That's great, but as a guy who has had to "repair" more than one Joyride weapon, it's a whole lot easier to make a 2-piece mold.
For this exercise, you'll want to grab the Starter Kit from Smooth-On, the top maker of mold and resin products. Get this one:
Step 1: Make a box large enough to hold just under an inch of space all around the item you're casting. I use Legos to make my box.
Step 2: Insert modeling clay. Fill up the box halfway. (WARNING: if you use polymer clay like Sculpey, it will eat off the paint that Joyride uses to finish their toys. Not critical, but you will have to repaint.)
Step 3: Take your item (we'll say pistol from now on here), and push it halfway down into the clay. Use a small cone of clay to make a spout from the edge to the pistol grip to pour the resin in later. Also, IMPORTANT: take the round end of a pen and poke a few holes down into the clay in any pattern all around the item. These are your registering points.
Step 4: Spray the entire exposed surface inside the box with Mold Release. Let it dry completely.
Step 5: Mix the mold rubber. It's a 1:1 ratio, so even Grunts can't screw it up. Mix it well.
Step 6: Pour the Mold Rubber into the box. Start pouring from the lowest spot in the box, and let the liquid rubber SLOWLY cover everything, all the way up to the top of your box. HINT: now put the box on top of your clothes washer and turn it on spin cycle. It will vibrate all of the bubbles out without splashing the mold rubber. Let it sit for 6 hours.
Step 7: The rubber mold is now "solid." Turn the box over and peel out all of the clay. BE SURE not to disturb the pistol, though. That would mess things up. Spray the inside with Mold Release and let it dry.
Step 8: Repeat Steps 5 & 6 for this other side.
Step 9: When it's done, remove the box (or take apart all the Legos). The mold release makes sure the rubber does not stick to your pistol, but also that it does not stick to the other side of rubber. Feel with your fingers and you'll find the split. Slowly pull the two pieces of the mold apart and remove the original. Tada! You now have a 2-part mold.
NOW TO RECAST:
Step 1: Spray the inside halves of the mold with Mold Release. Let them dry.
Step 2: Put the mold back together. See how the holes in the one side became bumps in the other side? They fit into each other, keeping your mold lined up. Wrap a rubber band LOOSELY around the mold. WARNING: If you make the band too tight, it will distort your mold, and your cast.
Step 3: Put the mold in a shallow container, with the pour hole UP. This is to protect your kitchen table (or other surfaces) from the resin. Resin gets hot, and will destroy wood finishes.
Step 4: Mix the resin and catalyst. Again, this is 1:1, so it's pretty easy. WARNING: While the rubber has a pot life (the time before it starts to harden) of 30-45 minutes, the resin only has a pot life of 20-30 seconds. Resin needs a little more care to eliminate waste and get a good mix. Pour the proper amount in two paper cups. When everything is ready, pour them both into a third cup (large enough to hold all of both cups' contents). Mix it with a popsicle stick. Do it fast, but thoroughly. If the resin is not completely mixed, some parts of the cast may never harden.
Step 5: Slowly pour the resin into the mold. For all of this, you still only have 30 secinds, so to eliminate bubbles, try to pour a thin line at a good rate instead of dumping it into the mold. Stop when it's filled up. Tap on it, tilt it or flick it with your finger to jostle the bubbles out.
Step 6: Wait. The resin will turn from clear to white as you watch. It gets hot, so don't touch it. The reson is hard within three minutes, but wait five to make sure the inside is fully hardened too.
Step 7: Remove the band, split the mold, and remove your new pistol! A perfect match, with no glueing and no (or little) trimming!
Now you can repair your Halo weapons, or fully supply a 1:6 scale armory. Have fun. Resin casting is a BLAST!