It depends on gun, pressure levels, etc. For example in a .357 Magnum loaded with a 158 grain bullet peak pressure is already reached by the time the base of the bullet has reached the mouth of the case. Bullet expansion is already well in progress before the base passes through the throat. It is quite possible that the bullet base is first sized to the thoat diameter. As the bullet enters the barrel it will encounter some resistance which should cause it to expand further until restrained by the bore and groove dimensions.. I have seen many spark photos over the years of bullets being fired from revolvers with the barrels removed. It is quite apparent that even in low pressure cartridges like the .45 Colt that the bullet nose has slumped and the bullet shortened from the Gees of acceleration alone. If the bullet is shortening there must also be expansion.
Wouldn't it make more sense to make sure the bullet was large enough diameter to completely fil the bore, with or without expansion, and then to make the chamber throat fit the bullet. Instead we want to make the bullet fit the throat and the bore. A tough job especially if the throat is smaller than the bore. One of the original advantages of breechloaders over muzzleloaders was the ability to use bullets larger than bore size in order to get a tight fit. In muzzleloaders it was necessary to depend on the bullet bumping up to fit the barrel as it had to be smaller to be easily loaded. Not so with breechloaders. Nonetheless it appears that the old ways are, for some people, hard to give up.
In addition to using enough gun it also makes sense to use enough bullet.
