Looks like your standard, bolter-armed, poorly photographed Astartes, right? Well, in the photo below, he's got himself a flamer. Emperor's teeth! How did that happen!? Was it forgotten tech-lore from a bygone age? A new piece of holy STC equipment?
Nonsense! Magnets are the key!
This conversion's easy, just grab some 1/16" x 1/32" magnets and a pin-vise with a 1/16" drill bit. Assemble your space marine as normal, but leave off the weapons. Now, drill a hole in the top of your marine's hand and another hole where you cut off the bolter's grip. After checking your polarities, stick your magnets in, then you're good to go. If you look closely at the photo, you can see where I put the magnets.
I say again, CHECK YOUR POLARITIES! You'll learn to do so the hard way if you don't check 'em before-hand.
I like to drill my holes a bit deeper than 1/32" and get the magnet a bit lower than flush. That way the weapons sit flush with the hand. Be careful drilling out the holes for the magnets. You've got a bit of wiggle room on the hand, but not too terribly much on the bolter, and it's very easy to screw up.
I'll have a real, not "I did this in 10 minutes" tutorial up sometime later in the month, most likely after my Forge World order comes in so I can actually start work on my marines.
As always, all the best,