First, I like and prefer Forster full length benchrest dies and have nothing bad to say about them.
Second, I have had the same marks on case necks with new Redding and other make dies.
The problem is soft brass, lack of case lube and a sharp edge at the neck shoulder junction of the die.
Clean the the neck shoulder junction of the die with a good copper bore cleaner and remove as much copper as you can.
Next chuck a snug fitting shotgun cleaning mop in a drill and apply some J&B Bore cleaner, Flitz
, automotive rubbing compound, Mothers Mag and Wheel Polish, etc to the mop. Now polish the inside of the die with emphasis on the neck shoulder junction of the die.
Once any brass begins to stick to the die or bushing it just keeps building up with the brass sticking to itself.
If you are lazy then just take the die apart and put the die body in a vibratory tumbler with treated walnut media and let it tumble all night.
I have used both methods above to clean and polish the die to remove any dirt or grit stuck inside the die that is scratching the cases.
My guess to the problem is caused by not beveling/chamfering the outside of the case mouth enough and lack of lube.
I wet tumble my cases with stainless steel media and this can peen over the case mouth. And I trim and debur the case mouth to keep this same problem from happening.
Now watch the video below and see what brand of die has the best finish polishing at aproximatly 3:03 into the video.
Below a Redding steel bushing with brass sticking to the inside of the bushing causing vertical scratches on the case neck.
Bottom line, some good copper cleaner and polishing the surface fixes the problem. And you do not need to send the die back to the factory for something simple you can do yourself.
