Well, I've thought about writing this for a long time but I never got around to it, I'm sick today and missed school so I decided to do the forum members a little service here.
Just a picture showing my product of finished Heavy Bolter Bearers.
This is a trick to use when you want your big guns to be substitutional. Model the body.
Model the gun and the packs together.
Now 1 marine of yours can bear all the weapons without using extra models and magnets. This is also very convenient if you want your army to be very bapable to changing, you can try different things.
The only one that's slightly harder to do this trick to is the missil chick (since the pack does not connect to the weapon). But I figured out a method: make it substitude with the sergeant.
This one will require you to pin without gluing, and if you do it right, it will stay on unless you pull it off.
As well, glue the weapon and the hans together
And then just put it on!
Hope this helps guys, have fun trying this!