Everyone should have a bits box, if you are a serious modeler... this is your "go to" place to kit-bash, franky-build, etc. This is my Space Marine bits box, which is a sectioned-tackle box purchased from Wal-Mart!
You are going to need: One (1) Heavy Bolter; six to seven (6-7) Space Marine frag/crack grenade glue-ons; one (1) Heavy Bolter magazine; one (1) Orkboy Shoota; some plastic rods off of some flashing;
First thing you are going to want to do, is cut the Heavy-Bolter Belt off of the gun with some clippers...
You will want to notch, or cut the Heavy Bolter magazine so that it fits into the place where the belt was attached to the Heavy Bolter (Sorry gang, I forgot to take a pic); but once that is done, you will want to start gluing the grenade packs onto the gun and magazine...
Once as that is done, clip the end of the Orkboy gun barrel off and glue it to the end of the Heavy Bolter. Glue on the rods, and then Green-stuff the casings around the rods in!
Here is the finished Fragcannon on the right!!! On the left, is the "actual" Fragcannon produced by GW! Because the bits are all from GW products, and it is almost unrecognizable from a distance... it is totally legit for tournaments! Enjoy your build!!!
Here is another variation build on youtube that is even quicker... and while it doesn't look as authentic as mine (IMO) it still looks GREAT!