. First off I’m a right handed shooter and the gun came from the factory set up for a left handed shooter.
how does that happen?
I think it is the cast that is backwards. Not sure how a cast on gets put on a gun but it's probably the manufacturing process that takes the next stock whether it is a cast on or off.
With your new auto loader:Take the butt plate off. Take the nut off the stock bolt. The stock will pull off. Inside the stock is a small metal plate with a hole the stock bolt goes through. Take that plate out. Flip it 180°. Keep the top of the plate up. Put it back in the stock. Flip the shim that is between the stock and receiver. Put it back together. Now you have right handed cast. Fine tune the cast with the assortment of shims. Further tweaking can be accomplished by lightly sanding the shims. Flipping the stock plate top to bottom will add or take away drop before fine tuning with the shims to adjust point of impact up or down.
Edit to add: to know what adjustments to make.
Put a spot on a pattern board. Step back 16 yards. Stare intently at the spot. Shoulder the shotgun and shoot. Do it 3 times. The patterns should be on top of each other. If they are not, you need to work on tightening up your mount before proceeding. If your mount is not repeatable, fitting is an excercise in futility. Measure horizontal and vertical distance from the center of the pattern to the center of the spot in inches. 1/16” of added drop will lower the pattern 1”. 1/16” Less drop will raise the pattern in the same 1” increments. Adding or subtracting 1/16” cast will move the pattern 1” in the direction you alter the cast.
With your shotgun upside down, place the rib flat, running down the edge of a flat table or workbench. The bead needs to be off the end and pulled up against the edge of the bench so the rib is flat and you can return the shotgun to the same position. The edge of the rib needs to be flush with the edge of the table for its entire length. Use a carpenters square to measure the distance from the table to the heel. That is your drop at the heel. Now with the square perpendicular to the stock, mark the cast from the side of the stock at the heel on the table. Multiply the horizontal distance in inches you want to move your pattern by 1/16. (You obtained this number from the target you shot at 16 yards) Mark the measurement from the current cast mark on the table.That is where you need to adjust the cast to. Do the same with your elevation and drop. You can accomplish drop by simply measuring. Take your time and don’t over do it. It is a simple process. When done correctly, when squared up and mounting into a mirror, this puts the bottom of my pupil sitting on the rib dead center with almost no rib showing. The shotgun in the mirror should be square to the one in your hands. Factor in if you like a 50/50, 60/40, or whatever pattern you like for your preferred site picture.
Enjoy your new smoke pole. If you want to adjust your new Benelli to your mount, it is simple enough to do yourself. Older autoloaders and over unders that do not come with modular stock adjustment require more skill.