Also consider often reported shooting 2 3/4” shells in a 2 1/2” chamber cause higher pressures. I’ve been told up to as much as a 1,000 psi. 8,760 + 1,000 still puts you under the service pressures fr a modern 2 1/2” gun. So should be safe but not my first or second choice. But that’s why I loaded 5,000 Fiocchi shells trimmed to 66-67mm, 6,500-7,000psi. 66-67mm are a bit longer than 2 1/2” (63.5mm), but are commonly used in GB. It’s the pressure that is a bigger concern to me and that’s to not stress the stock.
So in a pinch you could use them. But far simpler to reload if you can to create a steady supply of safe ammo. I have 5,000 12 shells, 3,250. 20 gauge shells and 2750 28 gauge shells all loaded to safe lower pressures. I like RST but am not being held hostage to their ability or prices they need to charge. From their viewpoint $22.00 is not much above cost I bet. Powder $40.00/lb, shot $50,00+/25, primers $55.00/1,000, wads and hulls on top of that. Then equipment, labor and overhead. You might reload a box for $7-10/box with no labor or overhead.