Originally Posted by RockyMtnMT
What am I missing?
Nothing, I believe.
Bullet expansion isn't necessarily due to spin, but stability keeping it penetrating straight, point on, can allow it the opportunity to expand normally. If it turns sideways and tumbles before this has a chance to happen, obviously results will be very different.
This is very well documented for those of you who follow African DGR ballistic experiments where, especially with long monometal solids/semi-expanding bullets, twist has a HUGE effect on the outcome. Too slow a twist the bullet goes in, turns, tumbles and gives generally poor results. With a proper twist, the bullet stays straight and can even make a cup point solid expand a bit, which happens very slowly and gradually but won't happen at all if it tumbles first.
It has been a while since I followed the discussion closely, but at least a few years ago both Gerard of GS Custom bullets and Mike Brady of Northfork were very specific on the twist rates they stated their monometal solids required--not because they wouldn't stabilize flying through the air, but because the spin was required for them to stay stable and penetrate straight through the animal.
So yes, in my opinion, penetration tests with bullet RPM close to what it is when launched at max velocity from the muzzle, either by actually hitting the medium at long range or using a fast twist barrel at lower velocity will eliminate this as a possible source of error in testing.