So the tendency is for the round’s diameter to be reduced upon firing unless the jacketing is substantial? I’m talking about a round that is fired and travels through the air until gravity brings it down to a complete stop.
For copper jacketed bullets, the unfired bullet’s diameter is equal to the groove diameter of the firearm or usually .001 more to ensure a good gas seal without undue friction caused by a too large bullet.
The bullet’s diameter is swaged to the bore and groove diameters of the firearm. Upon exit at the muzzle, the bullet’s diameter does not appreciably spring back, but remains as the size it was in its journey down the barrel.