Bit of sneaky pre-work testing this morning (as little man decided 5am was a good time to wake us up).
Set the power to 10.5 ft-lb with a 250g active piston (the heaviest the active piston will currently go).
Then added in the dummy piston for total mass of 460g. Power dropped to 8.7 ft-lb
Then removed 50g off the active piston for a total mass of 410g. Power dropped to 7.1 ft-lb.
We roughly know that a 28mm pistons work best sub 300g, so those results don't seem to make sense. The 410g piston should make higher muzzle energy than the 460g.
But actually it does make sense, and I think it proves a point I was thinking about yesterday. With the dummy piston in place, there is a buffer seal (imagine the rear piston of the Diana twin piston rifles). Because the piston seal on the active piston compresses in use, the dummy piston seal/buffer must have an amount of pre-load on it, equal to that deformation. (If you read the Diana service manuals this is something like 0.3mm).
If the pre-load is too high, the active piston will not be able to complete it's full stroke. If the pre-load is too low, there will be excess load transmitted through the gears (causing accelerated wear) and you will lose the completely recoiless feel. Both pistons must come to the end of their stroke at exactly the same time.
In my testing this morning I have started off with a very high buffer pre-load which will be preventing the active piston making full stroke. However the 460g total piston mass is able to compress the buffer seal a bit more than the 410g mass, therefore completes a bit more stroke, and makes a bit more power.
Now I need to remove 0.1mm buffer pre-load at a time until I reach the correct amount. As I am actually using Diana piston seals, this is probably going to end up very similar to the 0.3mm that Diana specify!
Recently I seem to finding lots of reasons that other people don't already make this rifle
The real challenge will not be making it work, but will be keeping it as easy as possible to service. Still working on that!