Most people run their Sheridans on unregulated CO2, I don't recall ever seeing one with a reg. I've never tried to lower the operating pressure on one and I doubt you can go very low as the interior volume is probably not adequate and they are sprung for full pressure. You might be able to go a little bit lower on the pressure if you change spring tensions, but that might be hard to do as there isn't any such thing as a Nelson spring kit for a Sheridan. If you are having trouble making velocity on full pressure CO2 gas, your hammer spring may be tired or have been previously trimmed. I recommend freshening that first, you shouldn't need to run liquid.