WOW. I just spent 15 minutes at that schematic and I don't understand it at all. You are feeding DC power into a transformer and gettting high voltage out the other side? I could really use a walk through for this circuit if someone would be willing?
Here's the circuit for reference:
This is close to the simplest oscillator possible. Even so, or rather despite that, he got it seriously wrong and mutated it into an unsafe circuit, by feeding the HT side into the primary, which besides the obvious dangers of someone touching the "low voltage side" in good faith, might create flashover in the primary that probably isn't wound with enough (if any) isolation between layers, or any other precautions (like making sure there's no large potential differences in close by windings/layers), that are usually taken in HT circuits. Further, I wouldn't like to be the MOSFET in that circuit.
It's called a blocking oscillator and the simplified explanation is like this:
Assume power is just applied.
The transistor is opened (conducting) due to the upper resistor, starts drawing current through the primary winding.
As the magnetic field changes along with the electric field, the flux changes and the changing flux induces an electric field in whatever other windings there is on the core.
At some point determined by the resistance and the inductance, the core cannot hold more flux (i.e. it is saturated) and no matter how much further the current is drawn through the primary, the flux cannot increase.
No flux change
equals no electric fields generated, so the secondary winding shuts off the transistor. This would normally be done in a separate feedback winding, so the galvanic isolation keeps the voltage of the secondary side from venturing back into the primary side.
When the transistor closes (stops conducting), the magnetic field collapses for an even faster change of flux, generating a higher voltage in the secondary than before (even with a 1:1 transformer, the secondary voltage will be higher than the primary, as the sec. voltage is a matter of how fast the flux changes)
The feedback will now be negative and blocks the transistor (guess where it got its name).
After some time the field has decayed and the circuit is back to where it is turned on by the upper resistor and the cycle repeats.
It's the cornerstone of (nah, the entire) circuits for single cell LED boosters, whether they steal the lame name invented by the guy who stole the circuit from a reader supplied column called "Ingenuity Unlimited" that ran in the then named Practial Electronics* back in the eighties. (This reader didn't invent the blocking oscillator, just presented it used to light an LED from a single battery cell).
Practical Electronics later assimilated two other mags, "Electronics Today International" (which in its golden days was a brick of more than hundred pages mostly of quality stuff) and "Everyday Electronics" (which was always a lousy rag, only of value to the true beginner).
They promised to keep the colo(u)rs of E.T.I. flying up to the merger, but lo and behold, the E.T.I. stuff (and probably staff) was gone in 2 or 3 issues
But I digress... In the LED circuits that actually doesn't steal a thing, there is just the primary and the feedback windings and it's the flux collapse that boosts the primary (with inverted polarity of course). It s easy to add a secondary for a substantial higher voltage and that's how I'd do it for the coil gun, even if it has got a low efficiency (around 60% to 70% max due to the constant power take up, but it's cheap and easy to change as experiments goes). For the coil gun, it wouldn't take much extra to turn it off automatically when the caps reach a given voltage and that would lift the efficiency.
In another (even older) issue of P.E. was a similar circuit for driving a Weller WTCP50 (i.e. 24V/~2A)from a 12V car battery and turn on/off controlled by the irons Magnastat switch. This used a pot core around 2" in diameter (plus a 2N3055) for a 50W output- If a suitable secondary was wound on a core that size, those caps would be charged really fast
Well, this must be enough words for the simple explanation