Some weapons' fire rate fluctuates within the same run

[WhenTanksFly]

d_firerate_uncertainty_2and40s.zip At about 0:01.96 I started firing the Flamethrower continuously. With its supposed 10 rounds per second, I expected the 80-round magazine to end at 0:10, but the last shot was fired at 10.38, meaning ~9.5 rps.
I then waited until 0:40, and mouse1-ed the flamer again. The mag was over at about 0:47.90 - the weapon went back to its expected fire rate, and I haven't yet restarted the mission or even moved my marine!
The timing isn't always consistent though. But at least the issue is reproducible on near-vanilla settings.

The problem also applies to Autogun (ranging from 9.4/s to 10, again), Mining Laser (8.65-9.08/s), beta Crosshair Thrower (16.1-20/s), and maybe some other guns. However I've never managed to un-12rps the basic rifle.