Once you have obtained the BLUNDERGAT
you need to complete the following stages to make it a Magmagat, I found this video (not my work) really helpful
please also note:
if you are playing in a group:
Only one of you needs to complete the quest steps to unlock the magmagat, then the remaining players (as long as they have got a blundergat from the box) can instantly turn the blundergat into a magmagat by simply putting the gun into the machine where the first dog head spawns :)
As soon as you grab the gun back out the machine you'll be rewarded the achievement
I just wanted to add to this quick to give everyone a heads up: Me and my friends ran it again last night and we believe there is a slight bug with this … we had it where the gun disappears if you leave the house while its charging, also the timer on it, where you have to bash the barrels is off to. we had to run it twice as they kept randomly going out - which is a right pain.. hopefully this is something they will fix. the achievement is still obtainable it just might take an extra gun run if the barrels don't light or you run out of time