It's not a spring issue, it's an air seal issue.
this airsoft notion that leaving a spring cocked will cause it to fail is ridiculous, it may reduce it's life and rebound by a few FPS but will not kill a beefy main spring outright.
270 FPS is 80 FPS loss from what we expect of g&gs which are often hot. 80 is too much even for a tired, worn spring. The fact the previous tech has replaced the air nozzle and hopup unit reinforces it has a pre terminal air seal issue. They most likely threw in a stronger m110 spring because they couldn't fix it. You will possibly need a m140 to plaster over the crack, who will kill the motor.
My gut is a Cracked shell causing an un identifiable leak, but without stripping the box we will never know, hence why it's a project.
Certainly spend an hour with some fairy power spray, and maybe some paint to tidy it up will add a few quids.
Nobody should be buying 2nd hand batteries, we don't know the life cylcle or workl load of them, they are always a hassle, especially as they are so cheap.
Pack of 5 Mid caps are worth £30 +p of anyone cash. But again we don't know their work load.
Hi caps worth £5 a piece.
It wasn't a dig at all, and I'm not pushing anyone any direction, just sharing my experience.
I was the person buying this gun when I started, and I had my fingers burnt badly, I don't want to see someone else buy a broken gun and I don't want you to have the hassle when someone who does known what their are looking for comes back for your blood.
For reference the mags are £55 and the gun is £120 brand new.