Something else I thought about - one of the first lessons in Precision Measurement.
SuzukiRay knows EXACTLY what I'm talking about with this one, too. <thumbsup>
The very first thing you need to suspect is the measuring equipment - verify it, and know how to use it. When measurements come up different, you don't suspect the part - you verify the equipment, very first.
Are you using a digital vernier caliper to measure your COAL? Are you practiced in applying the correct pressure to the thumbwheel each and every time? Easy way to check this is with a verification piece that is a (machined) known length. Say the thing is precision machined to 1.750" - the length of a trimmed 5.56 case. You measure that known piece, and see what you get. If you get different numbers that what that verification piece is, then you're measuring wrong.
I'll get pics in here tomorrow of what I'm talking about. You can buy a HF caliper set that has these verification pieces in it, and it's cheap, and you can get your technique down.
Don't doubt your die set yet, brother. <thumbsup>