Hi Art,
A properly blown shell trumpet can produce an extremely impressive
sound.  It has to be blown like a bugle or trumpet though, not like a
saxophone or a police whistle.  When my youngest son was younger, I
made him such a horn from an old Charonia sauliae about 10 inches
long.  At first he only managed to create reptilian sounds similar to
those you described;  but eventually he could generate a blast that
would rattle the windows in the house - and this wasn't a big horn.
I also donated a shell horn to a camp.  This one was made from a
Pleuroploca gigantea about 22 inches in length.  When it is blown, it
can be heard in all parts of the camp, and is used to summon the
campers back from their various activities.  Across open water, such
a sound would carry a lot farther than it does in the woods.
Paul M.
Rhode Island