SAC bullet comparator question

[CCamp88]

When I purchased my comparator set from SAC I got a little confused about the bullet comparator inserts. Im starting out reloading 6.5 prc, 270 win, and 300wm. When you pick out the bullet comparator inserts you get the option of .30, .30 straight, .30 solid. When I spoke to SAC I told them the 6.5 and 270 were getting Berger elite/classic bullets and I hadnt ordered anything for the 300wm yet. They suggested I get the 6.5, 270, and 30. straight inserts. Can someone explain to me the difference in the inserts of the same caliber like youre trying to explain it to a fella from Alabama?