Many of the paradox gun, such as the Holland & Holland 12 bore tested by Ross Seyfreid, shot fairly light, shot-type charges of black powder to achieve velcoities in the 1,000fps range with the paradox bullets. The guns were light, not heavy and were not intended for, nor regulated or designed for heavy charges.

If you want to make them into magnum guns such as propelling 800gr. bullets at 1,700fps, them they'd better be heavy guns designed for such heavy loads.