This idea has merit when the bullets do not properly fit the cylinder throats of a revolver. The concept relies on this premise: The bullet is undersize for the cylinder throat, and therefore when the bullet hits the forcing cone, and is still in the mouth of the chamber of the revolver, expanding powder gasses leak around the base of the bullet when it hits the resistance encountered at the forcing cone. This leaking gas will cause "gas cutting" at the heel of the bullet, thus severely deteriorating the potential accuracy of that bullet. The theory, or practice here, is to use a bullet hardness that is matched to the pressure generated by the given load in order balance the pressure necessary to obturate (read deform) the base of the bullet to totally seal the chamber mouth before the bullet hits the forcing cone, thus preventing the "gas cutting" and enhancing potential accuracy of the load.
This concept does work to a degree, and can greatly enhance the accuracy of some load/bullet/revolver combinations. However, it is flawed in its conception of being the perfect scenario. Please consider that if the bullet is undersize for the cylinder throat, that it will most likely be laying at the bottom of the chamber (Yes, at the bottom of the chamber! Remember that the chamber has to be bigger than the cartridge for it to easily chamber, and that the cartridge will lay at the "bottom" of the chamber, even if it is just a few thousandths of an inch!) upon ignition of the cartridge. Consequently, when that bullet, that is undersize for the throat of the chamber, obturates under pressure of the expanding powder gasses, it is already out of alignment with the forcing cone and the central axis of the bore by a few thousandths of an inch. Now, considering this aspect, the bullet will already be against the bottom part of the throat of the chamber once the bullet's base obturates (deforms), the odds of that bullet uniformly expanding at its base are really unthinkable. It will expand into the unfilled space of the throat, and thus the bullet will remain slightly off center in relationship to the central axis of the bore, and will remain that way throughout its passage through the barrel! Yes, this bullet will most certainly shoot better than if it did not obturate, in that the powder gasses were sealed in the chamber mouth and the base of the bullet did not become eroded from escaping gasses. This concept does have its limited merits.
Now, consider a more precise, and predictable alternative. If the exact dimension of those chamber throats in the cylinder are known, from slugging and measuring, then a bullet of the exact same dimensions can be procured to fire in that particular firearm. This being the case, the bullet will be a firm, snug slip-fit through the throat of each chamber in the cylinder. With a bullet that already tightly fits the chamber throats, it is not necessary for the bullet to obturate in order to seal the mouth of the chamber when that bullet hits the forcing cone and pressures climb. It fit the throat BEFORE firing! Carrying this one step further, the loaded cartridge, with a bullet that is a snug slip-fit in the chamber throats, will automatically center itself in the chamber, due to the tight dimensional relationship between the bullet and the chamber throat. No longer does that loaded round lie in the "bottom" of the chamber, but rather is centered by the bullet in the chamber throat, establishing near perfect alignment with the central axis of the bore, (assuming good cylinder timing and bore alignment), before the projectile is even launched. The result is much more predictable ballistic performance and more forgiving load development, as well as reduced leading in the forcing cone and rearward portion of the barrel.
This is the main reason that Beartooth Bullets stresses custom sizing for our customers, and encourages each of our clients to determine the exact dimensions of their firearm chambers to ensure proper bullet size application. When this dimensional relationship described above is properly balanced, bullet obturation no longer plays a role in bullet accuracy or load development. The bullet will already fit the gun, and not need "pressure fitting" by obturation (read deformation).