How do you know it wasn't proofed for Steel? I didn't see any pics of the flats. And how do you know how thin or well struck the barrels are? Unless you've seen the gun in hand, actually looked at the bbl flats or measured wall thicknesse or mic'd the barrel diameters...., you're not offering much here.
This gun is modern enough to where the original purchaser could have specified everything from weight, type of proof etc.
If the gun is in proof, the barrels are sound, then shoot whatever size lead shot you want. If your primarily shooting upland birds with it, you won't realistically be shooting anything larger than 5's through it anyway. Have at it man.
In regards to being ok with Steel....you'll have to show the bbl flats. If your chokes are open enough, you should be ok..or there's always the bismuth route.
