Often 280 Pistons are out of round, both my .177 and .22 were, no drop in kit will correct this.

If the piston is sleeved you have to reshape the end of the cocking arm.

best spring I found for the 280 is the shortened Titan no11, you have to set it before shortening it, its a stiff spring but works well. You can reuse the OEM rear guide if machined to suit the spring...add a top hat and slip washer at the front

To get the best from a 280 you sleeve the piston , check it for round, add bearings, reshape the cocking arm, change the trigger adjustment screws, polish the transfer port..the gun then is a easy a match for a seriously tuned 95/98...mine are superb (I sold the 98 as the .177 280 was much much better)

Last, call RUAG and order the 16mm barrel weight they do (grub screw fitting) get someone to machine some baffles for you and you get/turn it into a nice silencer..its heavy and settles the gun a treat.

Im about to test Vortek 28mm Diana piston seals in mine, im not to keen on the OEM seal and feel it can be improved upon.