I should have added that the important point
was that Cadet Springer could not have
left the cannon cleaning rag in the gun
since he was not on campus the night
before, so the fact the Rumford was
at the cannon that morning shows that
it had to be him who put the rag in
the barrel. Columbo proved that a
power explosive charge was used instead
of the usual blank charge, so whoever
put the rag in the gun wanted to kill
Haines, and Rumford had a good motive
to do so since Haines wanted to fire
Rumford and turn Haines Military Academy
into a co-ed Junior College.
Most of us Yanks -- at least those of us of a certain age -- know this, but, for anybody who doesn't:
The Citadel, where (as you correctly said) much of this episode was filmed, became the focus of a big controversy some years ago, when its commandants fought extremely hard to prevent it from becoming co-ed -- a striking parallel to the "Columbo" plot (but without the urinal-based clue). It was really a case of "Life Imitates Art".