Nice 124!
Original owner of a beautiful 124D. Some years ago, had a retired machinist pard CNC mill the piston skirt for 3- 13% PTFE Delrin buttons and I also installed a Maccari spring kit. Have a target with three touching holes at 50 yards with 8.4gr JSB Exact pellets. It also stacks the 10.4gr Exacts. 10X mil-quad SWFA in double-strap BKL rings with parallax ranges noted from 13-55 yards. Shoot outdoor Field Target with it on occasion. Starlings fear it....
If your 124D was new to my hands, I'd tear it down and replace seals (check out Air Rifle Headquarters) and lube with Krytox GPL 205 as a minimum. James' spring kit is a very nice upgrade that really smooths/quiets the shot cycle. Then you'd know for sure the condition of the piston seal, particularly if it is original and potentially failing. Decent groups at 25 paces is a good indication it's still OK. Also, lay a piece of toi-toi paper over the breech seal when fired to check its integrity. If you end up with confetti, that needs to be replaced too as noted below.
As a final note for those that aren't savvy to springers, NEVER dry fire one, use one with a damaged breech seal or use a flammable lube in the cylinder. They are designed such that the piston will stop just before the end of the cylinder as the pellet begins its forward movement down the bore, allowing the piston to slowly settle against the cylinder's end. If there's no pellet or the breech seal is toast, the piston seal will be pounded to death in short order. The air in the cylinder follows gas laws where as the pressure sharply rises as the cylinder volume correspondingly decreases, the internal air temp rises rapidly just like a diesel engine.
