Toland would disagree. Also he isn’t much fun at parties.

I just assumed he used a generic scout rifle frame (same model was used for alot of green scouts in d1) but he could have possibly used a MIDA weapon. also why is Juju a pulse when it uses a scout frame? what would turn the mechanisms in the weapon to go from semi-auto to burst fire?

It is certainly possible. After all Toland had to start with a base weapon/frame to begin with. It is never stated that Bad Juju is a weapon of sorrow, however; it is very likely that Toland's obsession with the Hive is probably how he came up with the idea and means of creating Bad Juju. And all the weapons of sorrow, in their original state, were just normal weapons that were corrupted with Hive magic. I would have to speculate in the same direction that you have. Bad Juju's origin frame was most likely made by the same weapon smiths that created the MIDA Multi Tool. Also let's be honest here Toland my be considered Shattered by most, but he was no idiot and would probably seek out one of the best gun smiths to help him with project before he was exiled from the City. Before anyone comments that "weapons of sorrow don't need to be made from preexisting weapon frames look at Touch of Malice and Wisper of the Worm". All I have to say is yes they are weapons of sorrow, but they are crafted from the Hive which means they have already been corrupted with Hive magic and therefore do not need to be tainted with Sword Logic because it already exists as an item of the Sword Logic