velocity changes with same charge

[finnaddict]

I have been noticing large changes in velocity while using the same charge.

SAKO M/28-30 (0.3082)
7.62x53R
Lapua cases
Lapua D46 185gr
WLR or GM210M primers
51.0 grs W760
2.953 coal.

Avg velocity is 2535 with a SD of under 20.
However out of 10 rounds upto two may have changed in velocity by as much as 70 fps.

I cant figure this one out. Any help would be appreciated a

 

[ShootDots]

There could be a myriad of reasons BUT the first one that comes to my mind is your chronograph. On B-R-I-G-H-T sunny days, mine will show something like 2932, 2927, 2984, 2933 and 2929 for s 5 shot string. I have ZERO doubt in my mind it is my PACT Chronograph... Now if you're SURE it is NOT your chronograph, the list is almost endless as to why you may be getting e.s.'s like that... But my money would be on the chronograph...

 

[Lapua40X]

What chronograph are you using?
Is it under cover (e.g. in the shade) or exposed to open sunlight?
Is it a bright sunshiny day or is there cloud cover (overcast)?
What's the distance from muzzle to chronograph?
Is each round centered on the chronograph detector or are the rounds passing the detector right or left of center (or alternating sides) or at varying heights?

 

[jlow]

The unfortunate truth about MV is constant powder weight is only the start of getting small SDEV. There are so many things that can affect it like primers, case volume, seating depth, neck tension, temperature, whether you are on an accuracy node so that some bullets are not fling by the barrel. Welcome to the search for that single digit SDEV....

 

[finnaddict]

This is a borrowed chronograph that I am using and I am not sure who the maker is. It is set up outside in the open. I last used it yesterday and it was a mix of sunny, overcast and a period of rain. Usually when I have used it, it is sunny. The chrono is set at 10m and it is very possible I am not centering the shots each time simply because I use more than one target that are set up side by side. I have observed that between higher and lower velocities there is a change in impact so this leads me to believe maybe the charge weights are different and not just the velocity. I use a digital scale as a primary and a beam scale to cross check. Yesterdays purpose was to find a better seating depth because I was getting anywhere between 1 and 3 fliers at 200 yds with which I have not had issues with in the past. I did manage to find two accuracy nodes that tightend up right at 1" - 100 yds (10 round strings). Single digit SDEV's would be nice.

 

[langenc]

Do you really need two scales? Do you find a variation between charges when weighed on one and then checked on the other? They should agree to less than 0.1 gr.

As to the variation-some think chrono? I dont use one so I dont know.

You could use those two scales and weigh 10 or 20 cases and get them to within couple tenths of a grain. Assuming they are near same length, the main other reason for much weight variation would be wall thickness. Wall thickness variation would mean case volume difference and that could be one other cause of your variation, assuming all else equal.

I have noted that when using recycled range brass that Federal is heavier than most other, generally. From that I assume that Fed volume is less. Put all the Feds in a group by themselves and see, if you have/use that type brass.

 

[finnaddict]

The beam scale is just for spot checks to ensure uniformity. If there is a change in charge weight it is right around .1 like you said. The brass is weighed and separated with no more than 1.0 grs difference and the cases are all the same length. I have thought too that maybe case volumn could be the culprit. Federal brass is not a option because they are not available in my caliber. I use lapua brass exclusively. Winchester, PPU and even Norma don't last for more than 6 reloadings if even that. With lapua brass I get around 18. The only thing federal I use are the GM210M match primers when I can find them.

 

[jonbearman]

How long between shots are you shooting the rifle?One thing you could try is culling the cases that shoot odd and then collect them all and mark the speed on them and reload about 5 of them and retest to see if you are getting the difference in speed and poi. If this checks out then keep the ones that shoot slow together and reload once again and increase the powder charge to see if it makes a big difference,it should. From this you learn to cull the cases that are giving you a headache and use them for the slightly hotter loads.You can still shoot them just under different conditions.If they shoot all over the place then use those cases for foulers.

 

[finnaddict]

How long between shots are you shooting the rifle?One thing you could try is culling the cases that shoot odd and then collect them all and mark the speed on them and reload about 5 of them and retest to see if you are getting the difference in speed and poi. If this checks out then keep the ones that shoot slow together and reload once again and increase the powder charge to see if it makes a big difference,it should. From this you learn to cull the cases that are giving you a headache and use them for the slightly hotter loads.You can still shoot them just under different conditions.If they shoot all over the place then use those cases for foulers.

There is at least one minute between record / practice shots after the foulers. The foulers are two minutes between each over a ten minute period. Yeah. I never thought of pulling the flier shot cases off to the side and then test them separately and if they do test out to be odd, put them to use for foulers which I already do but with the older more often shot cases.

Thanks for this recommendation. It makes sense.

 

[bigedp51]

Your shooting a rifle dating to the 1930s, it could have a frosted and pitted bore from corrosive primers. What does copper clinging to a frosted and pitted bore and a bullet passing over this do to chronograph readings?

I do not mean to rain on your parade but I collected milsurp rifles for a very long time and the only perfect bores I had were on unissued rifles that never fired corrosive ammunition. Have you ever inspected the bore with a bore scope?

The average Russian noisy maggot is counter bored and has a badly pitted bore from war time use. What condition is your Finnish SAKO military barrel?

Below, one shot of foam bore cleaner in a 1943 British .303 Enfield after firing 50 rounds that has a good bore for its age and is only slightly frosted. The blue means it had a lot of copper in the bore.

 

[finnaddict]

Your shooting a rifle dating to the 1930s, it could have a frosted and pitted bore from corrosive primers. What does copper clinging to a frosted and pitted bore and a bullet passing over this do to chronograph readings?

I do not mean to rain on your parade but I collected milsurp rifles for a very long time and the only perfect bores I had were on unissued rifles that never fired corrosive ammunition. Have you ever inspected the bore with a bore scope?

The average Russian noisy maggot is counter bored and has a badly pitted bore from war time use. What condition is your Finnish SAKO military barrel?

Below, one shot of foam bore cleaner in a 1943 British .303 Enfield after firing 50 rounds that has a good bore for its age and is only slightly frosted. The blue means it had a lot of copper in the bore.

True. It is a 1932 dated barrel and the rifle is in near new condition including the bore. The bore showed signs of no use because tooling marks were visible. I scrounged up my last cent to purchase it mostly for that reason. The chamber cast came out clean and shows no throat erosion. Not counter bored either. By no means are you raining on my parade. I have had bores in far worse condition so I know where you going with the pitting. No I have not bore-scoped it as of yet but that is a step in the works. To date the round count is under 600 using medium charges.







This is the chamber end showing the beginning of the leades.

 

[bigedp51]

After seeing what great shape your SAKO M/28-30 is in the only mistake you made was not burning Vihtavuori powder and keeping it totally Finnish. ;D

And 99% of the people reading this posting here do not truly understand what a fantastic rifle you have pictured.

 

[Tim Singleton]

Are the flyers you mention the same shots that show the big velocity difference?

As another guy said mark those pieces of brass. Don't load them the next time you shoot. See if that makes a difference

 

[GSPV]

If you are shooting at different targets through an optical chrony, you should expect it to change things.

Repeat the test. Shoot under the as identical as possible conditions. Shoot on as identical as possible path through the chrony gates.

If you have access, you might use both a Magnetospeed and optical chronys to crosscheck the velocity data.