Mike, when I have a custom barrel chambered, the gunsmith always has to cut off 1" on the muzzle because the barrel maker laps the barrel from the muzzle end. So, I have the gunsmith run the reamer into that 1" section up to the depth of the shoulder only. This little tool used to be called a neck checker, if the round goes into the neck checker, then there is enough clearance on the neck. This "neck checker" can also be used to measure the shoulder length(some would call this headspace length) using a dial caliper. When the barrel is brand new, the neck checker can also be used to measure the seating depth on a brand new barrel.
Over on accurate shooter dot com in the gunsmithing section, there is a nice discussion on reamer design, relating to the dimension on the web that gives people fits, "clickers" on the bolt lift.
Mike, CIP spec reamers are larger in dimensions than SAAMI As a rule of thumb. Lapua brass will not work well at all in SAAMI spec chambers in 223, 22/250 & AI, all cases based off of 243 that I have found. I had major headaches with Lapua, RWS, Hertenberger, and Norma brass, until I got my reamers/brass/dies figured out.
Reamer makers have a +/- of .0005 on tolerance of neck dia, web dia, etc. I have found that JGS and Manson are very reliable regarding what you get vs. what you ordered.