I figured if one wants to only neck size his brass, he should fireform the cases concentric to the chamber. But if he doens't do the concentric fireforming, doen it hurt anything? Because anyway the brass expand to fill the shape of the chamber, only there's a different brass distortion with the misaligned case.

Theory says (actually engineering says) that the bullet needs to sit in the centerline of the bore.

What prevents this more than any other factor is variation in neck wall thickness. Variations in neck wall thickness also result in split necks, and an uneven release of the bullet as neck tension varies according to the thickness of the brass in the neck.

So, you can stick the brass on a mandrel and turn the neck to a uniform thickness. Fire forming and neck sizing should get this neck centered in the bore -- in theory.

Your chamber should be concentric. The case should fit in the midline of the chamber and center the bullet in the midline of the bore. This works for bolt action guns. But you can't expect this sort of precision when full-length resizing for a "generic" chamber.

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.223 Ackley Improved Wildcat Forum:
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Pyrotek,

I get coeccentric fireforming by wrapping narrow(2 mm) widths of aluminum tape around the case head, just above the extractor groove and below the expansion ring. This tape is about .002 in thick and I wrap enough to get a snug fit. This centers the case perfectly in the chamber.

If you fireform your cases, and they have minimal variation when measured on a gauge, this still won't guarantee the you will have good, centered in bore concentric ammo. A bad seating die can take a case with virtually zero runout on the neck and give it several thousandths of runout measured on the bullet. I've seen this with some of my dies.

Also neck tension can be a factor of how hard the brass is in the neck.

John Barsness has written up some fairly good articles on ammo concentricity in "Handloader" over the past year.

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I know the next rifle will be perfect.......

just an observation.. I reloaded some UMC
brass for my .308 and with 48 grains of H380
it was shooting 3/8" groups same load new Win. brass. groups went to over an 1" now fire formed the new brass, same load, it shoots around 3/4" groups. there is a diffrence in the brass ? maybe. I have gone over the loads with a fine thooth comb and out side of a few minor things it points to the brass.
add that the brass was neck sized with a lee collet die
dave

[This message has been edited by HIVELOSITY (edited 04-25-2002).]