I've used 'form' tape on occasion (though finding more CADPAT is nearly impossible). I could swap out the tape easier than re-painting if I had 'painters remorse'.
As was mentioned, breaking up the silhouette. For me it not so much making the gun invisible as to hide what kind of platform I had running. If my opponent doesn't know if I'm running a carbine or LMG, or something else, that's to my advantage.
