OK

for the mac cannons. you will need
1)some styrene trip OR just styrene sheeting bit it needs to be cut into 1cm strip.
i made mine 27mm but the length is up to your really.. remember to keep in mind the tubing needs to be as long.
2) 2 cotton buds ( i think the americans out there call them Q-tips) basically those things your not supposed to clean your ears with

3)STAPLES the secret ingredient

get a decent set of needle nose pliers itll take some trial and error, but lets face it you can afford to waste a few right.
4) those 3 circles off the back of a missile pod

5) a fusion blaster and a flamer you don't need any more.
6)Glue super and plastic.
7) either metal wire from a relatively large paperclip... ooor and this is what i used (gives a nice texture) the low E string (the fat one) from an electric guitar (this stuff is pretty cheap and a fantastic tool for converting stuff, id recommend getting some).
so to start with i would cut up the weapons. i made a diagram of how you want to cut up the fusion blaster, i think it explains it all quite well. and for the flamer you just need the fuel tank of the top. make sure to keep the attaching part of it on. cut closest to the flamer body and away from the fuel tank, you need it to give you some space between the Fusion blaster bit and the flamer tank.

Then cut the cotton parts off the cotton buds and measure and cut the length you need (don't worry about the ends being exact, the staples cover over the ends).
next glue the fuel tank off the flamer to the back end of the FB.
the next bit is tricky. i used plastic glue to stick the 2 bits of plastic rod to the body of the fusion blaster. making them stay in a straight line is a pain. but it is possible, once dry then repeat for the middle part of the FB, make sure you turn it the other way round so that the 2 grooves are at the end closest to the plastic rods.
let that lot dry then get your plastic strip and stick it so that it butts up against the plastic attaching points originally designed to let it slot into the crisis suits arms all along the length of the gun forwards of them.
next grab the metal wire (paperclip) or the much better guitar string and slot it between the plastic sheet and the plastic rod. then just put some superglue on at either end. do that both sides.
again let that dry and grab your trusty staples.
i used 2 lots. one was a group of staples 5 5 and the other was 7 wide. leave them all stuck together like they are when you get them. just break off a chunk of about 3 more than you need (some might break free. grab them with your needle nose pliers and bend them 90* about a CM appart. they will form a square. then simply get your clippers and cut the excess of. then superglue them to the gun.
next grab the metal wire (paperclip) or the much better guitar string and slot it between the plastic sheet and the plastic rod. then just put some superglue on at either end. do that both sides.
almost done now. get the back of the missile pod and glue that onto the side. took some of the spare staples i had left and ubent them into thape to make a connected between the 2 furthest circles. and to make a barrel i used a cross sectional cut off the end of the plastic rod
thats basically it. ill post pictures of it painted once its done.