This is for the 30-06 brass parent case. Have reloading data
but after resizing they get shorter than 30-06 length which I
think is normal.

Hornady shows a trim length of 2.484. My formed brass has been fired twice and it is still shorter than that.

Thats why I use 35 Whelen brass necked down.

Ken Waters' Pet Loads lists max case length at 2.480" and trim to length as 2.470"

Wildcat Cartridges II lists trim to length as 2.450"

I just miked some Lapua 30-06 at 2.455" and
I just miked some R-P 35 Whelen at 2.465"

In any event, there's plenty of neck to hold the bullet securely. I measure 0.400 on my 338-06 AI.

Any time you neck a case up it will be shorter than the original brass, no biggie, keep shooting it and it will grow in time...BTW I have seen this happen in the 35 Whelen also...

KEVIN,
Both the new Nosler and Hornady manuals show max case length as 2.494" and trim to length as 2.484". this is also the same length as the '06. I go along with Bear Claw on using .35 Whelen cases. The .270 Win case will also do the job and will get you closer to your desired case length. If your die is not equiped with a tapered expander button, Sinclair has them. Good luck.

There is no absolute "standard" for a custom chamber, which your .338-06 must be by definition. Only by measuring your actual chamber can you determine the maximum acceptabled case length. Cases somewhat shorter than maximum will be fine, as long as they are fairly consistent in length.