Forster Inside Neck reamers are designed to remove excess brass from the inside of case neck walls. The Forster Neck Reamer is manufactured from high grade, wear-resistant tool steel and ground .0025" to .003" over the maximum bullet diameter. Constructed with a staggered tooth design so that brass is cur smoothly and without chatter. Neck reaming should only be performed after the cases have been fired with a full load and before the neck or full length sizing operation. Wall thickness can be measured with a tubing micrometer, but a simpler test for excess brass in the neck area is to slide a bullet into a fired, unsized case neck. If you feel interference, the neck should be reamed or outside neck turned.