Here's the removable shoulder stock which still accepts my 2x UMB.
The bushing is inset for less deadspace.
I need to redo the seal (hilariously janky but pretty good), and tweak a few things. The ring catch is functioning very well and you can really bang the blaster about without accidentally firing. I like it so far but need to build a larger version! There's only a few things that differ from SGNerf's design but I'll do a write-up when it's well tested.