Please explain

[smokey66]

i need someone to try to explain the science involved here. i was working on some 30-06 loads the other day. the best results came from 57 grains imr 4350 with 165 gr Nosler partition oal 3.360. i was using a mixed batch of cases, winchester and federal. i noticed the federal cases were creating 50-60 fps more than the winchesters. i then weighed the cases and found the federals to be approx 15 grains heavier than the winchesters. can someone please explain this to me. thanks....

[Thomas Krupinski]

I think what you will find is that the internal capacity of the Winchester brand cases is a bit larger than the others.  This is probably due to the thickness of the brass.  That will effect the pressure and consequently an extremly slight varriation in performance.

[gwindrider1]

The heavier cases, which are giving you more velocity, have thicker case walls.  With the same powder charge as the other cases, you are getting more velocity because the thicker case walls of the Federal cases result in higher pressure.  Smaller combustion chamber area with a given load=higher pressure.  If you measure and weight the water volume of the various cases you are using, you will be able to see the difference.

This is neither good nor bad, just stick to one brand of case for that rifle.  All cases are not the same, and there are manufacturing differences within the same make of case.  Some handloaders go so far as to weight their cases, and sort them out according to weight.  Unless you are after a benchrest level of accuracy, that really isn't necessary.  

So now you know!  Good shooting to you, and be careful out there!

[smokey66]

thanks guys. that all makes sense. this was actually a friends hunting rifle,but it seems that we keep "raising the bar" the more we learn the smaller the groups get. sure, a gun that shoot's 2 inch groups will kill a deer just fine, but during the off season we do a lot of paper punching and with all that practice comes a lot of satisfaction....