If it is the older tube fed model then heavy slugs and heavy loads of .44 Special MIGHT work. A local commercial reloader used to offer such loads specifically for the tube fed Ruger with the idea that one could load an additional round.
I suspect these were the .44 Special equivalent of the old .38 SPecial .38-44 loads. that is they were loaded to near magnum pressures in the shorter cases, in the case of the .38-44 because the .357 had not been invented yet.
I unfortunately ended up with a box of this stuff which had no special marking and fired it in a .44Special N-frame S&W revolver. There was considerable spitting of lead and flame through the cylinder gap to the point that I immediately stopped and took the stuff back to the shop, where I was told about the heavy loading for the .44 carbine and the non owner clerk got a good talking too by the owner for selling it to me from behind the counter without explaining this in advance.
-kBob