A bespoke or other high grade double rifle will most probably have cast off for a righty or cast on for a lefty.
A modern production standard grade, entry level, off the shelf double probably has neutral (no) cast. Casting a stock costs money that buyers don’t want to pay for. Plus, any amount of cast is personal. One bend does not fit all. I may be wrong but I own a Heym, Krieghoff, and Blaser. None have any cast on or off. I’ve also owned a couple of Merkels and a Chapuis. You already guessed it, no cast on any of them.
A special order double from may have cast on or off. I think those rifles will be of higher grade with extra engraving and very nice wood. To check for cast, with an empty rifle, look down the rib from muzzle toward the butt. Cast on or off will be noticeable. Turn the rifle over and do the same to look for toe in or out of the butt. Both can also be measured but you get the idea, right?
If offered a great rifle with too much or the wrong direction cast, a good stock maker may be able to steam the stock and bend it straight. This is a little risky because too much pressure can crack a stock. The more figure in the grip area, the weaker a stock will probably be. I say probably because there are a lot of variables in both wood and the art of stock bending. The process isn’t hard and I am proof. A decade ago I was bending 100-year old SxS shotgun stocks to add or decrease drop. I never cracked a stock. Twenty years ago I had Glenn Baker of Woodcock Hill bend stocks for both drop and cast. He was very skilled at this. I don’t know if he’s still in business or not. If I had a high grade stock that needed bending, rather than doing it myself, I’d call JJ Perodeau.
There will probably be a cheekpiece on a double that looks nice but in my opinion, doesn’t serve much purpose. These aren’t 1000-yard target rifles where perfect cheek weld is important. Heck, I own a lot of super accurate rifles that do not have cheekpieces and some very nice rifles that do. If a lefty buys a right hand rifle with a cheek piece, leave it there or have a decent gunsmith remove it. You won't miss it!
Finally, there are triggers. Have your gunsmith swap them over. That’s a pretty simple task for a skilled smith familiar with SxS actions.
If “I” had questions about a “specific rifle” concerning what it would cost to change the cast, remove a cheekpiece, swap triggers or otherwise “convert” a right to left or vice-versa double rifle, I’d call JJ Perodeau for an estimate before buying that rifle.
Note: In theory, any change in the way a shooter presents opposition to recoil may change regulation and/or point of impact for the two-barrel group. Any change in results may however not be noticeable to the shooter. These rifle changes include bending a stock for more or less cast or drop, or even shooting a double rifle set up for a righty as a left handed gun. There is also a theory that for a right handed shooter, the right barrel recoils to the right more than the left barrel recoils to the left, and vice-versa The same applies for Joe or Bill shooting my rifle and me shooting theirs. We all shoot and absorb recoil somewhat differently. Like I stated, in theory any change could change regulation or point of impact. In theory…
