Jacko - I am delighted the chap from the other forum has offered to mentor you in casting & loading for this rifle.

I prefer not to have even .005" over loaded ammo diameter, but that is normal. Many factory guns are larger than this- even up to .008" to .010" over. WAYYYY too big - but that's a factory gun for you. Then, the dies squeeze it way smaller then need be, then the expander bullet opens it up - a LOT o brass working.

Search around and find the .303 brass with the thickest necks you can. Then, modify the die to a 'button' style sizer for the neck area, that merely neck sizes to the correct size - no expander used.

The die will have to be annealed for any work to be done to it - they are TOO hard as they come to you. It this sounds complicated, then perhaps you could mention it to your new friend - he might know I'm referring to.

Is the chamber the .35/303IMP or standard, merely necked up version?