You might not find a difference between a teflon or properly fitted rubber o-ring.
The correct o-ring size is 22 x 1 mm. It should fit a bit the ring groove should be a hair bigger than the 1 mm o-ring thickness.
This way when it fires, the pressurized gases push the o-ring down and get behind it in the groove and force it out to firmly seal at the cylinder wall.
When it is free wheeling the o-ring retracts a bit and has near zero friction in this state.
After your experiment, please let us know if teflon can do any better than that.
Most of all...... have fun...!!!
Gil