28mm piston efficiency?

[cooper_dan]

Bit of a long shot (no pun intended) but wondering if anyone has calculated the efficiency of any rifles with 28mm diameter pistons.
So spring measurements, total pre-load, stroke, and muzzle energy.
(Piston mass and barrel length would be a bonus too).

I've hit a bit of a wall at 30% efficiency on my project rifle (in recoiling mode). Granted I'm currently using a short .177 barrel which isn't helping, but I was hoping for around 33-34%.
The plan is to make sure it's making power properly with just the working piston (hence recoiling mode), before rebuilding it in recoilless mode and continuing development from there.

Many thanks.

[Shed tuner]

not got anything on 28, but it sounds the right ballpark... how long is your stroke ? short = innefficient