According to W.W. Greener in "The Gun and Its Development" (Cassell & Co., London, 1910, page 436): "The barrels of a breach-loader should never be jerked down, nor should they be thrown back into position with a snap. The proper manner in which to load a gun is to drop the stock under the elbow, and press it firmly against the hip or the body, unfasten the lever with the right hand, and with the left grasping the barrels a few inches in front of the fore-end, lower them easily. Close the gun in a careful manner after putting in the cartridges, bringing the stock up to the barrels."
That said, I use my SXS's in Cowboy Action Shooting and they get opened with the right hand by snapping the barrels down very hard, while pulling two shells with the left, loading them and then slamming it shut as fast as possible. I also shoot black powder. Needless to say, I don't use mint collectables for this little exercise. An ideal gun is a Win. 24 or a Miroku-built one with double triggers.
