I believe the first and only correct question is; what do the barrel walls measure from chamber to muzzle? Many guns have been cleaned up by honing and reaming so you you can't depend on frame size by itself.

P.S. I was always told that was one of the live decoys in the bottom corner of the picture.

I believe everyone at my club shoots low pressure loads in the old SxS's. Mark, with his heavy 10's may go a bit higher with hunting loads. In my 12s it's 3/4oz of shot with Promo or 700X. In the 10s it's 1oz or 1 1/16 oz and the Promo or 700X - everything at or below 1200FPS. These aren't duck or goose loads, but fine for target shooting. Loading for the 10s are a bit more because of the cost of getting shells, and the wads are a little more, but not out of range. Once empty shells are bought, they can be reloaded for just pennies more a shell than 12ga. I can get 12ga wads for a penny and a half where 10ga wads are 6.8 cents each. About a buck and a quarter more a box. Not too bad.

I normally shoot low pressure smokeless loads through vintage guns. For hunting I will use modern factory loads in guns with both fluid and Damascus barrels that have sufficient minimum barrel wall thickness! I have on occasion even shot Federal steel shot through thick but tightly choked Damascus barrels with no ill effect. Oh, do not do this with a gun that you cannot afford to destroy!

Before I started blasting with modern ammo I bought the Hosford Barrel Wall Thickness Gauge set to determine the minimum barrel wall thickness at 1/2" intervals from breach to muzzle. Also, my guns are Parkers, LC Smiths, Lefevers, and AH Fox's.

Legal disclaimer: Do not try this at home!