Looks mostly cosmetic to me, when you black the back of the barrel is there plenty of black on the back of the inlet? I fixed the barrel inlet before I did the tang in this picture.
Mostly like this, in my opinion this is good enough, you can glue in a thin shim to close the gap, I have done this several times.
After you shoot the gun a bunch of times the recoil will seat the barrel just a little further back.
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Barrel, tang shims, I have closed a gap or two at the breech the same way, they are paper thin where they are glued in and don't show on a finished gun.
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