Bruce's own words on this particular phenomenon:
"Some of the songs on this album have what we call The Molossian Guitar. A Molossian is this ridiculous dog which weighs about two hundred pounds. It's a fucking monster! Our guitar tech got given one by a friend to look after, and it wrecked his car. It totally ruined the suspension because it was so big, then it ate half the car from the inside out. He moved out of his house in the end because his wife was going to divorce him over this dog! So when we came up with this guitar sound we called it The Molossian Guitar.
We took the D sting off a bass guitar and put that as the E string on a normal guitar and started with that, tuning the other strings in proportion. this guitar was just a fucking monster! At first Roy and Adrian were getting terrible hand pains because the strings were so heavy, but the sound...! We put it through an old Laney valve amp or something and it blew the roof off! People would walk into our rehearsal studio going, 'What is that sound?!' That's how our guitar sounds, mate. 'Yeah, what pedal is it?' No pedals. Straight into the amp. 'WOW!' It was absolutley ridiculous. It has low end, it has everything, and it just expands the sound enormously."
https://mariseb0.tripod.com/interviews/bd_terrorizer_98.htm