It seems after much research the magmasaur gathering is affected by temperature. In Arctic it will farm 150-300 metal per rich node. In Volcanic (outside cave) it will farm about 3-500 metal per rich node. Inside cave 4-500 per node, and during eruption upto 700 per rich node.
My new favorite is farming ambergris on south spawn lunar (no radiation, easy evading rex and raptors, bring bloodstalker on neutral for raptor attacks). Bring him inside cave and follow left path all the way. Meet golems? Ambergris and 100% spits.
I farm upto 20k metal per run. I spawn on SE volcanic with skiff and bring him to entrance.
These guys are actually quite good climbers, I have a mining outpost on top Eldritch Isle on Crystal Isles and can easily keep two industrial forges filled and going at all times. Just watch out because they take quite high fall damage.
Magmasaur is able to destroy ALL STRUCTURES utilizing its searing spit. While it is a walking forge it produces metal at the same rate as a refining forge. You can use the jump button to taunt other creatures and when holding searing spit use crouch to erupt. (This attack pushes creatures back and leaves magma everywhere) it is breedable in lava. Hope this helped