Calculation of freebore for ordering a reamer

[Cassidy]

Erik, on an earlier post you indicated that for a particular bullet, freebore can be calculated as follows: [length of bullet bearing surface] minus [case neck length minus 0.050"]. You indicated that this will allow the bullet to jump 0.020 (if needed) and also have 0.030 between the heel of the bullet and the case neck-shoulder junction. My assumption is these 0.020 and 0.030 dimensions are to allow an adequate bullet seating depth range when testing for optimal bullet seating dept. What is the reason for the 0.030 dimension between the heel of the bullet and the case neck-shoulder junction; is it only to allow a greater powder load (id needed), or is there another reason?

 

[dbramley]

This is where a "donut" forms if it is going to form one. A narrowing inside the neck. You don't want you bullet touching it.