My design for a RepRap MILESTAG gun requires that the barrel is prevented from rotating about or sliding along its axis. Sliding would result in the barrel becoming longer or shorter which is obviously ridiculous. Rotation is a problem when there are attachments to the barrel (such as my aesthetic flash suppressor) or muzzle flash LED wiring.
I originally designed two rings, one of which had small tabs which mated with the barrel housing to prevent rotation. These tabs were woefully inadequate and I snapped one off whilst installing the barrel.
Here is a picture of the two rings installed. Note the small (i.e. inadequate) tab on the right hand side barrel ring.
I redesigned the holding brace and nophead printed it to his usual high standards:
And here it is fitted.