When I shoot at 30m I'm getting a per barrel group size of 1.5inch. I shoot each barrel at a separate target. Both groups land in the same 2~3 in circle.

It up s shooting about 5~6 inches to the left. It has a Pedersoli style "Ghost sight". To correct for such huge error I have to move the sight 3~4 mm (over an eight of an inch).

However, today I went to the range with the purpose of diagnosing it further and I found some additional clues.

I swapped the short 25.5in .58 cal barrels for longer 28.5in .50 cal barrels and those were shooting right at the point of aim. The gun is a lot more front heavy with long .50 cal barrels.

Then I swapped the short .58 cal back and I tried to vary my hold. I discovered the following:
- Holding strongly the grip with left hand (like a double shotgun) and pulling pretty hard into the shoulder I'm getting pretty good group sizes, but the gun prints to the left as described.
-Holding strongly around the front of the grip with fingers around the barrels (the sight is pretty tall I assume to accommodate just this kind of grip) as described by Cal Pappas in his book. Pulling the gun into the shoulder a lot less I'm getting classic "crossed barrels" with the right printing about 3~4 inches to the left and the right an equal amount to the right.
- Finally, holding with fingers wrapped around the barrel and the grip same as above, but focusing attention on the left hand to ensure it is NOT pulling the gun into the shoulder, just holding it on target (even pulling forward a little bit). The right hand on the pistol grip rests the gun into the shoulder, almost no pulling force. This way I managed to hit the point of aim few times with both barrels.

This very elaborate hold is pretty difficult to replicate exactly. I managed it quite a few times when shooting at 30m, but none at 50m. I was expecting shorter lighter barrels will be easier to shoot. It turns out it is the opposite :-o

However, I have a 11in howdah in the same caliber. I shoot it successfully with left hand supporting the bottom of the pistol grip (no holding to the barrels) letting it recoil up. (With replaced front sight) it shoots right at the poa.

I guess I have an answer why this happens. Still if there are any double rifle shooting technique books or tips I would be grateful to hear about them