on.

Then start turning necks, annealing, and all the rest.


Quite chasing numbers!