How the Shoei FG42 blowback works:
Shoei's FG blowback works using an open bolt for both semi and full auto. The bolt itself is fixed in the 'unlocked' position by way of a spacer that is fitted behind the fake firing pin (circled in pic below), which stops the bolt running down the camming channel and rotating to lock.
The locked bolt system simply would not work for PFC firing, as the system relies on gases driving the gas piston back, which then rotates bolt to unlock it.
I got this info from Tomio, which shows some mods to the bolt and gas piston for PFC firing.
The first pic shows an extra cut out on the gas piston. i am presuming this is for open bolt semi auto firing, and to lock bolt back further than the original semi cut out, but not as far back as the auto cot out. On the pics I’ve seen of the conversion kit, the gas piston does not have this extra cut out on all of them, so maybe someone has added this to improve semi auto function.
The second pic is the info page, in Japanese. sadly i dont have a clue what it reads but the pics are quite helpful. it shows dimensions for the new cut out in the gas piston, and a pic of . mod to the bolt lugs, which im thinking is to help the bolt from maybe snagging on the corners of the locking cut out in front of the chamber. this makes sense as the bolt could possibly have some slight rotational movement, and may snag at battery position.