Thread: Case capacity effects on velocity

I've been weighing my target cases.

The Lapua cases are noticeably lighter than the ADI and PSD cases I used before.

So this got me thinking, if you use the same amount of powder and the same weight projectile, what effect would a lighter case (assume it has more internal capacity ?) have on velocity ?

Asked another way, if you batch your cases and projectiles by weight, would you use the heavier projectiles with the lighter or heavier cases, and why ?

May have more capacity or brass is just thinner.

Rem, Lapua and Norma cases all shoot the same with the same load in my .260
With very little velocity variances.
I get the best life out of Rem and Norma brass, go figure.
I have had major velocity effects with different brass that was similar in weight in other calibres tho.

You are going to go to shit with all this indepth analysis ebf😆



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Hehe Dave, no TV, so lots of time to read and think about these types of things.

The physics of it fascinates me... Would you get higher or lower pressure (and resultant velocity) for a given power weight and projectile weight if the combustion chamber is larger or smaller ?

Tbh I am not sure. There can be many factors that effect results and if I had to guess I would say probably less due to the same charge unless there was a marked difference.

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I'd go with higher pressure and velocity, same as if you have a longer barrel ?

Yup hamsav, nowhere near shooting one hole groups... There is this nigly little thing called wind

Found some interesting comments here : Internal Ballistics

By using a cartridge case with greater capacity we provide for more chamber volume with the same bore volume, thus the expansion ratio (see definitions above) becomes less. As a result, the powder gases have been through less expansion by the time they reach the muzzle and muzzle pressure is higher. As the muzzle pressure is higher, the average pressure along the bore is higher and the bullet has a higher velocity. There is a trade off for this. Because the muzzle pressure is higher, more energy is carried off into the air unused (as flash and muzzle blast) and thus the efficiency of the load (the getting of energy out of the powder charge) is less.


You get the same effect with seating depth, further down, less pressure and velocity.

For the same charge the lightest case will have the greatest capacity and lowest pressure and velocity, but loaded to max the lightest case will take more powder to reach the same pressure and greater velocity due to more volume.
Think of your heavy case as a standard 308 and a light case as a 30-06 max load in the smaller capacity would barely be a starting load in 30-06 and be slower yet have potential to be loaded higher and have more velocity for the same pressure.

Originally Posted by veitnamcam

For the same charge the lightest case will have the greatest capacity and lowest pressure and velocity

Tx VC, and yup Hamsav, looks like my earlier comment about seating depth was the wrong way round... found an article on the Berger blog that discusses seating depth differences for same powder charge, and seems to say the same (provided you not jamming into lands and increasing pressure that way).

More volume, less pressure, less velocity.

So now taking it to the next level, if you sort cases by weight and projectiles by weight, and your end goal is to have consistent reloads that minimize vertical issues, would you load the lightest projectiles in the lightest (greatest internal volume) cases ?

Yes, I know it's hyper anal, so humour me

Originally Posted by ebf

Tx VC, and yup Hamsav, looks like my earlier comment about seating depth was the wrong way round... found an article on the Berger blog that discusses seating depth differences for same powder charge, and seems to say the same (provided you not jamming into lands and increasing pressure that way).

More volume, less pressure, less velocity.

So now taking it to the next level, if you sort cases by weight and projectiles by weight, and your end goal is to have consistent reloads that minimize vertical issues, would you load the lightest projectiles in the lightest (greatest internal volume) cases ?

Yes, I know it's hyper anal, so humour me

Bro you need help
Off topic but I just read this, might be of intrest to you.
http://www.arcanamavens.com/LBSFiles.../Downloads/DA/

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interesting, but way way beyond me

heeelllppp !!! my brain just imploded...

Originally Posted by ebf

Tx VC, and yup Hamsav, looks like my earlier comment about seating depth was the wrong way round... found an article on the Berger blog that discusses seating depth differences for same powder charge, and seems to say the same (provided you not jamming into lands and increasing pressure that way).

More volume, less pressure, less velocity.

So now taking it to the next level, if you sort cases by weight and projectiles by weight, and your end goal is to have consistent reloads that minimize vertical issues, would you load the lightest projectiles in the lightest (greatest internal volume) cases ?

Yes, I know it's hyper anal, so humour me

If your goal is max velocity then lightest projectiles in lightest cases and find new higher max.

Minimise vertical means minimise ES velocity, don't think capacity is really relevant here as a bigger or smaller... just (and only if you are really really anal) weight sort brass and cull the extreme heavy and light cases.

Originally Posted by R93

You are going to go to shit with all this indepth analysis ebf

I think that OCD mate of his has infected ole @ebf somehow ... then again maybe he was like that all along ...

The only real way to check volume differences in cases is to measure the volume, not case weight.

Use a digital scale put the case on and tare the weight. Then add water until level with the mouth record the mass. You will need to leave the primer in. If you don't want to use water then a very fine grain powder will work.

The density of brass (134.7grains/cm3) is such that it doesn't take much brass to add up to a few grains in weight.

Ha ! The OCD reloading paid off...

Shot my first 60.nn on a 800yd target this weekend up in Gisborne. So that's 10 consecutive shots into a 255mm circle