Same bullet????? 308 bullets are 0.308" diameter ---- 303 bullets are 0.311".
I think that If you're shooting 308 in a 303 you may get blow by & probably low chamber pressure.
The bullet may obturate & seal the gap but it may not. (Lead muzzle loading bullets (99% pure) do seal, but are necessarily very soft to successfully seal in the bore) - I suspect jacketed bullets would be too hard & would leak.
If you're shooting 303 in a 308 you may get excessively high pressures.
Take care!!!
I'd be interested to hear the views of those with specific experience of this.
Ian