resizing new brass

Depending on the caliber your MAY need to resize. The new brass MAY not be perfectly dimensioned for your gun. After it is formed it is polished. It MAY get deformed during the polishing process. Some calibers are thicker or thinner than others. For example, .45 ACP I would not worry about it as it is fairly thick. .44-40 would need to be sized as it is very thin and easily deformed. The resizing should not be difficult as the brass is very close to proper dimensions already.

Is it "necessary"? No. Is it advisable? I think so.

thanks guys now I don't feel so dumb. I bought new star line brass for my 45-70 and 41 mag. and was arm weary by the time I sized a hundred cases.

It is completely unnecessary to full-length size new cases for almost all firearms. (You don't think that the ammunition manufacturers do this, do you?). The exception would be a custom-barreled benchrest or match rifle with a very tight chamber.

If your new brass is boxed, so that the cases are separated during shipment, no sizing at all is necessary. If your new brass is bulk-packaged (i.e., loose in the box), it is good to run the necks through the expander ball to round out any dents or eccentricity in the case mouth that may have occurred during shipment. However, there is no need to full-length size.

Most of the time you can bet your life that you are going to have a few bent necks. For this I go and resize first no matter what.

I always do but I'm not sure why

When you resize new brass, all you are doing is uniforming and making sure that the necks aren't dented.

I say that it's necessary.