Just wanted to insert that brief editorial before we go anyfurther, alright? Wonderful.
So, anyway, making wrist-mounted weapons. It's not too difficult, quite quick, and doesn't take much in the way of resources. Heres the step-by-step guide, including pictures.
Step 1.
To start off with, take a (chaos) marine melee-weapon/pistol arm. *Carefully* remove the weapon, and then the hand. The weapon isn't needed again, so you don't actually have to be careful with it, but you will need the hand. I find it helps to cut a ridge in the gauntlets of chaos arms as a guide for the weapon. Step 2.
Now, take the chosen weapon - I've used knives and shotguns, but similar sized weapons ought to work - and glue it to the back of the forearm. once in place, re-attach the hand. If using a chaos arm, this will require you to shave the back of the hand, otherwise the nuckles will stop it from fitting properly. Ater this, cut two narrow strips of plasticard - about half as wide as the hand is deep, and one-and-a-half times as tall - and attach one to either side of the fist, holding the weapon in place. 
Step 3.Nearing the end now, cut out 3 rectangles of plasticard. Size is harder to explain that to demonstrate, so simply look at the picture. For reference, one should be about as wide as the two strips are seperated, with the other two slightly thinner. the length depends on how much of the weapon you want covered, but as a general rule of thumb, it should go from about the elbow to the far end of the hand. Once done, glue the widest one on top of the weapon, and the others along the sides of the arm.
Step 4.Final part - cut out a second, slightly wider rectangle, and glue it on the very top. this will leave some gaps between the top and sides, which I fill in with little right-angle triangles - point at the hand, base at the elbow - but greenstuff could equally be used. Same applies for the gap at the back - though given this size, a rectangle of plasticard would probably be easier.
And there you have it - Wrist-Mounted Weapons, in 4 easy steps. The uses of this are quite widespread, from converting models into Sanguinary Guard or Callidus Assassins through to making cool HQ choice - an example of which we have here.
for him, the only difference was that I used the ends of scout shotguns rather than knives for his weapons. As you can probably tell, I had been playing rather too much New Vegas at the time, but it did push me to create this technique, so I suppose it all worked out in the end.
I'd be happy to answer any questions or comments you may have.
