Is the recvr front ring of the action notched for the bullet point for loading from stripper clips?
Is the rear recvr ring stamped '30' ?

If so,,
A possibility is that the rifle may have been one that was orig in 7.65 Argentine Mauser and then converted to 30-06 in the 1950's by FN
That is the type I described in the first 2 lines. These used the orig 7.65 Argentine cal (.313) bbl. & just a rechamber to 30-06.
An ample supply of surplus WW2 ammo so they'd be our friends post WW2.

Perhaps someone has since used a chamber insert on one of these 30-06 chambered Argentine rifles to (further) convert to 308/7.62 NATO caliber.
Lots of 7.62 Nato surplus ammo around in the 80's, 90's and early 2000's.

If you remove that insert, you'll have a 30-06cal rifle with the orig .313 Argentine bbl spec.,,not a 7.65 Argentine chambered bbl.


The Argentine Mausers in their orig 7.65 Argentine MAuser caliber converted to 7.62 NATO (as I understand it) were rebbl'd to the NATO 7.62.

The 7,62 NATO is slightly larger in dia at the bottom of the shoulder and upper body than the Argentine round and of course a couple M/M shorter.
On some 7.65 Argentine chambered rifles you can actually force a NATO round into the chamber and with some force close the bolt and fire the round.
The NATO round is jammed against the sidewalls of the Argy chamber, so it is defacto headspaced in that it sits firmly against the bolt face.
Firing the round will fire form the brass to a 7.65 Argentine case but with a very short neck on it becasue of the too short Nato brass compared to the Argentine round.

The .308 Nato bullet flys down the ,313 Argentine bore with no issue so pressure is low. Accuracy can be anything with a .308 bullet in a .313 goove dia bbl.
Sometimes very good, othertimes not so good.


The NATO round being slightly larger would have needed a chamber clean up rechambering recut operation if an insert was used to avoid the above,,the slightly larger in dia NATO rd from refusing to chamber in the Argentine chamber.


Just some thoughts before you go digging in there.