I use 2 methods for replacing bolt pistols with bolters depending on which arms I have available.
If the bolter is on the usual right arm but I need to put it on the left arm on a model I cut the whole bolter away from the hand (including the trigger). Ater cutting the bolt pistol off the model's left hand (being careful not to damage the hand itself) I then just file the contact surfaces to be flat and glue the bolter onto the hand as with the model on the left in the picture below.
If the model needs a bolter on the right arm that's even more simple as I just cut the bolt pistol hand and replacement bolter hand off at the wrist, leaving both as a a hand with the weapon attached. Then just file the connecting wrist joints to be flat and glue together, which was done with the model on the right.
(Ignore the paintwork, they're still in progress)
It takes a couple of minutes to do a batch of 5 and once you've added paint I don't think it's noticeable that they've been cut and replaced.
I use the wrist chopping method a lot to change both melee and ranged weapons or to kitbash as you really can't see the join with power armour wrists once painted. Just depends if weapons are already attached to the correct sided hand or not.
Edited by Thoridon, 16 April 2018 - 11:55 AM.