There is also the velocity phenomenon known as "freebore boost". To explain: When a fired projectile uncorks from a rifle bore, the gas expands and dissipates into the atmosphere, causing the blast that we are familiar with. When a suppressor is fitted, the projectile uncorks inside the body of the suppressor and gas continues to work against the base of the projectile, increasing velocity slightly until the projectile exits the suppressor front cap.
Well frankly it looks like I can't fault your
use of that term, but I will say that Mr Robbie Tiffen (Canterbury Gunsmith and Suppressor Manufacturer), is using a definition of "freebore" that would draw a chuckle from most rifle builders and pursuers of bottle necked cartridge accuracy. They know what freebore is and have dealt with it and argued over it's merits for generations. It is that portion of the chamber beyond the end of the cartridge mouth, but before bore rifling engages the bullet bearing surface.
In any case, I know little about suppressors, so there may be jargon there with which I am unfamiliar.
Normally, any expanding gases that "chase" the bullet out the muzzle will impede accuracy as a general rule. The gases are moving at a much, much
higher velocity than the bullet and do not stay in a nice neat column of pressure once free of the confines of the rifled bore. Once free of the rifling, any pressure buildup is relieved
and great turbulence ensues. There is the reason for a suppressor. Unless a suppressor has a gas-seal tight bore of it's own, the behavior of these very hot, expanding gases can upset the flight of the bullet at the very beginning of it's flight. Bleeding this pressure off, in order to suppress the report, seems contrary to the claim of increased velocities too.
There may be much to learn here, but it seems a strange claim to make.