More balanced valve development:
Version 2.1 with a stronger spring and a shoulder that bottoms out before the base of the valve hits the end of the balance chamber.
At 1100 psi, again using 45 grain 0.310" lead balls, the chrono read 789 fps for 62.2 ft lbs.
At 1500 psi, 860 fps / 73.9 ft lbs.
At 1600 psi, 163 fps / 2.7 ft lbs - obviously beyond what the hammer can reasonably open.
Not bad, but I was sure it could do better, so a coil was cut off the spring and further testing was carried out
At 1100 psi there was a slight improvement, up to 795 fps / 63.1 ft lbs.
At 1500 psi, WHOOOOOOOSHHHHH... no reading from the chrono as the chamber dump blew it three feet along the floor.
Subsequent investigation revealed that the spring had slipped over the valve shoulder, obviously the maximum diameter was a little too close.
This inspired the final balanced valve version 2.2 with as wide a diameter as could be made without compromising flow vis a vis the barrel diameter.
At 1100 psi it remained in the same ballpark at 786 fps / 61.7 ft lbs.
At 1600 psi now the hammer opened the valve comfortably so 4 shots were fired:
1) 869 fps / 75.4 ft lbs
2) 873 fps / 76.1 ft lbs
3) 860 fps / 73.9 ft lbs
4) 858 fps / 73.5 ft lbs
I thought I'd push my luck and took it to 1750 psi, another 4 shots were fired over the chrono:
1) 726 fps / 52.6 ft lbs
2) 883 fps / 77.9 ft lbs
3) 884 fps / 78.1 ft lbs
4) 878 fps / 77.0 ft lbs
The first low shot suggests that it's at the hammer's limit in this configuration.
More shots starting at 1700 psi showed similar results. I think I can happily call this "done", muzzle energy has been effectively doubled compared to the original and I'll wager it's some sort of record for an antique air cane. Also having fired about 25 shots with this last valve without failure, it looks like it's up to the job.
Here's one interesting result from the 786 fps shot at 1100 psi:
The lead ball entered the base of the aerosol can and ricocheted off the base at a 45 degree angle, still with enough energy to exit the can.