The open tip does two things. It does create a small pressure bubble and mostly it shifts the bullets center of gravity rearward.
It also allows the bullet be longer for its weight than would otherwise be possible. This increases the BC and increases the possible range.
The boat-tail lets the passing air close in behind the bullet with a reduced amount of drag. I flat base tends to create a vortex behind the bullet which induces drag like the tail-gate of a pickup truck.
Other bullets like the Nosler Accu-bond and Ballistic tip are also made with sealed tails as the nose is the last part being finished. They simply have a plastic nose cone placed into the nose cavity area.
Sierra holds most of the target accuracy records due to their superior quality control as well as the bullet design. Years (decades) ago they figured out the best ratio to use in their jacketing material, so the bullets have a good gas seal while having limited drag within the bore.
They also pioneered the checking process to make all of the match bullets super close in weight and concentrically,.
The Ballistic Coefficients of a bullet change at different velocity ranges.
Here is how a 142 grain, 6.5mm (264) Sierra Match King changes.
595 @ 2850 fps and above
.580 between 2850 and 2400 fps
.575 between 2400 and 2050 fps
.550 @ 2050 and below
Now here is the 140 grain SMK. It is only slightly shorter.. BUT, look how much the B.C. change has become.
.535 @ 2800 fps and above
.526 between 2800 and 2000 fps
.521 @ 2000 fps and below
Here is a photo showing how much the bullet lengths can vary. This really makes a big difference when different rifling twist rates are applied.