Im casting a magazine for a paintball pistol using a two part mold made with CS2 silicone rubber and Fast Cast Polyurethane Casting Resin. The problem is that fast cast isn´t very strong. These magazines needs to be strong enough to be dropped down on a concrete floor. I have tried that with the fast cast mags and they just shattered. 

Fast Cast isn't ideal for that application sadly.  There isn't much you can add to it to make it less brittle. You could experiment with adding carbon fibres into the mould then pouring the resin around it.

I wouldn't recommend putting loose carbon into the resin itself, as when you mix it, it goes like custard and won't pour.

We don't currently stock any stronger PU casting resins. It is something we may look at in the future if there was sufficient demand and we could find a decent product.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant