I've tried seating .30 plain base, with a Lee sizer, and had nothing but problems.
Lots of the time the check would tear.
i did polish out the die. Chamfer the opening.
I got some 9mm plain base checks and instead of using the Lee sizer, I used the Lyman lube/sizer.
Worked great.
I would lube/size the boollet first, then put the check on, and push the boolet down just enough to size the check.
Not all the way down like when lube/sizing. Just enough to size the check.
Haven't tried it with the 30 boolets yet, but I think it would work the same.