I have a .50 cal GM 1:28 flinter barrel and a GM .58 1:66(?) caplock barrel.
I was wondering if it would be difficult to swap breeches on them. My concern is that the flats wouldn't line up and I have no means to machine the face(s) if they don't.

Can a shim be used?

That's what they make files for.

I'd never get it straight with a file, but i'm sure some guys can.

I could possibly put the hook of the breech into my 1/2" drill press and clamp a file in the vise and work it down like that.
But I need the breech removal tool.
Another thing I could do is sent the .50 out to get reamed out to .58 with a 1:70 twist???

You could also give the breech and barrel to a gunsmith.

I believe Track of the Wolf will do the breech plug swap and fitting. Worth a phone call.

Bronko,

It's not that hard. I wouldn't touch the barrel, I'd work on the breech plug. Trust me, you're going to have to work hard to get this too much out of square. That metal is probably hard. So, first you do a trial fit. Don't crank it down really hard. Then begin filing. I would like a small bench vise for this. When you've taken a bit off, do another trial fit. Honestly, unless you get really lucky, this might be a very time consuming job. The suggestion to send it to Track might start looking very good.

Which barrel is going to be the orphan?

OldBob

I will first do a trial fit for sure. But then I was thinking, I will likely have to work either the barrel or the breech depending on where the mis-alignment is. I have to keep the sights on top and the vent liner on the right side.
Neither barrel will be orphaned. I am going to switch breech plugs.

Why not buy another BP and keep them both breeched?