They won't always list pressures as it is not in this case all that relevant they just need to say that they are suitable for nitro proofed guns with 2 1/2" or longer chambers. I should think your Ithaca was built for much heavier pressures and unless in poor condition I cannot see a problem. I wonder if the velocity is a bit higher to compensate for the light weight of the Bismuth shot against lead. Lagopus.....