Some experimentation was definitely in order, so I machined a Delrin cup to fit the chamber with a 1/4" orifice through the base:
I fired a series of shots and the results were quite interesting.
The first thing I did was repeat the tests done in the first post, at 16x with 0.25g airsoft BBs and with photo paper burst disks.
1 layer - 1903 fps (up from 1525 fps without baffle)
2 layer - 2012 fps (virtually no change from 2010 fps without baffle)
3 layer - no burst (1808 fps without baffle)
I then upped the mix to 20x, with a single aluminum disk it failed to burst, whereas normally even two layers would burst without the baffle.
With 2 layers of photo paper I got 2075 fps with the baffle and 1400 fps without it.
In the latter case the muzzle energy with the baffle is 50 J up from 23 J without it, just over double, with all other conditions being equal. That being said, just looking at these preliminary results, what the baffle seems to do is make up for performance loss from a disk that bursts too early rather than increase the energy up from the optimum disk burst pressure.
That's enough experimentation for this session but I would also like to try cutting down the baffle to make the "second chamber" smaller to see if that makes a difference, I wonder if it will increase or reduce peak pressure at the disk.
Of course it's not just putting BBs through the chrono, here is some effect on target at various velocities, more to follow eventually: