Hey-

From experience (painful):
        Mark the number on the outside of the trigger guard in clearly
visible ink (paint a white acrylic patch on top of the B-72 and then write
the number in black if the metal finish is dark.)  Our collection areas and
exhibit areas are not well lit (well, they're not suppose to be, are
they?) and trying to squint at a number skillfully hidden on the inside
of a trigger guard leads to a lot of flipping of said longarm leading to
accidents like hitting the cabinet next to you or something on the
table.  Alright, maybe not if it's on exhibit....

Also, about the comment of reversibility and thieves:
        Unless you have skillfully written your museum's name on the gun,
the catalog number is not going to mean much to thieves or buyers.  We
have a collection where the donor painted numbers on the
stocks on messy white splotches of paint (pretty much irreversible.)  I
would suspect that guns in circulation would have a lot of such
marks--your museum's mark would not draw comment so it is not a security
feature in that sense.  It is a security feature in keeping it
identified--without it's number and attached records, in a hundred years
when we're all dead and gone would a new worker still identify the
longarm as Napoleon's gun? (Unless his name is engraved on the stock like
our Sam Houston pistol and Mirabeau Lamar shotgun...Texas guys....)
Collections care includes control of information.

Sally Baulch
Collections Manager, Anthropology/History
Texas Memorial Museum

PS: Since you're asking about renumbering guns, you ought to ask how to
number the horn chairs on exhibit :)