If the bullet diameter is the same size as the bore diameter--and it should be--there can be no expansion within the bore.
If the bullet is a slight bit smaller than the bore, it will be upset slightly by the initial heat and pressure when the powder ignites. In this case, a flat-based bullet will be more likely to expand to bore diameter than any boat-tailed bullet.
One of the bad things in rifles--or pistols--would be for the flame front to be able to get past the bullet. It would erode the bullet in a non-uniform manner somewhere along the jacket as it did this, and thus ruin any hope of accuracy. It would be even worse for lead bullets--which is why lead bullets are commonly one or two thousandths of an inch larger than the bore diameter.