The simple explanation for how the D-Vise reduces group sizes is that it ensures the bullets are well-guided at the moment they exit the barrel.  We know what you’re thinking:  Rifle barrels already guide the bullets well because the bullet is forced into the rifling and is tight in the bore.  It’s an interference fit.  Well, that’s not quite true.  Bullets actually have a slightly loose fit and this is a MAJOR cause of bullet dispersion.  One estimate states that this issue accounts for 50-65% of shot dispersion.

Ballistics engineers have known for many decades that bullets will be slightly loose and tipped in the bore.  But there hasn’t been a solution until now.

Now you’re probably asking two things:  (1) How can the bullets become slightly loose and tipped in the bore?, and (2) How loose are we talking about?  Good questions.  One provable cause is the pressure of the gases expanding inside the barrel.  Another is variation in the bore diameter and interior finish due to manufacturing imperfections and barrel wear.  Another is believed by many to be longitudinal shock waves that travel in the barrel causing expansion and contraction of the bore as the bullet is traveling through the barrel.  As far as how loose, around 1/2 of 1/1000th of an inch, or 0.0005 inch, for a typical hunting rifle, which is enough to account for a few MOA dispersion.  If you really want to geek out, go to the detailed explanation which goes into much more detail and  includes references.  

So, how do we get the bullets to be well-guided in the barrel?  It turns out that you don’t have to except for the last little bit of travel right at the crown where the bullets exit the barrel.  You do that by applying about 40,000-60,000 PSI of pressure to the crown to shrink the bore by about 1/1000th of an inch, which is enough to ensure a perfect fit between the bullet and the bore at the all-critical moment when the bullet exits the crown.