Printable NERF Sledgefire Shell

New file 2/7/19 that should address length and dart fit issues I had in earlier version. All versions work in mk18. This is a remixed version of slug's shell modified for printing. It should replace stock shells. I made one and it worked, tweaked a few things and have posted here. Test one for both dart and blaster fit before printing several and let me know here or on /r/nerf if there are issues. Should print with the front down no support but requires good bed adhesion as this is a tall print with a small contact patch.

Also works in my [mk18](https://www.thingiverse.com/thing:3219077) break-action barrel.