In my head I feel like laying the barrels in the crosshairs and pushed up against the backstop of the miter saw will "have to be" a straight cut, but will a barley uneven (not noticeable by looking at it) cut really make a difference? I mean I wont have chokes so I am just going to be spraying anyway at this point...

100%

Especially if you don't solder the barrels and rib together before hand.

I used a hack saw on mine and went slow and then followed with a file. Wrap a sheet of notebook paper around the barrels and keep the paper aligned when wrapping, it gives you a true square edge as a reference.  
I did solder the barrels at the cut area.
Mine is a 9" 20ga Stevens 311. Both barrels still hit the same and pattern well.

You guys talking about solder I am assuming it is done after the cut right? Got me a little 20ga Ithaca that I kind of want to cut down.

I just assumed they were talking about that empty space that is usually left in between the barrels. If you solder before the cut do you just heat up the rib or what? I guess I was thinking you might undo whatever is in there already.


You could fit everything I know about these old shotguns on a matchbook

When I did mine, I used a horizontal band saw. After which, I had the space between the barrels soldered. I can't see how one would solder, prior to cutting the barrel. The rib runs the entire length of the barrels?????

There is a top rib and a bottom rib, so you can't get between the barrels without removing one of the ribs. Or, I suppose you could drill the bottom rib to allow you to drip solder between the ribs.

Cheaper shotguns have more parallel tubes and barrel sets are built on jigs, expensive shotguns have tubes which are more curved and barrel sets are built by hand. POI on expensive SxS shotguns is adjusted by the curve of the tubes, the adjustment of the barrels by heating the solder and moving the barrels and by honing the muzzles.

At the muzzle there is a steel fitting with roughly 1/4 moon shape on each side for the tubes which is soldered to hold the muzzles. If possible, removing that fitting and placing between the cut off barrels and soldering would be the way to go rather than just filling the gap with solder.

Fwiw, some cheaper shotguns are brazed rather than soldered and removing the muzzle fitting would be difficult.

JPK

Topic Moved

Some pics for reference. Regarding the "dirty" pics. Barrel (left over from my SxS SBS conversion) was in a pile of metal, keeping the "barn" cat company!

Anyways.

Original barrel end, with solder joint from the factory:


Other end of the barrel above, post chop:


New "business end" of the SBS -not finished, end is getting milled and a bead sight installed-:


This is with the top rib peeled. You can see the small area that the solder populates! It is all about surface area. Notice the "spacer" left in, after the top and bottom rib had been soldered?



Hope the above help, in being able to tell how a SxS barrel setup is put together...