I recently picked up a Spanish shotgun in a 16gauge with marking on barrel 15.8 on left and 16.4 on right and I cant find a conversion chart from millimeter to us chokes. Any help would greatly appreciate

15.8 mm (.622") is a full choke and 16.4 mm (.646") is an improved modified per the 16 ga Briley chart that lists a typical 16 bore dia as .673".

That said, the few Spanish shotguns I've measured have had a bore a bit tighter than Briley's chart....probably making the chokes full and modified. Best to go pattern the gun for choke performance with the ammo you plan to use as well as point of impact (POI) vs point of aim (POA) for each barrel. The shotgun may or may not be well regulated and may or may not shoot high. Testing will tell you this.

Thanks

If you have a metric (MM ) number and want to covert it to inches, divide by 25.4 (fyi)

If it's an older gun - built before the modern plastic shot cup wad - it was probably choked slightly tighter than modern specs. Absolutely go pattern it and see exactly what it is doing with modern ammo. Then you may get it honed slightly to tailor it for your intended purpose (turkey, ducks, grouse, pheasant, etc.)