Another useful method is to go to a supplier of machine-grade drill
bits, the kind that are supplied it thousandth's of an inch. Take a new
barrel, and buy a drill bit that just fits inside the barrel.
Unfortunately, standard SAE and even metric drillbits don't really fit
well enough. Almost any damage to a barrel can be fixed with a drillbit
that fits well enough, but be careful not to destroy the plastic cap at
the bottom of the barrel while removing dents, etc.