What is an exeptable variation in case weight?

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Endeavor to Persevere

Oddball,
If we begin with the ratio of the density of brass compared to that of smokeless powder we see that it takes about 7.5 grains of brass to occupy the same space as 1 grain of powder. Then you can make a choice as to how much of a difference you want in available powder space. If you are loading to 100% of capacity then you will want to keep variations to a minimum. If, on the other hand, you are loading at 80% of capacity or less then variation is not as critical to uniformity.
I like to keep my general purpose brass at +/- 1 grain for low density loadings but for real accuracy I keep cases to +/- .5 grains. That way the difference in the pressure curve is like having less than .14 grains variation in the same volume case. With a case weight of say 200 grains that's about 1% variation.
On a side note the general purpose loads keep bullets in groups under 1 MOA at 100 yards. The best loads are about .3 MOA at the same distance (rifle loads)

The more air space you have the less difference small deviations in powder volume make - the air space acts as a buffer for the expandng gasses and modify the pressure curve.

Hope this helps

PaulS

[This message has been edited by PaulS (edited 01-27-2002).]

Yes it is very helpful. Thank you!

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Endeavor to Persevere

Some things to think about, just in case you're loading for super accuracy and getting anal about the whole process:

Neck turning will reduce the weight of the case without changing interior volume.

Powder residue in the case will make for weight variations -- and the weight is residue, not brass.

In large caliber cases this is less critical than in smaller calibers. I weigh cases and bullets for accuracy.

I wash the brass in the ol' soap, water, salt, vinegar, lemon juice mix. That removes powder residue. Be sure to decap the brass before washing with this mix. Otherwise the steel in the primer will turn the brass a pink/salmon color. Looks nasty, but doesn't affect the character of the brass.