More on headspace - why does gun shoot better?

[Lilswede1]

my CZ 527 Varmit .223 seems to shoot a very consistent 5 round group when I resize the case to -.008" to -.010" shoulder length.
Using the Precision Mic Gage a fireformed round from afore mentioned rifle is .003" to .004" below the "0" mark on the gage. I normally resize cases to .006" below "0", they chamber and shoot nicely.

I didnt realize the die was out of adjustment and resized about 50 military brass (all brass is LC) to .010" to .012" below the "0" mark on gage.
I loaded 5 rounds to "fireform" them and was amazed at the improvement in accuracy.
All other factors were the same. Powder, primer, OAL (use Hornady's gage).
Twice more I have gone to the range with 5 batchs of 5 ea. test rounds in small zip lok bags. 2 bags with the shortened brass and 3 bags with brass resized to fit my chamber. Each time the bags of 5 rounds were all shot using a blind method where I did not know which bag was which. Both times the shortened cases were more consistent and accurate (1/4" minus) then the longer ones (3/4" minus).
Does anyone have an idea why this is happening?

[dardascastbullets]

To be honest I lost you in your text concerning the dimensions that you were stating. However, the answer to your general question as to why your rounds are shooting more precisely lies in the fact that cases that have been sized to 'closer' to the chamber's dimensions does not afford the cartridge to move forward upon the firing pin striking the primer. As opposed to a case that has 'excessive' headspace whereby the cartridge will move more upon the firing pin strike. The less the cartridge moves the tighter the sytem and the more precise will be the result.

[wncchester]

I followed your description. For your question, I have only ONE answer: "MAGIC!"

Dardas gives the normal, "conventional wisdom" answer. But YOUR experience runs counter to that! So, your experiece simply proves the adage that each rifle is a law unto itself, if we want to know if something will work better or not we must try it because there is NO WAY what you stumbled onto should work the way it has.

Meaning, now I got to go do some more experimenting myself to see it it works for me too! Oh well, life's hard, ain't it?

[Brian Pfleuger]

Forgive my ignorance if this suggestion has no basis in reality as I know next to nothing about these things, but I thought that it might be possible.

Are your bullets touching the lands at the OAL you are using? If so, that in itself would prevent the round from being forced forward by the primer strike, regardless of case/shoulder length.

If the case is shorter but the OAL is the same then the bullet must be farther out of the case. Farther out means lower pressure, right? Which would effect the "time-in-barrel". Closer to an optimum muzzle exit time?.... http://www.the-long-family.com/OBT_paper.htm

[FrankenMauser]

The extra expansion of the case (seen here as lengthening) allows pressure spikes to be 'smoothed out', and give the projectile a nicer launch.

[amamnn]

What Frank says can certainly be true. You may find the effect on accuracy to be transitory, however. As you work the brass it will become less elastic and the expansion will be limited which will change the harmonics of the vibrations, changing the point which the muzzle has reached in its travel when the bullet exits the barrel. Since you can't have any control over how each case hardens, bullets exit on different trajectories. Annealing fairly often can mitigate the effect.

A better cure is to form the brass in such a way as to limit the amount of expansion, controlling pressure by bullet seating and/or powder charge, or even bullet slection, as Dardas mentioned.