I think repairing or filing it opens up a whole can of worms.
1: It's pitted & as someone said, you file it down & you have clearance issues.
2: It could be just as bad pitted Inside.
3: The barrel is made in Spain so it is probably metric on the breechplug & drum.
4: You may break off the drum & breechplug trying to get it out.
5: You can buy a barrel for it on eBay for $100-125 or so, & eliminate the issue.
All that being said, I would replace the barrel. Hang the old barrel on the wall saying
"This was a good ol barrel".
Note: Also, from the condition of the barrel, the Nipple threads may be eroded as well.
So IF you decide to use it, I would invest in a oversize tap & oversize nipple for it.
When nipples blow out, they don't give you a Warning, they come Out With Velocity. Loose
wobbly nipples in a drum are a No No....