The non-Busycon egg mass Nancy described, as I picture it, is probably
from a Fasciolariid, either Fasciolaria tulipa or Pleuroploca gigantea.
If it is football-sized, I would lean toward Pleuroploca.  Also, the
sides of the individual capsules of Fasciolaria are smooth, while those
of Pleuroploca have a series of encircling ridges.  The masses of
capsules are pretty impressive looking, but there really isn't much
substance to them.  If the mass is already hatched out, and you dry it,
it weighs almost nothing.  So a football-size mass really requires the
secretion of only a few ounces of capsule material.  If the capsules
have not hatched out before the mass is beached, then each capsule
probably contains many embryonic shells, which add to the weight.  If it
"rattles" after drying, it probably contains shells.  Break open a
capsule and see.  The capsules are made of a secreted proteinaceous
substance either identical to or similar to conchiolin, which opercula
are made of (and which is, as you observed, a very strong substance).
It is somewhat similar to the material of human fingernails.  Each
string (of Busycon capsules) is the work of a single animal.  Some
northern species like Neptunea decemcostata sometimes do produce
"communal" clumps, containing capsules from several individuals.  I
don't know if any southern species do this.  The individual capsules are
formed one at a time, and the string gradually grows longer (or the
clump larger) as each capsule is added.

Paul M.