I was wondering if it is possible to use a 1,5 farad power cap capacitor as the energy source for a rail or coil gun.
Don't know if it is a electrolyte or a cheramic capacitor.

Tech sheet:

http://www.dls.se/english/tillbehor/dls_power_caps.htm

max charge voltage 20 DCV
max temperature 95 *c
capacitance 1.5 farad

You have to consider other things.

The application is to send a projectile if I understand you correctly. Well, that means giving it kinetic energy. This energy comes from a coil, I'm guessing, so it is converted from magnetic energy to mechanical kinetic energy. Then, you have to consider the energy for the coil is, I'm again guessing, coming from your capacitor. To sume up:

E_cap -> E_magnetic -> E_projectile.

In both conversions, some energy will be inevitably lost, I don't care how careful you are, but that is not to say that you shouldn't be careful. Actually, being careful about this in your design is the essence of engineering. So, how to be careful?

First, the type of cap will be important. You want the lowest possible series resistance, because this will be the main reason on the cap side of things why you have loss.

Then, things get a lot more complicated because the cap, coil, and kinetics of the projectile are all interrelated. You have to design your coil for a particular inductance (no simple calculation in a real physical case) that gives you the proper discharge rate. As you are calculating you inductance, you must consider the geometry and coupling of the flux between the coil and the projectile. Also, it probably won't work very well unless you design a flux gradient into your coil, so this complicates the inductance.

Of course, on the projectile side of things, you will want to minimize your friction and such.

This is just a brief overview of the problems that you will need to address. Can you just blindly slap something together without these considerations? Of course, but let's not confuse coincidence with principle.

I guess I really didn't aswer your question. My answer is, I don't know; it would have to be calculated. But, as a first guess, I would say "no," I don't think you will be able to store enough energy in the cap.

For a capacitor:
energy = 0.5*capacitance*(voltage)^2

This is the maximum energy that would be converted to kinetic energy of the projectile if the whole thing were 100% efficient. I would estimate that it will probably be at best 50%. According to your specs:

0.5*m*v^2 = 50%*0.5*(1.5 F)*(20 V)^2 = 150 J
=> v ~ 500 mph (assuming 1 g projectile and no calc. mistake)

This is the speed of a good pellet gun. The mass of the pellet is on this same order I'm sure, so the result would be approximately the same. And notice, that's after you go through all the pain of design. Not to mention that you can probably fire a good pellet gun (CO2) several times whereas this accounts for using all the energy in the cap for only one shot.

You will probably either want to use a higher voltage cap or a higher capacitance.