Flyback transformer driver - working IRL 40KV 1-1.5 cm arc length not fatty tho

Your flyback has a 16.2 mH primary? That seems off by 1 or 2 orders of magnitude.

Yeah seems wrong to me. Currently I have no way to measure it so I just hooked it up to 10 VAC, plugged some numbers into RL impedance formula and this came out.

Hmm, yeah I've tried a similar approach to finding an unknown inductance. Maybe try building a colpitts around your primary with stable, known caps (ceramic or film) and measure the resonant frequency. If you have suitable caps to get it resonating in the audio range I recommend using a crystal earpiece to hear it and a spectral analysis phone app to find the frequency. If you can't find any that are large enough you can also use an AM radio. Just try to find a frequency on the dial that goes very quiet when held close to the oscillator.

The trouble with using line frequency to find very small inductances is that the inductive reactance will be tiny compared to the resistance of even the winding itself, let alone the separate resistance.

primary inductance value isn't that important to make silly HV. primary can be anywhere from 10μH - 1mH depending on core material, number of turns, spacing. counter-intuitively you will find higher output power from increasing the primary inductance. the operation of the transformer in this way is more about getting a strong magnetic field than working like an actual transformer, so more turns work better than less. your problems with this circuit will be: 1. rectifier diode resistance and heat death; 2. gate drive circuit doesn't have any nuts at all and the impedance is too high; 3. power supply not able to handle enough current; 4. high voltage spikes on the primary caused by current spikes from shorting the secondary with sparks, destroying your diodes and 555. if you overcome these and can feed about 10A through it, you'll have more than enough power to go past 45kV output, which is enough to arc over the secondary winding and burn it out. have fun and be safe; don't forget to short the HV terminals together before you touch the thing, output capacitors can store enough energy to kill you even when unplugged and will jump across you to get to the other secondary terminals or to ferrite core.

Update: burned the 555, now I need to redesign the gate driver. Power supply is a modified ATX that can supply 16A on its 12V rail so that's not the problem. Also that diode is 30A rated so it will take a while to blow.

Might be safer to use a isolated driver for your mosfets. I did it with optocouplers, and it works well. As for the fatty arcs, I am still working on it too. For some reason, flybacks do not give their all with NE555 single coil drive