Brass rails, bunting bearings ok for x,y axes?
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Extruders tend to be quite light. An E3Dv6 complete with fan, shroud and wires is under 100g. Unless you use a Bowden extruder, you also have to add ~300g for a stepper motor. In any case, it will be quite light.

Sintered bronze bushings can definitely be used in a 3D printer; they're used in the (quite popular) Ultimaker 3D printers. I can't tell whether your particular bushings will work well, but I think the most important consideration is that there isn't too much play between the rod and bushing (but I'm assuming you took care of that). 3D printers don't place high loads onto the motion system.

"Proper" LMxUU bearings aren't very expensive either (reasonable ones can be had for less than $2) so if the bushings don't work out, you could always try those.