I have just used the FL length sizing die and bumped the shoulder, I've FL sized and collet neck sized and my groups shrunk compared to FL sizing alone, I've just neck sized and groups were worse than FL sizing, I bought Redding body die and used that with collet neck die and got consistent good groups 1/2 or less at 100, I found that the body and collet die gave the least runout, very rarely over .002 and usually right around .001, also got low run out just neck sizing but my groups were often over an inch, FL sizing seems to be more hit or miss in terms of runout, also with the body collet combo my flyers are almost gone. I kept everything the same as far as brass weight, once fired, uniformed primer pockets, trickled powder exactly, I can shoot 100 yds in my backyard and made a shed with rest I can shoot out of with heat, the body and collet die was a huge improvement for me, felt like I solved runout problems and shooting way better.