Weighing cases to make volume and velocity constant.

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joeb33050 posted this 2 weeks ago

 

Weighing cases to make volume and velocity constant.

 

A 223 Rem case, full to the top with Titegroup, holds 24 grains of powder.

 

The case, filled to ~ 1a quarter inch from the top with Titegroup, holds 22.3 grains of powder.

 

24 – 22.3 = 1.7 grains.

 

 A 22-250 case, full to the top with Titegroup, holds 36.3 grains of powder.

 

The case, filled to ~ a quarter inch from the top with Titegroup, would hold 36.3 – 1.7 = 34.6 grains of powder.

 

36.3/24 = 1.51

 

34.6/22.3 = 1.552

 

 223, 40 gr. bullet, 5.5 Titegroup, 2063 fps

 

22-250, 40 gr. bullet, 5.5 Titegroup, 1857 fps

 

2063/1857 = 1.11

 

1857/2063 = .90

 

@ 5.5 grains of Titegroup, 223 vs. 22-250; volume decrease ~ 66%; velocity increases ~ 11%

 

223, 40 gr. bullet, 8.5 Titegroup, 2678 fps

 

22-250, 40 gr. bullet, 8.5 Titegroup, 2418 fps

 

2678/2418 =1.107

 

@ 8.5 grains of Titegroup, 223 vs. 22-250; volume decrease ~ 66%; velocity increases ~ 11%

 

Between 5.5 and 8.5 grains of Titegroup, 40 grain bullets, velocity varies, inversely, about 1% for each 5% change in case volume.  

 

How does case volume vary with case weight?

 

A 223 case weighs 96 grains, a 22-250 case weighs 159 grains.

 

Case weight and volume are:

 

223         24/96 = .25, 22.3/96 = .23

 

22-250   36.3/159 = .23, 34.6/159 = .22

 

223 and 22-250 cases have internal volume of about 23% of case weight.

 

Case volume changes, (inversely), about 23% as much as case weight varies.

 

A case weight goes up 5%, internal volume goes down ~ .23 X .05 = .115 = 1.15%.

 

As internal volume goes down 1.15%, velocity goes down .0115 X .2 = .0023 = .23%

 

5% of a 223 case ~ .05 X 96 = 5 grains.

 

5% of a 22-250 case ~ .05 X 159 = 8 grains.

 

Why do we weigh cases?

 

joe b.

 

 

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BHyett posted this 2 weeks ago

Weighing cases was presented to me forty years go as the key to accuracy. by my uncle who was fanatic 600-yard and 1,000-yard shooter. He stated that 6 grains of powder occupied the same volume, therefore he bought one thousand cases and sorted them into groups with +/- 1.5 grains.He awas also the first person I Know who promoted case-neck turning to even wall thickness.

I had a .222 Remington 788 that shot consistently and experimented with the same approach. I was never able to establish a correlation between accuracy and the weight of the case, .

My thought si this this is covered by a reloader so fanatic that he does everything well and sees this as a possible influence. As we gain in epe\perience, we do not consider that we are also improving our technique, wind-reading skills, reloading consistency and other factors.

Case-neck tuning is a factor I have found will eliminate flyers. riing .  

Country boy from Illinois, living in the Magical Pacific Northwest

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John Alexander posted this 2 weeks ago

Joe,

Nifty approach to showing why weighing cases is unlikely to improve accuracy.  However, I believe the case you are making is even stronger than what your numbers show because of a faulty assumption.

Your shooting and comparing the volume of the 223 and 22-250 has indicated that: "velocity varies about 1% for each 5% change in case volume."

However, your contention that "+5% case weight = -5% case volume" isn't true.  A +5% in case weight = a -5% in BRASS volume.  But case volume is made up of brass volume + air volume and air volume is many times larger than the brass volume. So a given percentage change in brass volume results in change in case (brass +air) volume much smaller than 5%.

If the air volume is 20 times the volume of brass, the change in velocity for a 5% change in brass weight would be about 1%/20 or .05% for a 5% change in weight.  

John

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Ross Smith posted this 2 weeks ago

All that case weighing I did in another post suggests that 5% difference in case weight/volume is negligble. But there are cases out there that are more than 5% different, much more. The 5% is very easy to achieve even for novice plinker shooters. If you are looking for achieving a world record, that's a different story.

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joeb33050 posted this 2 weeks ago

Joe,

Nifty approach to showing why weighing cases is unlikely to improve accuracy.  However, I believe the case you are making is even stronger than what your numbers show because of a faulty assumption.

Your shooting and comparing the volume of the 223 and 22-250 has indicated that: "velocity varies about 1% for each 5% change in case volume."

However, your contention that "+5% case weight = -5% case volume" isn't true.  A +5% in case weight = a -5% in BRASS volume.  But case volume is made up of brass volume + air volume and air volume is many times larger than the brass volume. So a given percentage change in brass volume results in change in case (brass +air) volume much smaller than 5%.

If the air volume is 20 times the volume of brass, the change in velocity for a 5% change in brass weight would be about 1%/20 or .05% for a 5% change in weight.  

John

Right, John, Thanks. Put up a revised paper.

joe b.

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Ken Campbell Iowa posted this 2 weeks ago

simple ken here ...

i like math problems ... mostly because when god handed out * math * cells i thought he said * bath * and so i ducked out of that line ...

so after reading the  joeb-johna conversation i have spent hours writhing in pain ... feeling like a monkey playing with a rubik's cube ... 

...air density ... specific gravity ... volumn of average of  mass relative to acceleration ...  philosophy of structured approach to quantum considerations of spectrum analysis in the narrow range of 1 to 2 valence exhibitions ...

owch ...... so how about:

 

since brass weighs about 10 times as much as gunpowder .... for every 10 grains of brass added inside the case, it displaces 1 grain of gunpowder ...

worry about per centages later ... my powder scale isn't marked in per centages ...

thanks joeb and johna ...

ken

... and to atone for my guilt ... i sorted my brass by mfg .... and my 222 groups got better than mixing brands  .....  

 

 

 

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RicinYakima posted this 2 weeks ago

Even I, who loves to fuss with his ammo, quit weighting brass years ago. If they were from the same lot, turned the necks to somewhat close thickness, and kept the necks stress relieved, they shoot good enough my '03 couldn't tell the difference.

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