FWIW, I'd NEVER put ANY modern loads through my Ithaca Flues, since the reason the Flues Models are so light is that the inside of the receiver/fame is cut out for the cocking mechanism, and not strong enough for any modern ammo made after 1925, when two things happened:
1) All the ammo companies changed/upgraded shotgun loads significantly, from previous loadings, with different/newer powders w/diferent pressure curves, etc.
2) Ithaca discontinued the Flues Model, due to point 1 above, in favor of the much stronger N.I.D. (New Ithaca Double).
That's not to say I don't shoot/hunt with my Flues - I do - but I use specialty (commercial) low-pressure shotshells from the likes of RST, PolyWad, Kent, GameBore & others - I prefer the RST loads, mostly because it's available locally.