Forster micrometer 308 die issues.

[Idaho-Lefty]

Measured a bunch of the SMK’s their lengths were all over the place. Varied by as much as .007.

Sierra MK's CANNOT be Measured "well", in normal OAL Form with, a Mike ( Huge Bullet length, Variations from, bullet Base to, Points / Tips ! ) See Jeff Porter's explanation.
Use, the Hornady CBTO Method, as Sierra MK's ( my Pointed, 107's, anyway ) were, Plus / Minus, .005
( .010 total on,.. Length ). I use, Forster Dies to, Produce TINY Groups in ALL of, my Rifles from, .22-250 to, .270 WSM.
My 6 XC with, the 107 MK's loaded, Vary only, .001- .002 on Readings of, CBTO using, Forster Dies and TIR is, .002 or, less.
CBTO stands for, Cartridge Base To Ogive and really is, the Only way to Measure, Match Kings.

 

[Idaho-Lefty]

The method of drawing out the hollow point tip will have a lot of variation even in SMK , Hornady and Nosler match bullets. Not uncommon to see .010” bullet lengths with these OTM bullets. The bullet base to ogive is typically less than 0.002” so if your seater is on the profile of the bullet and not the tip your cartridge base to ogive should be consistent.

If you are running a compressed load in a Forster you can fracture the seating cup and cause the results you are seeing.

OP,.. HERE's the Problem in, FIRST, Paragraph ^^^^^
It's,.. the Bullet,.. NOT,.. the Dies