I'll probly end up doing the carbine. While I do have access to a 3d printer the Rainbowpup and ESLT need it more than I'd like to get into for this project. The main reason for wanting the bullpup design was so I didn't have loads of extra barrel out the front but I think I can work around that. Could also do the pump snap but not sure if I wanna try and get the Alpha Trooper's pump working with that.Anything snap is easier in my experience, I just really like the carbine because no need for plunger rod Meaker.
I also would like to add, I agree with Langley that you should work on a beginners homemade. Those are just easier in my experience, and shouldn't be too hard. If you're really insistent in trying to put a homemade in a nerf shell, see if you can fit 1-1/4 in PVC in it. You'd probably have to cut out a lot of supports too. The only thing besides putting it in an LSK is a nitefinder rifle, and I don't remember if those hop.
I'm pretty sure 1-1/4 tube will fit in the Alpha Trooper. I was already planning on gutting the inside. I like doing things the hard way.
The first pick is what I'm going for the second is the internals, the lower one being the AT. I also have a deploy to work with too. I have a longshot, would way rather a longstrike though.