On one point, you are exactly right and that's why classification is always the key to great gold recovery. A 2 mesh piece of magnetite will both weigh more and displace far more volume of material then a 100 mesh piece of gold. But put a 100 mesh gold next to 100 mesh quartz and the gold wins every time. That's one reason I put the 4 mesh hardware cloth over mine to keep out the bigger rocks. On the other hand, you dont have to worry about the bazooka clogging unless you have it setup way wrong with too little flow or are in an area that has unusually heavy rocks in the river. Even with the stock design, there will usually be some room in the trap for heavier gold to settle in, especially at first and especially with bigger pieces. And once any gold settles down in the fluid bed, the only way to get it out is to displace it with something heavier. Also the smaller rocks tend to be pushed out and only the big ones left because as the trap fills up with black sand and heavies, the smaller lighter rocks will float on top of the fluid bed and eventually be pushed out with the gangue.
So you can see the bazooka works pretty good from the factory, but there is always room for improvement. Any piece of recovery equipment should always be tailored to the type of ore you are working. For example if your gangue is much heavier say you are in a place with a lot of cassiterite then you need increased flow, screening, and/or slope to compensate for the heavier concentrate. Likewise a lighter gangue means you could run less of said properties and still do well.
A bigger problem for me, and what I don't like about the stock bazooka grizzly is it lets in large flat rocks that may only be 1/2 inch wide but also 2 inches long. These rocks then turn sideways and lay flat in the trap and due to their heavier weight they are not pushed out by the fluid coming up from the lower pipes. Fine gold that is entering the traps can then hit these rocks and slide right out over the top with the lighter slurry since it has no place to settle down into the trap. But the 4 mesh hardware cloth zip tied over the trap fixes that so you dont have to worry about it.
Another fix you can do is use punch plate, which works well also.
