Basic sheet metal repair.....
When this problem shows up here(not limited to firearms).....theres a cpl ways of doing the job.
Its going to be either an expanding arbor,works well with round thingys...but can also be devised on sq/rectangular tubes utilizing,"sliding" wedges.
The "trick" is where to apply dedicated hammer strikes.You expand,then litely tap around the perimeter of dent.The metal got stretched....whether its on purpose(as in this case),or by accident.Then expand a bit more...tappy tap....Wash,repeat.You just have to be aware of the stretch and subsequent "shrink" techniques.Do it wrong and you can have a bigarse mess.
Hence the need for an expanding arbor.Google-foo shotgun brrl dent removers.Also look into tailpipe expanders....obviously too big,but look at how they're made.Further...look at the handlebar stem on a bicycle.They're a good example of how we making sliding wedge style expanders(for sq stock).
Above is not an endorsement on modifying a firearm.Presenting it from a mechanical/skill standpoint.
P.S.We also do right much of simply sectioning the part,and Tig'n in a straight pce. when you can't access or reach it from an end.