I used to work for Bushmaster, and now that I don't represent the company anymore, I can throw out some hints.
First, yank the spring out of the middle of the trigger. It really doesn't need to be there at all and only increases the trigger pull. This is the spring between the trigger and the cam in the middle of the trigger. Removing that will help a lot.
Lubing all contact surfaces will also help, especially where the trigger cam rides on the inside of the frame, lots of friction right there.
If you're really handy, you might want to make another trigger cam out of aluminum or something, change the location of the holes slightly, and try that. You can change the leverage that way and change the trigger pull a lot.
Because of the linkage, smoothing the contact surfaces in the firing pocket usually doesn't really accomplish much, and I always recommend leaving them alone. You can do a lot more just by smoothing the link rod itself and lubricating it well.
That's pretty much it without getting too crazy and making custom springs and stuff like that. These fairly simple mods will improve the stock trigger a lot, but it will still be heavier than a regular trigger. Bullpup triggers are tough to tinker up, it's the design that complicates matters. Anyway, hope this helps.
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With my shield or on it...