I've been waiting for one of the gun experts to weigh in,
but in the meantime perhaps I'll give it a shot, so to
speak.  (My puns may be bad, but I won't be muzzled.)
 
Perhaps the analogy of fixed/attached extended to small
guns with stocks.  I seem to recall instances of guns being
sold as metal hard-ware only; the buyer then made (or had
made) wooden stocks which were custom fitted.  If this
were the case, the terms "fixed" may have referred to guns
with stocks, and "unfixed" to those without.  (This is
just a shot in the dark, based on a hazy recollection.
Well, I got to go... time to get the lead out...)   :)
 
Marty Perdue
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