|Now, the safety issue...  Well, that's easy.  Don't use the visible portion
|of the beam spectrum.  Most HeNe lasers operate at 628nm wavelength - red.
|However, you can get laser diodes and small gas lasers which operate at
|+700nm, extending well into the IR.  Use one of these, and an IR filter as
|the last stage in  your optics to filter out the harmful radiation.  You
|could look at one of these all day and it wouldn't hurt you.

I agree with most of your posting, but must disagree STRONGLY
with you here.  At a given power level around the range we're
discussing (1-3mW or so) a visible laser is far SAFER than an
invisible one.  With an visible laser, someone who accidentally
gets hit in the eye will naturally flinch; this flinch will
occur before any damage is done.  With an invisible beam laser,
someone who's hit in the eye will not notice unless or until
real damage is done.

Otherwise, I was wondering what would happen if you could use a
pair of lenses of different focal lengths, seperated by the sum
of their focal lengths (e.g. use lenses with 5mm and 50mm focal
lengths, with 55mm between their centers).  It would seem that
this would give a fairly collimated beam, but 10x the diameter
of the original.

As others have suggested, for targeting you probably don't want
a laser even though for sighting it could be nice to have.  I
think for targeting it would probably be best (if people didn't
mind the odd-looking gun) to have a 3" diameter lens with an
IR LED at about the focal point.  This would give a beam which
wasn't perfectly colimated at 3" diameter, but was fairly close;
if you placed the LED so the beam was 4.5" at 10', then at 20'
it would be 6", and at 30' it would be 7.5".  By contrast, if
you use only a small aperture then a beam which was 6" at 20'
would be only 3" at 10' and 12" at 30'.