Trion barrel retention

Thingi's remix-linking thing is busted again. This design uses the MuzzleNut_v4 from https://www.thingiverse.com/thing:4929499/files

Adds Sillybutts' o-ring-based barrel retention mechanism to your Trion. Replaces the orange muzzle tip that comes stock on the Trion. Technically works with the stock barrel but not particularly useful for that. Works very well with longer barrels.