The basis for the whole buggy was the fact that it needed to move quickly over any terrain, so Wolfe didn't want to weigh it down any more than he had to with Mana Crystals, but it did need enough to sustain its defences if it was attacked by a Rank Two or Three Monster.
They hadn't seen many, if any, Rank Four monsters in the wilderness, which was good for him, since they hadn't distributed any weapons that would seriously threaten a Rank Four monster without sustained fire.
Just like with his own armour. One Rank three or four Witch was no real threat. But dozens of them at once, or like the machine gun, firing offensive spells ten times a second, was a very different matter.
So he used a higher density mana crystal in the floor pan to increase the capacity, and then started to layer the defensive Inscriptions under the driver's seat. It was a good spot that wasn't going to see a lot of wear and tear, so the spells should be fairly safe there.
