Check the Library for Armstrong.
I cut dovetails with a hacksaw, stepping cut by cut over until almost wide enough. Cold chisel the fins left by sawing flush with bottom of dovetail. File base smooth. Then I take a SMALL chisel, and start stabbing in side-by-side blows for the actual dovetail part. This raises material on the barrel surface that you can dress up with a file, and it gives the undercut for your dovetail. But the raised metal is all steppy and rough, so I then use a big chisel to even up the cuts made by the small chisel. Alternate big chisel and 60 degree triangular file until the dovetail is clean and the width you want.
To try to raise the dovetails with big chisel alone is very stressful on a barrel.