All -
I tend to agree with 6lbgun that the preponderance of the gun would not have been so great as to make the operation of the elevating screw/wheel that difficult.
The first 7" Brooke Gun tube that Brooke shipped to Jones for use on the CSS Virginia weighed about 14,000 pounds and had a preponderance only 300 pounds, or 2.07% of the total tube weight [less charge, projectile, lock, sights, etc. - WEL note] (from an unpublished article by "Selma Brooke Gunner" excerpted from George M. Brooke, Jr.'s book of John M. Brooke's letters - "Ironclads and Big Guns of the Confederacy", University of South Carolina Press, 2002). All you really wanted to do was keep the breech of the gun "seated" on the screw rest so that it wouldn't bounce around and send hot projectiles off into places not intended. In this case if we assume a similar ratio of tube weight to preponderance (always a dangerous thing - the making of assumptions!) with a crudely assumed [again, by WEL] tube weight of 600 lbs then the weight on the screw rest would only be about 12 (and a fraction) pounds. Employing the "inclined plane" of the screw this would reduce the perceived load to a pittance at anything near this low number. IMHO & FWIW.
Also, did anyone else note the significant split in the carriage timber (base) aft of the elevating screw? Must'a been a heck of a load to open a crack up like that!