ER2 mods: Spyder threaded barrel adapter (not currently 100% compatible with all barrels), Buffer tube and mounts for AR style milspec butt stocks, new larger pump handle with integrated weaver rail, and a clamping feed neck. I also made two inserts that go into the breach that fill the voids in the plastics housing halves so that the balls will roll more smoothly into the chamber, but my ending measurements were a bit off and I had to do some sanding. So that file isn't posted yet.
Clamping feed neck is made out of Taulman alloy 910. It's strong and flexible and works perfect for this part. (highly recommended) You will need an M4 bolt with a length of 25mm, with the corresponding nut. (for reference, it takes an 8mm socket) Also the bottom uses set screws to mount the feed neck tightly to the feeding tube. You can use any size you like. I just drilled out my holes to the size I needed and threaded them with a tap. IMPORTANT NOTE Alloy 910 is stringy and if you allow your printer to cross the open path for the paintball, getting the strings removed can be darn near impossible. My slicer (simplify3d) has an option to avoid crossing open paths. This is great for making parts with hollow voids with alloy 910. So check to see if your slicer has this option before printing in alloy 910. Unless your really good at retraction settings. The feed neck is designed to be printed without supports in it's upside down position.
All other parts were made out of Proto Pasta HT CF. The buffer tube mount uses a 10-32 allen bolt that is 1 inch long (25.4mm). (For reference, the nut takes a 3/8" socket.)