Ok I went to the range again. I shot both barrel sets interchangeably and the difference in difficulty of getting good results is like night and day.

After coming back I decided to inspect the barrels closely using my workshop measuring tools (surface plate etc.). I noticed .50 cal barrels are straight as expected. .58 cal both barrels are bowing inward mid way. One around 10 thou the other 40 thou. This is in addition to a twist of around 10 thou. This is definitely wrong.

I also have to say that there was a cosmetic problem with the barrels. When I got it first I noticed they were painted not blued. Local Pedersoli distributor offered to swap them for me or give me a discount on another purchase for almost thf entire value of spare barrels. As individual barrels were shooting fine and I thought I can always reblue them I chose the latter and I got the .50 cal barrels for a very low price.

So I decided to fix it. I plan to remove the ribs first, then to straighten both barrels (hopefully without separating them) and finally to regulate them so they shoot
I removed soft solder from the muzzle side by heating it gently with a propane torch and blowing it out with compressed air. I planned to de solder the ribs, but it turns out they are silver soldered :-o

Still I should be able to fix them hopefully.