Thanks for working on a bullpup, I like them.
Obviously you are much more experienced in this area than I am, but I played with some designs and ergonomics for pull pups and silhouette pistol layouts, as these share some common ergonomic challenges. Just for fun, I will throw in some ideas for you to use / abuse / discard as appropriate:
1) Bolt Cycling
- Since the right hand will be firmly and pleasantly on the pistol grip, the natural tendency will be to cycle the bolt with the left hand.
- Consider the Savage Striker and other silhouette pistols vs the original XP-100.
- Pick one up and fire it from a standing position - see for yourself how quickly cycling left handed makes more sense.
- It won't take long before many right hand shooters desire a left hand action, rather than twisting the rifle over counter clockwise to cycle the bolt left handed.
- If nothing else, please consider to make the platform friendly for left hand bolt actions.
2) Pistol Grip - Adjustable Position
- What I found (for me) was that the ability to move the pistol grip forward / backward) was much more useful than adjusting the stock position forward / backward.
- Consider to mount the pistol grip on a rail rather than a fixed position.
- Yes I know this makes the trigger mechanism issue bigger, but it also lowers the mass since you don't need such a complicated adjustable stock.
3) Trigger Mechanism
- One of the more successful designs is the FN P90
- A big reason for their success is that they didn't design it based on a single rod passing through precision bushings (usually not stiff enough) but rather a wider "ladder" type design.
- There are many machines in the world that have failed trying to do motion with a single thin rod in bushings. It can be done, but it is expensive and subject to dirt infiltration.
- Consider to use an electric trigger. Yes, non traditional, but it is just what the doctor ordered for a design like this.
This is one of the times when the much maligned Remington etronix system would be really handy, but that might be a bridge too far. Just keep in mind that when it comes to reliability, modern artillery doesn't use a hammer smashing into a primer.
Just some ideas to ponder, but don't let me throw your design into a mess.