Shrouded Walnut 77K

[Deadeye Dick]

Very nice looking gun.

 

[JamieUK]

Is there a general formula you used to arrive at the correct TP, piston diameter, piston sweep, piston weight?
I'm seeing more and more skirtless, lighter piston conversion for the <12 FPE rigs.
I assume this was started by Tony Leach? and the lost volume crowd?
I have his set up in my TX...night and day difference in recoil and shot cycle.

Not really a formula as opening up a TP or increasing piston mass for eg can have a similar affect on cycle and output, so there is more than One way to skin a cat. It's always a good idea to keep an eye on the static compression ratios as you go though.

As long as the TP isn't horribly long 38 to 40cc of swept volume at 25/ 26mm diameter for sub 12 is plenty and a safe starting point. As the piston diameter drops peak pressure can inrease, so it's possible to go down to 32cc (and lower if everything else is optimised).

People only really go skirt less as its a convenient way to lose weight (turning down hardened pistons is no fun) and in small bore builds (especially 21mm) it allows for more spring options. It's also far easier to make a skirt less piston than full skirt