My electromagnetic BB gun project, 130 m/s velocity, 20 rpm rate of fire

[BravoV]

I wonder if you could use an energy recovery winding  for every turn on the barrel. This way you may be able to recover the energy that isn't transfered to the projectile.

An energy recovery winding might work, but I suspect that copper volume in the coils is critical.  Instead, you could try a two-switch circuit (see below).
To fire, close M1 and M2.  The DC rail is applied to the inductor and current rises.
To hold current, open M2.  Current flows through M1 and D2, 0V applied to inductor, current holds steady.
To release, open M1 and M2. Current flows through D1 and D2, energy is transferred from inductor back to the DC rail.

 

Fyi, this topology is proven and worked at the inductor saturation tester circuit designed and built by our forum member Jay_Diddy_B, it was started as an idea to recycle the energy used in the inductor when biased with current to find it's saturation limit.

Details here -> Inductor saturation tester

[LaserSteve]

As a target shooter, I think you should be very proud....

Steve

[rpopeye]

An energy recovery winding would not only take copper volume, which indeed is critical, but might also mess up the pulse in the main coil.
The two-switch circuit is a much better choice, it has also an additional benefit besides energy recovery, the electromagnetic field in the coil would decay much faster because of the much higher voltage potential between the coil terminals.

[ez24]

Very interesting 

[max_torque]

Considering that a magnetic coil acts on a iron object in such a fashion as to always try to maximise its inductance, then there must be a load of design optimisations for the coil geometry that depend upon the velocity of the projectile?  Ie you want to be be able to build the maximum coil current before the projectile reaches the max inductance location, and then ideally, immediately reduce the magnetic flux to zero to avoid creating a drag force on the projectile as it exits the coil.  Hence, so sort of iron magnetic flux steering might make an improvement?

The problem with recovering some of the firing energy, using an H bridge type driver is that the voltage available to reduce the magnetic flux is limited to that of the storage medium.  Using a simple on/off switch (of a suitably high voltage rating) means the flux can be destroyed by a much higher voltage that that used to generate that flux (there is no penalty (in terms of barrel velocity) with turning on coils early, compared to a significant one with turning them off late!

In all cases, you need to maximise the amp turns for any given inductance value

[jbb]

Hmm, if it’s really important to discharge the inductor quickly, there might be some interesting (I.e. complicated ) resonant snubber tricks which could still ring the energy back into the main rail.

Or maybe just adding a (non inductive!!!) discharge resistor in series with one of the diodes (and increase the voltage rating of the opposing switch) would do the trick. Lossy but easy :-).