I won't speak for the other mfgr's offerings but the Ruger single action revolvers in 44 special can safely be loaded right up to the 44 magnum's doorstep in power and velocity.
Ruger will not comment on the 44 special max pressure but the 45 Colt/45ACP flattop blackhawk is rated for 45ACP+P pressure which is 23kpsi. I figure that the flattop in 44 special may be perfectly safe up to 25kpsi because the chambers are smaller diameter, cylinder walls are thicker, webs are thicker, and the bolt cuppets are thicker at the bottom. This level of power with a 240gr boolit in 44 special will fit right up under the lower end of the 44 magnum performance with the same boolit.