Wiki says...
In a depletion-mode MOSFET, the device is normally ON at zero gate–source voltage. Such devices are used as load "resistors" in logic circuits (in depletion-load NMOS logic, for example). For N-type depletion-load devices, the threshold voltage might be about –3 V, so it could be turned off by pulling the gate 3 V negative (the drain, by comparison, is more positive than the source in NMOS). In PMOS, the polarities are reversed.
So for a depletion-mode PMOS it is normally ON at Zero volts but you need 3V or more on the gate higher than the supply voltage to turn OFF. Where do you get that voltage? I think , that's why it is uncommon.
In practise now we call them High Side Switches or Low Side switches for power MOSFETs. They prefer not to combine enhancement and depletion mode in the same chip as the processing costs are almost double. This patent defines some innovation and better physical desc. than I can remember. http://www.google.com/patents/US20100044796
It is possible though what you are suggesting and performance are key issues. However when it comes down to low ESR, MOSFETS are like voltage controlled switches with ESR changing over a wide range of DC voltages unlike bipolar transistors which are 0.6 to < 2V for max peak in some case. Also for MOSFETs it is constructive to think of them as having an impedance gain of 50 to 100 when looking at loads and ESR of source. So consider you need a 100 ohm source to drive 1 ohm MOSFET and 10 ohm source to drive a 10mΩ MOSFET if you use 100:1, Conservative is 50:1. This is ONLY important during the transition period of the switch, not the steady state gate current.
Whereas bipolar hFE drops dramatically so you consider hFe of 10 to 20 good when saturated for a power switch.
Also consider that MOSFETS as charge-controlled switches during transition, so you want to have a big charge available to drive the gate capacitance and load reflected into gate with a low ESR gate drive, if you to make a fast transition and avoid commutation ringing or bridge cross-over shorts. But that depends on design needs.
Hope that isn't too much info and the patent explains how it works for all modes of P N type depletion and enhancement in terms of device physics.
My advice is not to start with this, or at least minaturise it - 70A is a huge amount of current to be pulsing through a coil, and some pretty nasty things could happen if you get things wrong (burns, explosions, electrocution, etc)
If you really want to make it, I'd start with the same circuit, but make the coil and current smaller (use a small bank of AA cells or something, keep the current under 5A or so) It will still work with a small coil, just be less powerful but much safer, and you will still learn as much whilst large designing it.
You need a (reasonably hefty - check the maximum Id specs in the datasheet) N-ch MOSFET for the coil switch, and an optocoupler as Gustavo has mentioned.
The reverse biased diode across the coil as Wouter mentions is a very good idea, certainly if it's your first project of this type (otherwise seriously high voltages can appear at the drain of the MOSFET - we are talking possibly in the region of a few kV, so please be careful and start with low currents)
A resistor in series with the diode will actually increase the current discharge speed, but you need to calculate it carefully (it depends on the current switched, how fast, and the coil and MOSFET specs). This coilgun related page discusses switching an inductive load. If you are not sure about the resistor value, don't include it.
Do a lot of reading before you start out, don't rush things, be safe, and have fun ;-)
Best Answer
The picture you show is probably from a TTL-style AND gate, where the multi-emitter transistor was used to function as a bunch of diodes (each from B to En), where each emitter could be used to pull the base connector low, thus implementing the AND function.
The reason this was preferred over separate diodes is that a multi-emitter transistor takes up less space on a chip than the corresponding set of diodes. And once you have such multi-emitter transistors on a chip, nicely integrated in a complete AND port, why would you want to build an (inferior) AND port yourself from its components? (If you wanted to do so, using separate diodes would be easier because they are small and cheap.)
A totally different use of muti-emitters is current splitting. With careful manufacturing a transistor can be made so that the current on one emitter is always a fixed part (let's say 0.01) of the current of the other emitter. This is used in OpAmps to create current sources, and in low-drop voltage regulators to implement overcurrent protection without a sense resistor in series with the main current (which would add to the dropout voltage). Again, no separate multi-emitters of this kind are (widely?) available because no-one wants to build something from scratch that is available better and cheaper in one package.