Bloodnative.

IMHO

As I see it we have two criteria in opperation here.
Firstly the action size in relation to its ability to resist breech thrust.
Second the physical diamiter of the doner actions barrels at the monoblock or lumps. For any given cartridge you must be left with enough metal surrounding the cartridge, both in terms of the shank diamiter of the new barrel and what is left of the old blaoc/lumps after they have been reamed out.

So whilst it might be theorieticaly possible in action strength terms to hypothetically chamber such as a 20 gauge action for say .577 the walls of the barrel would be too thin to contain the hoop stress and would fail radially.

I have researched this extensivly and there is no difinitve data that I can locate. There are so many factors in play.

With old actions you have questions about the steel quality.

You have the inponderables of chamber friction v breech thrust. (see Varmint Al's web site, he is a good engineer and has graphicall illistrated stress calculations)
You have the weight issue. ( Too light a gun is going to beat hell out of you).
I have not read a thread or article where an action has failed. Barrels yes, actions no (so far). The reality is that one would need to destroy a gun in testing to find out where the limits really are.
We are extrapolating from shotshell pressures, but again there are variables. Shot shells use faster powders, so the pressure/time curve is different. Shot shell cases have no structural integrity in the walls. They are shorter so the force/area calcs are different.
Rifle bullets have to engrave on the barrel, smoothbores do not Pressure/time again. Shotgun barrels are choked, again Pressure/time.
I think in reality that many modern actions will tollerate such conversions because of the quality of the steels used nowdays and the computer controlled hardening and tempering the stells go through.

To give you an embaressing example. Whilst building my nitro powder muzzleloading .38 revolver based on a SAA clone frame with a custom built cylinder, I decided to see just what it would do flat out. It has a 7" barrel I was getting 1600 fps or there about with a 158 grain TCFP, when the recoil was so severe that it caused 2 other caps to back out and detonate or the flash back was so severe that it got under the other two caps. I never could determine which. Thus three cylinders discharged simutaniously. The ejector rod had been previously removed so one head extruded 2/3 out of there the rest heven knows where, and the head on the left of the cylinder adjacent to the one I was acually firing extruded out, and about a 1/4 of it made a nice semi circle around the cercumference of the protruding barrel shank.

That cylinder was unfluted and mechined from 4140 at RC30. Nothing was damaged except the loading gate which tore off and scored the back of my hand, my pride, my underwear, and my belief in my own lack of stupidity.
Incidently I am not proud of that piece of lunicy, I only write it in praise of the steel manufacturer.
I dread to think what might have happened if that had been an origional cylinder.
I would imagine that the pressure for a single cylinder was way above book let alone three, two of which had only about .125 before encountering an immovable obstruction.

As to definitive data. I am going to build this and then let the London proof house try and blow it up with a proof load. Thus we will know by the spring at least that there is or is not, one named action that will convert to .450 No.2

Regards