Originally Posted by 2152hq

Maybe you already have done so,,but take the barreled action out of the wood and look over the underside of the barrel for any markings. Sometimes you'll find a caliber, date and even a gunsmiths name on the bottom.

Short of that,,the advise to make a chamber cast is the best. Then carefully compare to known data to determine what you have.

A chamber cast can reveal the throat dimentions and the bore/groove also all in one cast.

Even after determining caliber, make a careful check of headspace. Custom barreled guns were/are often barreled and then chambered using nothig but empty cases as headspace gauges.
Even with a bit of excess headspace, carefull reloading techniques can overcome that and taylor the brass perfectly to the chamber.

Make sure the '03 action you're dealing with is a later one and not a low#.