I don't seem to be able to reproduce the issue in my test setup.
AMM:
Missile Size: 1.0000 MSP (2.50000 Tons) Warhead: 1 Radiation Damage: 1 Manoeuvre Rating: 129
Speed: 20 000 km/s Fuel: 150 Flight Time: 2 minutes Range: 2.84m km
Cost Per Missile: 3.13932 Development Cost: 314
Chance to Hit: 1k km/s 2580% 3k km/s 860.0% 5k km/s 516% 10k km/s 258%
Target:
Missile Size: 3.00 MSP (7.500 Tons) Warhead: 0 Radiation Damage: 0 Manoeuvre Rating: 10
Speed: 40 000 km/s Fuel: 1 675 Flight Time: 18 hours Range: 2 556.3m km
ECM Modifier: 60% ECCM Modifier: 60%
Cost Per Missile: 9.00 Development Cost: 900
Chance to Hit: 1k km/s 400% 3k km/s 133.3% 5k km/s 80% 10k km/s 40%
The hit chance would be
25800/40000 - 60% = 4.5%
Event log:
So the displayed hit chance is correct (with rounding), and the AMMs actually hits something.
Of course, AMMs being slower than the target means there is a non-negligible chance of interception failure. But once the interception is successful, the hit rate seem to be expected.
I had multiple suspicions on why this could happen, but it seems the above disproves all of them:
- ECM/ECCM is not displayed in the event log (never happened in previous versions), which is disproven by the log showing the theoretical hit chance correctly
- The displayed hit chance is 100x (at one point this was a bug), which is disproven by the same observation
- There is hidden mechanics on missile hit chance if missile speed is lower than target speed, which is disproven by my AMMs actually hit something