Damzur is right,c get your self a 5/16-36 tap ( special thread you will have to buy it from a machine tool supplier, I got mine on-line) and the correct drill size for the tap. Its easy to do. I full length size my case and then take my dremel tool and a fine cutting wheel and cut a slot in the case neck down to the start of the shoiulder. the tension is perfect. You may have to de-burr the edge with a small file. You can make any number of modified cases when you need them.
IMHO all reloading benchs should have a dremel tool on it. I would be lost and hamstrung without mine.
If you have time to do it twice, then you have time to do it once right and put your name on it