I have been going to some of the gun shows and they have been using oil filters on the end of there rifles as a dust cover , they sell an adapter to put on the end of a threaded barrel for a specific filter. Says its a grey area, don't know how they do it...
https://www.youtube.com/watch?v=g0tjGg2tO2g
https://www.youtube.com/watch?v=7t_pcWPdSDs