Coastal / Arctic testing done in Alaska as I recall. The cold, sea-level air being as dense as atmosphere tends to get on this planet. Easy to imagine having the same problem in the thick humidity of SE Asia, so the 1-12" conversion was probably the best compromise to retain "universal" function while still retaining the M193's tendency to tumble and fragment on impact.
The 1-7" rate did indeed come along with the M855/SS109 62 grainer, but the need to stabilize tracer bullets made longer by the need to contain their combustible element may have been a greater factor. But again, I don't know for certain if they changed the throat/leade.