My rifle arrived today. I quickly found that the cases I made from 303 B will definitely have to be neck turned. Even with bullets sized to .408" they will not chamber.  The sized brass chambers fine without a bullet seated.
I tried my old Forester turner but the pilot is too long and only allows turning about half the neck length that needs turned.
I thought I still had it covered with a Hornady neck turner but found they only make pilots up to 35 caliber.
I have a pair of old RCBS trim pro's but no neck turning attachment.  I can find those on line but I can not find any neck turning pilots for it. Very frustrated !
The rifle looks great. I tapped a .410" diameter two drive bands into the muzzle and turning that portion of the bullet in a mic I get .403" at the low end and .409" at the high end.  If I can get cases with a .408" bullet to chamber I will be happy. 
  I think if I shorten the stud end of my Forester pilot it will seat deeper into the cutter shaft and that might solve the problem with the Forester neck turner not turning enough case neck length.  We shall see.
OH BTW I did shoot it once.  I seated a soft .408" bullet then sized the neck in a 41 mag die. That worked a charm for chambering but I imagine the bullet was squished pretty skinny.

What I have so far discovered with this rifle is that with a .408" bullet seated in a case the case neck diameter can not exceed .433" for the round to chamber.   Cases that have not been turned having a .408" bullet seated measure .440" at the location of the bullet base in the case. So for this rifle with the 303 brass I used (PPU) and using a .408" bullet diameter case necks have to be reduced in diameter .007" to chamber.
I push a slug all the way through the barrel and turning that slug in a mic I get .403" x .408" so it looks like a .408" groove barrel.
   Since my neck turning tools did not work for this case I fudged a method of getting it done.    What I did was to fully seat a long .408" bullet in the case then chuck the bullet nose in a power drill and turn the necks into a file until neck diameter to the base of the seated bullet is .433".  Then the bullet used as a mandrel is pulled the case is re-sized , primed, charged and a new bullet seated.  That method is working fine but I wouldn't want to do a large volume that way.  I made 15 cases and that is enough to get started with until I can get my neck turning tools in order.