Personally I wouldn't even bother with that - with all my 'solid' projects (weaponry, some helm peaks and 'holes') I've found it's much more effective and efficient to inject self-expanding foam in there - of the sort you can find in any local hardware store. Wall filler, and the like. Just leave an end 'open' before you resin, fill that sucker up carefully, then seal, resin, and whatever. Makes a hard 'core' for the product, gives it the support it needs, and it'll lend your weapon a nice amount of weight to make it feel authentic.