Just wondering, why are you wanting to use substitute propellant in a classic design such as one of the '61 models? At the range and in the field, I've noticed that most of the sub propellants are used in the non-traditional units that will never see a genuine grain of B/P. You've already 'graduated' to the real deal! Consider using the 'real' propellant!
Before anyone begins to sharpen their knives: My comment is in no way intended as a 'slam' toward anyone using or advocating the use of non-traditional firearms or substitute propellants, Ok? We are all 'shooters', just traveling different avenues . . . . . .