I just got in a new book, "Guns of the FBI" by Bill Vanderpool, so I looked to see if I could find any information on this, finding that he said (speaking of the Md 13) that "most agents stilled carried the .38 special load in them (a 158-grain lead hollow point +P) but could carry the Bureau's magnum load, the Winchester 145-grain Silvertip, if required." He further said "The magnum rounds were a handful and some PFIs (Principle Firearms Instructors) in the field required the agents to demonstrate their ability to handle the hot load before being allowed to carry them. During new agent training, the trainees were only required to fire a small number of magnum rounds."
Looks to be an excellent book and I can't wait to get the chance to sit down and actually read it, rather then just searching through it!